St. Mary's County, MD: Owner-Builder Allowed

Unincorporated St. Mary's County, Maryland requires a building permit for a new single-family home, including modular and mobile homes¹. County Permit Services handles building applications and Development Services provides zoning determinations; inside Leonardtown, the town issues building permits and sets zoning²³⁴⁵. Septic applications throughout St. Mary's County go to the county Health Department's Environmental Health Division⁶.

At a glance

Building permit: Required for a new single-family home; $20 application fee plus other agency fees and excise tax¹⁷
Zoning: County zoning review in unincorporated areas; Leonardtown has its own zoning³⁵
Septic: Handled by the county Health Department's Environmental Health Division; permit valid 2 years⁶⁸
Owner-builder: You may name yourself as builder, but permits, inspections and occupancy approval still apply⁹
RV on your land: One camping trailer allowed on a 1+ acre lot for intermittent recreational use only, up to 7 consecutive nights, never as a permanent residence¹⁰
Wells: A well construction permit is required statewide, issued only to licensed well drillers¹¹
Septic soils: Only 1% of county soils rate "not limited" for conventional drain fields; site evaluation is essential, and septic may still be feasible¹²
Property tax: Effective rate about 0.9%, typical bill near $3,392/year¹³¹⁴

Who to call

Building permits

St. Mary's County Department of Land Use & Growth Management, Permit Services Division

Building permits and other permits including environmental (Critical Area and Floodplain), residential entrances, home occupations and sign permits for St. Mary's County

(301) 475-4200

41770 Baldridge Street, Leonardtown, MD 20650

Zoning

St. Mary's County Development Services Division (Land Use & Growth Management)

Zoning determinations, and multi-agency review of site plans, subdivision plans, boundary line adjustments and confirmatory plats for St. Mary's County

(301) 475-4200

41770 Baldridge Street, Leonardtown, MD 20650

Floodplain permits

St. Mary's County Floodplain Management (Land Use & Growth Management)

Floodplain development permits for construction wholly or partially within, or in contact with, a flood hazard area in St. Mary's County

(301) 475-4200

41770 Baldridge Street, Leonardtown, MD 20650

Septic permits

St. Mary's County Health Department, Environmental Health Division

Individual septic system construction, expansion and repair applications, perc tests and pre-covering inspections for St. Mary's County

(301) 475-4321

smchd.env@maryland.gov

21580 Peabody St., Leonardtown, MD 20650

Contact details are copied from each office’s official page and were checked against it on October 12, 2026.

Off-grid gear

Going off-grid in St. Mary's County means bringing your own water and power. The kit we’d start with:

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Do you need a building permit in St. Mary's County, Maryland?

Yes: unincorporated St. Mary's County requires a building permit for a single-family dwelling, whether stick-built, modular or a mobile home, as well as specified residential work such as additional living space, accessory apartments and townhouses¹. The county uses a Drop Off process for these residential applications; a project may also need review for septic or a private well, public utilities, floodplain or Chesapeake Bay Critical Area issues, and soil disturbance over 5,000 square feet¹. Permit Services routes the application to the agencies whose approvals are required under the building code, zoning ordinance and subdivision regulations². Expect a $20 application fee, a $0.16-per-square-foot new-dwelling fee and a $6,697 single-family excise tax, plus applicable agency charges⁷. St. Mary's County incorporates the 2021 International Residential Code alongside the 2021 building, energy, existing-building, and swimming-pool and spa codes, with Maryland amendments¹⁵. The ordinance adopted the 2018 fuel-gas and mechanical codes pending state updates; later editions take effect six months after the later state effective or publication date, so confirm the applicable editions with Permit Services¹⁵.

Who issues building permits in St. Mary's County, Maryland, and how do you reach them?

The Permit Services Division of the county's Department of Land Use & Growth Management processes unincorporated-area building, environmental, residential entrance, home occupation and sign permits, then routes permit information to other agencies for required review and approval². Reach Permit Services at (301) 475-4200, 41770 Baldridge Street, Leonardtown, MD 20650; applications are accepted Monday and Thursday from 8:30 a.m. to 2:00 p.m., and Tuesday, Wednesday and Friday from 8:30 a.m. to 4:30 p.m.¹⁶. Inside Leonardtown, obtain the town's building permit before erecting a building or addition; septic applications still go to the St. Mary's County Health Department⁴⁶. Development Services handles zoning determinations and coordinates reviews of site plans, subdivisions, boundary-line adjustments and confirmatory plats at the same address and phone number¹⁷¹⁸. Floodplain development permits for work in contact with a flood hazard area are also handled there¹⁹. A new home may be occupied only after required final sign-offs and issuance of the Certificate of Use and Occupancy²⁰.

Does St. Mary's County have zoning?

Yes: county zoning review applies in unincorporated St. Mary's County, and Development Services provides zoning determinations on request³. Inside Leonardtown, the town's own zoning ordinance applies; septic applications remain with the St. Mary's County Health Department⁵⁶. District-specific uses and dimensions must be read with the zoning map and the district assigned to the parcel²¹. A detached house in a qualifying residential cluster development has a 6,000-square-foot minimum lot size, but that cluster rule is not a general minimum for every parcel²². Minimum lot sizes and principal-structure setbacks are set by zoning district and, for setbacks, road classification in Schedule 32.1, and Critical Area rules can add requirements²³²⁴. For example, a Commercial Marine parcel needs at least 1 acre, and a Critical Area RL-T parcel with the footnoted entry needs 2 acres²⁵²⁶. The 720-square-foot minimum dwelling size applies only to mobile homes in the PUD-MHP mobile-home-park district, not to every dwelling²⁷. We could not confirm lot-coverage or building-height limits; ask Development Services for a written parcel-specific determination¹⁷.

How do septic permits work in St. Mary's County?

The St. Mary's County Health Department's Environmental Health Division administers applications to construct or expand individual septic systems and reviews repair applications for failed systems⁶. A perc test is required for new construction or repair, and new construction requires a site plan from a Maryland-registered land surveyor; construction may begin after application approval, and the permit is valid for 2 years⁸. The Health Department must inspect the system before it is covered²⁸. A property without public water and sewer needs a perc test before construction begins on any structure with plumbing²⁹. Only 1% of St. Mary's County soils are rated without severe limitations for a conventional septic drain field, so make septic feasibility an early contract contingency and allow room in the budget for site-specific design and work¹². A new well is a separate permit: Maryland requires a well-construction permit for any well, and the St. Mary's County Health Department issues permits for new wells only to licensed well drillers¹¹.

Can you live in an RV or tiny home on your land in St. Mary's County?

An RV cannot be a permanent residence in unincorporated St. Mary's County¹⁰. One camping or recreational trailer may be occupied on a lot of at least 1 acre in an allowable zoning district only for intermittent private recreation, for no more than 7 consecutive nights; it must be screened from neighboring dwellings and have an authorized self-contained sewage holding tank¹⁰. We could not confirm a separate tiny-home occupancy rule. A small second unit may instead qualify as an accessory apartment, but it needs permit approval and is limited to one per lot or parcel; the owner must occupy one dwelling unit³⁰. Ask Development Services for a written determination before buying or placing a tiny home¹⁷. A tiny home used as a dwelling remains subject to building-permit, inspection, septic and occupancy requirements¹²⁰.

What are the steps to build a home in St. Mary's County, Maryland?

Ask Development Services for the parcel's zoning district and a written zoning determination; inside Leonardtown, start with the town's planning and zoning authority³⁵.
Before dividing unincorporated land, obtain subdivision authorization or confirm an exemption; no lot may be sold or transferred and no building erected until the plat is approved or otherwise legally recorded³¹³².
If public water and sewer are unavailable, complete the Health Department's perc-test and septic process before building a structure with plumbing⁶²⁹.
Apply through Permit Services using the residential Drop Off procedure; Permit Services will route the file to other agencies whose approval is required¹².
Obtain a floodplain development permit if the site is in contact with a flood hazard area³³.
If soil disturbance exceeds 5,000 square feet, submit a Sediment & Erosion Control Plan prepared by a licensed surveyor or engineer¹.
Keep permitted work visible and accessible at each inspection point until it is approved, then obtain every final sign-off and the Certificate of Use and Occupancy before moving in³⁴²⁰.

What should you check before buying land in St. Mary's County, Maryland?

Zoning and subdivision: obtain the parcel's zoning designation and a written determination; inside Leonardtown, confirm requirements with the town²¹⁵. A division of unincorporated land needs subdivision authorization or an exemption before a lot is sold, transferred or built on, and each new lot must have required water and wastewater service³¹³². Septic and soils: only 1% of county soils are rated without severe conventional drain-field limits, and land without public utilities needs a perc test before construction of a plumbed structure¹²²⁹. A parcel in the MetCom service area may receive a waiver or deferral for interim private service but later be required to connect when public service is feasible³⁵. Check flood mapping before purchase because a floodplain permit may be required and new flood-prone building sites are prohibited in covered subdivisions³³. Direct residential access to a major collector or arterial is allowed only when no feasible alternative exists, with 150 feet between entrances measured from centerline³⁶.

Watch-outs

A new single-family home in St. Mary's County carries a $6,697 excise tax on top of the $20 application fee and per-square-foot dwelling fee, a cost many buyers don't budget for⁷.
A St. Mary's County accessory apartment requires permit approval, one owner-occupied dwelling unit and an additional parking space, and only one accessory apartment is allowed per lot or parcel³⁰.
St. Mary's County's building code has a narrow agritourism exception, including an existing-building occupancy exception for subordinate use by no more than 200 people with an egress-width condition; ask Permit Services whether your project qualifies¹⁵¹⁶.

Sources

Written October 12, 2026 from the rules quoted below, each taken from an official source and re-checked by an independent reviewer. This is research, not legal advice: rules change, so confirm with the office before you buy or build.

Compared with neighboring counties

Like all 6 of its neighbors, St. Mary's County has countywide zoning.

Rule	St. Mary's County	Calvert County	Charles County	Dorchester County	Prince George's County	Talbot County	Anne Arundel County
Building permit	Required	Required	Limited	Required	Required	Required	Required
County zoning	Yes	Yes	Yes	Yes	Yes	Yes	Yes
Septic permits	County Health Dept.	County Health Dept.	County Dept. of Health	County Health Dept.	County Health Dept.	County Health Dept.	County Dept. of Health

How St. Mary's County scores

Seclusion25100% ev.

Self-Sufficiency75100% ev.

Admin Boringness42100% ev.

Durability80100% ev.

Property Autonomy43100% ev.

Carry Cost55100% ev.

Evidence strength: 100%. This reports direct-source completeness and fallback use; it is separate from the score and is not a confidence interval. How it works →

$440kTypical home

0.9%Property tax

$8,652Land / acre

45.3"Annual rain

1.6% of yrsSevere drought

4.4 / yr (+4.2)Mid-century 100°F days

13 daysMid-century dry spell

2.16" (+5.6%)Mid-century 1-day rain

1.45×Wildfire change factor

100%Forward climate coverage

$1,748Home insurance / yr

3.2% / yrInsurance growth

0.3%Nonrenewal rate

county median (2+ published ZIPs)Insurance basis

58,850People within 30 min

6,612,918People within 2 hr

119,176Radius comparison (15 mi)

321.5People / sq mi

3.9%Federal land

89.8%Land at 15% slope or less

1%Soils rated "not limited" for septic (strict USDA rating)

2,058 acWorkable land screen

57.5%Wooded land

78.8 tonsBiomass / forest ac

57.4°FAvg temp

Answers from the rulebook for St. Mary's County

What the ordinances and state agencies actually say — one answer per question, each quoted from the official source and independently re-checked.

St. Mary's County rules

Building permitRequired
Residential land in St. Mary's County, including construction of a single-family dwelling such as a stick-built, modular, or mobile home.
- The county uses a Drop Off process for construction of a single-family dwelling, additional living space, accessory apartments, townhouses, and specified other residential development.
- Additional reviews may be required for septic or private wells, public water or sewer, soil disturbance exceeding 5,000 square feet, floodplain, Chesapeake Bay Critical Area, or other environmental features.
- A building permit application requires the applicable application materials and a $20 application fee; other agency fees, bonds, and excise tax may also apply.
application fee: 20 USD (Other agency fees, bonds, and excise tax may also apply.) · soil disturbance threshold for Soil Conservation District review: > 5000 square feet (A Sediment & Erosion Control Plan prepared by a licensed surveyor or engineer is required.)
“Use this procedure to obtain a Building Permit on Residential land if the following kind of development is proposed: • Construction of a single-family dwelling (stick built, modular, mobile home)”
How to Apply for a Residential Building Permit ↗Re-checked Sep 24, 2026
Owner-builder exemptionConditional
An owner-builder applicant for a residential construction project in St. Mary's County.
- The county permit application allows the applicant to identify the builder as either a contractor or 'Myself'.
- Selecting 'Myself' does not exempt the project from the building-permit requirement, inspections, certificate-of-occupancy requirements, or other applicable approvals.
- The official sources reviewed do not establish a broader exemption from Maryland home-builder registration, trade licensing, or other licensing requirements.
“Who will be the Builder? Contractor_____________________________________ Myself____”
St. Mary's County Permit Application ↗Re-checked Sep 24, 2026
Building code adoptedAdopted (with conditions)
Building construction regulated under Chapter 203 of the Code of St. Mary's County, including commercial and residential construction, existing-building work, energy conservation, and swimming-pool and spa work.
- The county ordinance incorporates the 2021 International Building Code, 2021 International Residential Code, 2021 International Energy Conservation Code, 2021 International Existing Building Code, and 2021 International Swimming Pool and Spa Code, each as amended by the cited COMAR provisions.
- As of adoption, the State Plumbing Code had not been updated to the 2021 International Fuel Gas Code; the ordinance instead incorporated the 2018 International Fuel Gas Code with amendments, with the 2021 code to become effective six months after the later of its state effective date or publication date.
- As of adoption, the Maryland HVAC-R board had not incorporated the 2021 International Mechanical Code; the ordinance instead incorporated the 2018 International Mechanical Code with amendments, with the 2021 code to become effective six months after the later of its state effective date or publication date.
- Future editions of referenced codes, as modified by the State, are incorporated and become effective six months after the later of the effective date or publication date.
- The ordinance provides an agritourism exception for certain agricultural-building construction, alteration, or modification; an existing agricultural building used for agritourism is not considered a change of occupancy requiring a building permit when the subordinate use does not require more than 200 occupants at one time and the stated egress-width condition is met.
- The ordinance does not apply to work covered by a valid building permit issued after the ordinance's effective date when the permit application was filed before that effective date and manifest commencement of work occurs within 12 months after the effective date.
- Permit fees are set by ordinance or resolution and collected by the St. Mary's County Department of Land Use and Growth Management.
incorporated building code edition: 2021 IBC (as amended by COMAR 09.12.51 et seq.) · incorporated residential code edition: 2021 IRC (as amended by COMAR 09.12.51 et seq.) · incorporated energy code edition: 2021 IECC (as amended by COMAR 09.12.51 et seq.) · incorporated existing building code edition: 2021 IEBC (as amended by COMAR 09.12.58 et seq.) · incorporated swimming pool and spa code edition: 2021 ISPSC (as amended by COMAR 09.12.56 et seq.) · agritourism occupant threshold: <= 200 people (the subordinate use of agritourism must not require more than 200 people to occupy an individual building at any one time)
“Sec. 203-3. Incorporated by Reference. A. In this Chapter, the following are incorporated by reference: 1. 2021 International Building Code (IBC), as amended by COMAR 09.12.51 et seq.; 2. 2021 International Residential code (IRC), as amended by COMAR 09.12.51 et seq.; 3. 2021 International Energy conservation code (IECC), as amended by COMAR 09.12.51 et seq.; 4. 2021 International Existing Building Code (IEBC), as amended by COMAR 09.12.58 et seq.; and 5. 2021 International Swimming Pool and Spa code (ISPSC), as amended by COMAR 09.12.56 et seq.”
St. Mary's Building Code, Ordinance No. 2024-11 ↗Re-checked Oct 1, 2026
One more reviewed finding on this topic
- Adopted (with conditions) — Countywide building-code administration by St. Mary's County Department of Land Use and Growth Management under the local Building Code Ordinance.
  “During their business meeting on June 25, 2024, the Commissioners of St. Mary’s County (CSMC) approved proposed updates to the local Building Code Ordinance to repeal and reenact Chapter 203 entitled, "Building Construction" and repeal Chapter 223 Article IV entitled, "Inspection Fees.” These changes will take effect on July 15, 2024.”
  Updates to Local Building Code Ordinance Effective July 15 ↗Re-checked Oct 4, 2026
InspectionsRequired
Construction or work for which a St. Mary's County building permit is required.
- The work must remain visible and accessible for inspection until approved.
- Work may not proceed beyond each successive inspection point without approval.
- The permit applicant must bear expenses associated with removing or replacing material needed to allow inspection.
“Construction or work for which a permit is required shall be subject to inspection by the code official and/or the building inspector and such construction or work shall remain visible and able to be accessed for inspection purposes until approved.”
St. Mary's Building Code ↗Re-checked Sep 24, 2026
Certificate of occupancyRequired
Occupancy of a newly constructed dwelling in St. Mary's County.
- The certificate is issued after construction is completed and the building has been inspected and found compliant with applicable regulations tied to the building permit.
- The applicant must obtain required agency final-inspection sign-offs; if an agency sign-off is not required, permits staff mark the card N/A.
- The final inspection card must be brought to the Department of Land Use and Growth Management after all required agencies have signed it.
“For an applicant to occupy a newly constructed dwelling, regulations administered by the Department of Land Use and Growth Management require issuance of a Certificate of Use and Occupancy pursuant to Section R110 of the Building Code and Section 22.7 of the St. Mary’s County Zoning Ordinance.”
Certificate of Occupancy - Residential ↗Re-checked Sep 24, 2026
County-wide zoningPresent (with conditions)
County-administered zoning applicable to permit and development review in St. Mary's County.
- Permit processing is subject to applicable Building Code, Zoning Ordinance, and Subdivision Regulations.
- The county's Development Services Division provides zoning determinations upon request.
“The Permit Services Division’s principal function is to process building and other permits including environmental (Critical Area and Floodplain), residential entrances, home occupations, family daycare, change of business occupants, and sign permits in accordance with the applicable regulations, such as the Building Code, Zoning Ordinance and Subdivision Regulations.”
Permits - St. Mary's County, MD ↗Re-checked Oct 1, 2026
Local zoningPresent (with conditions)
County zoning review and determinations for development and permit applications in St. Mary's County.
- The Permit Services Division processes permits in accordance with the applicable Zoning Ordinance and Subdivision Regulations.
- Development Services coordinates review of site plans, subdivisions, boundary line adjustments, and confirmatory plats and provides zoning determinations upon request.
“The Development Services Division coordinates multi-agency reviews for Concept, Major and Minor Site Plans, Subdivisions Plans, Boundary Line Adjustment and Confirmatory Plats, and provides zoning determinations upon request.”
Permits - St. Mary's County, MD ↗Re-checked Oct 1, 2026
Zoning districtsPublished
St. Mary's County zoning districts and zoning ordinance materials published through the county's Development Review and legal-resources pages.
- The county publishes a Zoning Ordinance PDF link as part of its official Development Review materials.
- District-specific dimensional and use controls must be determined from the applicable zoning district and official zoning map; this claim does not assign one district's controls countywide.
- The source page does not itself provide a complete district list or parcel-specific zoning designation.
“The Development Review division guides responsible growth within St. Mary's County. We evaluate proposed commercial and residential site plans, subdivisions, and infrastructure projects to ensure strict compliance with public works standards, adequate stormwater management, and effective traffic mitigation. Adequate Public Facilities Report Form Bond Estimating Worksheets Construction and Utility Permits Design Standards (PDF) Easement Agreements Environmental Site Design (PDF) Fee Schedule Ordinance (Current) (PDF) Format Guidelines (PDF) General Discharge Permits Grading Permits Inspection Request Form MDE Design Manual (PDF) MDE SMW Update (PDF) NOCC Form (PDF) NOCC Form Instructions (PDF) Permit Extensions Permit Release Permit Services Center Plan Revisions Private Roads Public Roads Record Plats Road Ordinance (PDF) Stormwater Mgmt Inspection & Maint Agreement (PDF) Subdivision Ordinance (PDF) Submission Conferences SWM, Grading, E&S Ordinance (PDF) Zoning Ordinance (PDF)”
Development Review - St. Mary's County, MD ↗Re-checked Sep 24, 2026
Minimum dwelling sizeDistrict specific (with conditions)
Mobile homes used as dwellings in the St. Mary's County PUD-MHP, Planned Unit Development—Mobile Home Park district.
- The 720-square-foot minimum and 12-foot minimum width apply to mobile homes in the PUD-MHP district.
- Double-wide mobile homes are also listed as permitted structures.
- The cited section is district-specific and does not establish a single minimum floor area for every dwelling type or zoning district.
- PUD-MHP development is also subject to applicable county zoning, subdivision, health, sewerage-disposal, building-code, and permitting requirements.
minimum dwelling floor area: >= 720 square feet · minimum mobile-home width: >= 12 feet
“2. Permitted Structures. The following structures may be permitted in the PUD-MHP: a. Mobile homes with a minimum of 720 square feet that are a minimum 12 feet in width; and b. Double-wide mobile homes.”
St. Mary's County Comprehensive Zoning Ordinance, Article 4 ↗Re-checked Oct 9, 2026
Minimum lot sizeSpecified
Single-family detached dwellings in residential cluster developments under Section 32.3
- The minimum site area for any cluster development shall be three (3) acres unless adjoining similar residential development.
- A structure containing three (3) or more dwelling units in a cluster development shall be a minimum of 75 feet from the boundary of the site where the site adjoins a single-family dwelling development.
- Proposed residential cluster developments must obtain preliminary plan approval or site plan approval from the Planning Commission before proceeding to final approval.
minimum lot size for a single-family detached dwelling in a cluster development: 6000 square feet
“Minimum lot size [...] shall be 6,000 square feet.”
stmaryscountymd.gov ↗Re-checked Oct 4, 2026
3 more reviewed findings on this topic
- Specified — RL-T zoning-district parcels in the Critical Area.
  “Minimum Lot Area none none none None17 none none none none none none none none none none none 1ac [...] 17. In the Critical Area, minimum lot size shall be 2 acres.”
  St. Mary’s County Comprehensive Zoning Ordinance, Article 3 ↗Re-checked Oct 6, 2026
- Specified — Commercial Marine (CM) zoning district parcels.
  “Minimum Lot Area none none none None17 none none none none none none none none none none none 1ac”
  St. Mary’s County Comprehensive Zoning Ordinance, Article 3 ↗Re-checked Oct 6, 2026
- District specific (with conditions) — Minimum lot dimensions and related lot-area standards governed by Schedule 32.1 of the St. Mary's County Comprehensive Zoning Ordinance, varying by zoning district.
  “The Planning Director has approval authority to apply the standards for variance to requests to relax: 1. the minimum lot dimensions or minimum setback standards of Schedule 32.1 of the Zoning Ordinance”
  Board of Appeals - St. Mary's County ↗Re-checked Oct 7, 2026
SetbacksDistrict specific (with conditions)
Principal-structure setbacks governed by Schedule 32.1 of the St. Mary's County Comprehensive Zoning Ordinance, varying by zoning district and road classification.
- Schedule 32.1 provides the applicable minimum setback standards.
- A variance may be available through the County's administrative or Board of Appeals process.
- Critical Area regulations and other ordinance provisions may impose additional requirements.
“The Planning Director has approval authority to apply the standards for variance to requests to relax: 1. the minimum lot dimensions or minimum setback standards of Schedule 32.1 of the Zoning Ordinance”
Board of Appeals - St. Mary's County ↗Re-checked Oct 7, 2026
Living in an RVLimited
Occupancy of one camping or recreational trailer on an owner's lot in an allowable St. Mary's County zoning district.
- Recreational vehicles may not be occupied as permanent residences.
- The parcel must be at least 1 acre.
- Use must be intermittent and for private recreational use and enjoyment of the owner's property, such as camping or hunting.
- Occupancy may not exceed seven consecutive nights.
- The trailer must be completely screened from dwellings on adjacent lots by a fence or evergreen buffer.
- The trailer must have an authorized self-contained sewerage holding tank.
- Recreational vehicles must comply with the Ordinance's floodplain provisions.
minimum parcel size: >= 1 acre · maximum consecutive occupancy: <= 7 nights
“Recreational vehicles may not be occupied as permanent residences.”
St. Mary’s County Comprehensive Zoning Ordinance, Article 5: Regulation of Uses ↗Re-checked Sep 24, 2026
Accessory dwellingsLimited
Accessory apartments, including units within a principal dwelling or accessory structure, regulated by the St. Mary's County zoning ordinance.
- Permit approval is required.
- Within a principal dwelling, the apartment must share at least one wall with the principal dwelling's living space.
- The minimum gross floor area is 300 square feet.
- The maximum gross floor area is the lesser of 40 percent of the principal dwelling's gross floor area or 900 square feet; attached garages are excluded from the calculation.
- A separate entrance must be on the side or rear, secondary and less visible than the primary entrance, and may not face the street except on a corner lot satisfying the section's other provisions; on a corner lot it may not be accessed by an exterior stairway on the street-facing sides of the principal dwelling.
- A basement apartment may consist of the entire basement but must comply with all applicable codes.
- The principal dwelling must retain the appearance of a single-family dwelling.
- Within an accessory structure, the minimum is 300 square feet and the maximum is the lesser of 40 percent of the accessory structure's gross floor area or 900 square feet.
- A separate accessory-structure entrance may not face the street.
- Building materials and architectural features must be compatible with the principal dwelling or accessory structure.
- The owner must occupy at least one dwelling unit, except for bona fide temporary absences determined by the Planning Director, and must sign an affidavit before receiving the Certificate of Use and Occupancy.
- No more than one accessory apartment is allowed per lot or parcel.
- All accessory apartments require approval through the building permit process by the Department of Land Use and Growth Management.
- One additional independently accessible parking space is required.
- Within the RCA, the apartment must be within the primary dwelling or have its entire perimeter within 100 feet of it, use the same sewage disposal system, be no larger than 900 square feet total enclosed area, and not require a Critical Area variance for the dwelling or appurtenances or a variance for clearing developed woodland or forest cover.
minimum accessory-apartment floor area: >= 300 square feet · maximum principal-dwelling accessory-apartment floor area: <= 40 percent (Also capped at 900 square feet, whichever is less; attached garage excluded) · maximum accessory-structure accessory-apartment floor area: <= 40 percent (Also capped at 900 square feet, whichever is less) · maximum RCA accessory-apartment enclosed area: <= 900 square feet · RCA accessory-apartment distance from primary dwelling: <= 100 feet (Only when not within the primary dwelling)
“one accessory apartment on a lot or parcel.”
St. Mary's County Comprehensive Zoning Ordinance, Article 5: Regulation of Uses ↗Re-checked Oct 1, 2026
Who permits septicCounty
Individual septic-system construction, expansion, and repair applications in St. Mary's County.
- The St. Mary's County Health Department Environmental Health Division administers the application process.
- State requirements under COMAR 26.04.02 apply.
“The Environmental Health Division receives applications for the construction or expansion of individual septic systems. When a septic system fails, the staff also reviews applications for repairs.”
On-Site Septic Systems ↗Re-checked Sep 24, 2026
Septic permitRequired
Construction or expansion of individual on-site sewage disposal systems in St. Mary's County.
- A perc test is required for new construction or repair.
- For all new construction, a Maryland-registered land surveyor site plan must be submitted for review.
- Construction may begin upon approval of the septic construction application.
- The approved permit is valid for 2 years from issuance.
permit validity: 2 years from issue date
“Pursuant to COMAR 26.04.02, I hereby apply for a permit to construct an On-Site Sewage Disposal to serve the below property. This application serves as the permit for installation and must be present while onsite. This permit expires 2 years from issue date.”
On-Site Septic Systems ↗Re-checked Sep 24, 2026
Septic inspectionRequired
Individual septic systems installed or repaired in St. Mary's County.
- The Health Department must be contacted to conduct the inspection before the septic system is covered.
- Inspection occurs after construction approval and during installation; additional requirements may apply to particular system types.
“Contact the health department to conduct an inspection of the septic system before covering”
On-Site Septic Systems ↗Re-checked Sep 24, 2026
Off-grid sanitationLimited
Private on-site sewage disposal for properties in St. Mary's County that are not served by public water and sewer.
- Properties not served by public water and sewer must complete a perc test before construction of any structure with plumbing.
- An approved septic construction application is required before construction begins.
- A Health Department inspection must occur before covering the system.
- The private system remains subject to site-plan, septic-permit, and applicable state requirements.
“A perc test is a soils test to determine if the soils and groundwater conditions on a property are suitable for use as an on-site sewage disposal system. All properties not served by public water and sewer must have this test before beginning construction of any structure with plumbing.”
Building Permit Process ↗Re-checked Sep 24, 2026
Mandatory utility hookupConditional
Properties subject to public water or sewer connection requirements in the St. Mary's County Metropolitan Commission service area.
- A waiver or deferral may permit interim private water or sewer service.
- The private service must be approved by the Office of Environmental Health.
- The private service must be designed for future connection to a public system.
- When MetCom determines connection is feasible, the private service must be discontinued and the property connected to the public system.
- The cited project document identifies the requirement as applying when a waiver from water and sewer connection is required.
“The applicant has obtained a waiver or deferral to connect to the public _I__, and subsequently the resulting private _2_ service shall be: a. approved by the Office of Environmental Health; and b. designed for future connection to a public system when the Metropolitan Commission determines that a connection is feasible and the private _3_ service shall be discontinued and the property shall be connected to the public system.”
stmaryscountymd.gov ↗Re-checked Sep 24, 2026
Subdividing landRegulated (with conditions)
Subdivision plans and subdivision-related development review in St. Mary's County
- The county's Development Services Division coordinates multi-agency reviews for subdivision plans.
- Permit information is routed to other required agencies for review and approval.
- Applicable requirements may include the county Building Code, Zoning Ordinance, and Subdivision Regulations.
“The Development Services Division coordinates multi-agency reviews for Concept, Major and Minor Site Plans, Subdivisions Plans, Boundary Line Adjustment and Confirmatory Plats, and provides zoning determinations upon request.”
Permits - St. Mary's County, MD ↗Re-checked Sep 24, 2026
2 more reviewed findings on this topic
- Regulated (with conditions) — Partitioning, use, subdivision, and resubdivision of land within the unincorporated areas of St. Mary's County, Maryland.
  “The provisions of this Ordinance shall apply to the partitioning and use of all land within the unincorporated areas of St. Mary’s County unless expressly and specifically exempted or provided otherwise in this Ordinance. No subdivision may be undertaken without prior authorization, or exemption from regulation, by this Ordinance.”
  St. Mary's County Subdivision Ordinance ↗Re-checked Oct 4, 2026
- Regulated (with conditions) — Subdivision and creation, sale, or transfer of lots in unincorporated St. Mary's County.
  “No subdivision may be undertaken without prior authorization, or exemption from regulation, by this Ordinance.”
  St. Mary’s County Subdivision Ordinance ↗Re-checked Oct 4, 2026
Driveway accessLimited
Residential driveway access governed by the St. Mary's County Subdivision Ordinance, particularly subdivision entrances and access to public roads.
- Residential driveway access is limited to cul-de-sacs, local roads, and minor collector roads.
- Multi-family and cluster subdivisions must have direct access to a public road.
- Direct residential access to major collectors and arterials is permitted only when no feasible alternative exists.
- Where direct access to a major collector or arterial is allowed, minimum separation between driveways or entrances is 150 feet measured from centerline.
- Entrances onto public roads must comply with the Department of Public Works and Transportation Manual of Design and Construction Standards and the ordinance.
minimum separation for permitted direct residential access to major collectors or arterials: 150 feet (Only when no feasible alternative exists; measured from centerline)
“Residential driveway access shall be limited to cul-de-sacs, local roads and minor collector roads. Multi-family and cluster subdivisions shall have direct access to a public road.”
St. Mary's County Subdivision Ordinance ↗Re-checked Sep 24, 2026
Building in a floodplainRequired
Development or construction wholly within, partially within, or in contact with a flood hazard area in St. Mary's County.
- A permit must be obtained before starting covered development or construction.
- Covered activities include filling, grading, new structures, substantial improvement or repair of substantially damaged structures, placement or replacement of manufactured homes, erecting or installing temporary structures, and alteration of a watercourse.
- Creation of any new flood-prone building site is prohibited in any subdivision governed by the County's Zoning Ordinance regardless of size, number of lots, or location.
- For a permit to build in the floodplain, the applicant must agree to have an Elevation Certificate prepared by a licensed engineer or surveyor.
- Structures in Special Flood Hazard Areas that are substantially improved or substantially damaged must comply with current floodplain regulations.
- Substantial damage includes repair costs equal to or exceeding 50 percent of original market value.
substantial damage threshold: >= 50 percent of original market value (Repair cost threshold described by the county)
“A permit must be obtained prior to starting any development or construction which is wholly within, partially within, or in contact with any flood hazard area including filling; grading; construction of new structures; the substantial improvement of any building or structure including the repair of a substantially damaged building or structure; placement or replacement of manufactured homes including the substantial improvement or repair of a substantially damaged manufactured home; erecting or installing a temporary structure; or alteration of a watercourse.”
Floodplain Management - St. Mary's County, MD ↗Re-checked Sep 24, 2026
Permit feesPublished (with conditions)
Published residential permit fees administered by St. Mary's County Department of Land Use and Growth Management.
- The source is marked Revised 8/24/2023; this is treated as a revision stamp rather than an effective date.
- Fees vary by permit type and may include application, square-footage, excise-tax, review, inspection, entrance-bond, and utility-connection charges.
- Health Department Review Fee is $21.00 to $90.00; Soil Conservation District Review Fee is $75.00 to $500.00; MetCom connection fees require calling 301-737-7400; Building Inspection Fees require calling MDIA at 301-884-4547; Entrance Permit bond is $5,000.00 or $10,000.00.
application fee: 20 USD · new dwelling fee: 0.16 USD/sq. ft. ($20.00 minimum) · new dwelling excise tax - single family: 6697 USD · new dwelling excise tax - multi family: 3218 USD · residential addition fee: 0.16 USD/sq. ft. ($20.00 minimum) · detached residential accessory structure fee: 0.03 USD/sq. ft. ($20.00 minimum) · health department review fee: 21-90 USD · soil conservation district review fee: 75-500 USD · entrance permit bond: 5000 or 10000 USD
“FAQ #12: What fees can I expect to pay for my residential permit? Application Fee $ 20.00”
FAQ #12: Residential Permit Fees ↗Re-checked Sep 24, 2026
Permit processPublished (with conditions)
County permit processing by the Permit Services Division for building, environmental, residential entrance, home occupation, family daycare, business occupancy, and sign permits.
- Applications are processed under applicable Building Code, Zoning Ordinance, and Subdivision Regulations.
- Permit information is routed to other required agencies for review and approval.
- The page does not establish that every permit type follows identical steps or timelines.
“The Permit Services Division’s principal function is to process building and other permits including environmental (Critical Area and Floodplain), residential entrances, home occupations, family daycare, change of business occupants, and sign permits in accordance with the applicable regulations, such as the Building Code, Zoning Ordinance and Subdivision Regulations. The Permits staff must also coordinate the routing of permit information to other required agencies for their review and approval.”
Permits - St. Mary's County, MD ↗Re-checked Sep 24, 2026

Maryland rules that apply here

Manufactured homesAllowed (with conditions)
Placement and resulting residential use of qualifying manufactured homes in zones that allow single-family residential uses.
- The manufactured home must meet the Maryland statutory definition and applicable federal safety standards.
- The home must be converted to real property by attachment to a permanent foundation, identical ownership of the home and parcel, and recording an affidavit of affixation.
- Local jurisdictions may apply building codes, zoning codes, and other local requirements.
- The rule does not require local jurisdictions to allow mobile homes on wheels or temporary foundations.
- Local jurisdictions may require the same historic-district standards applicable to traditional single-family homes.
“A legislative body may not prohibit the placement of a new manufactured home or modular dwelling in a zone that allows single–family residential uses”
mgaleg.maryland.gov ↗Re-checked Oct 5, 2026
One more reviewed finding on this topic
- Limited — Placement of a new manufactured home in a Maryland zoning zone that allows single-family residential uses; this claim does not establish unrestricted statewide occupancy on every parcel.
  “A legislative body may not prohibit the placement of a new manufactured home or modular dwelling in a zone that allows single–family residential uses”
  Maryland Land Use Article § 4-104 ↗Re-checked Oct 5, 2026
Well permitRequired
Construction of any well in Maryland, including wells serving private or non-community water supplies.
- A hard copy of the permit application must be provided for the County, Survey, Driller, Well Owner, and Department to the Approving Authority at submission.
- If a new well replaces an unused well, the unused abandoned well must be sealed.
- For residential and other establishments not served by a community water supply, the well must satisfy COMAR 26.04.04; alternatively, a permitted surface-water system meeting COMAR 26.04.01 may be used.
- A well may not be constructed to provide potable water to property already connected to a community water supply unless authorized by the Department.
“A well construction permit is required for any well that is constructed in the State of Maryland.”
Well Construction Permit Applications and Forms ↗Re-checked Sep 11, 2026
One more reviewed finding on this topic
- Required — Any well constructed in St. Mary's County, under Maryland's statewide well-construction permitting requirement.
  “A well construction permit is required for any well that is constructed in the State of Maryland.”
  Well Construction Permit Applications and Forms ↗Re-checked Sep 24, 2026
county zoning authorityGeneral
Maryland counties adopting and enforcing zoning and planning laws, including for unincorporated county territory within the county's jurisdiction.
- The authority is exercised through local laws and remains subject to other public general law, public local law, and any applicable State-government regulatory authority.
- The statute states that the State policy is that planning and zoning controls are implemented by local government.
“A county may enact local laws relating to zoning and planning to protect and promote public safety, health, morals, and welfare, including: [...] It is the policy of the State that planning and zoning controls shall be implemented by local government.”
Maryland Code, Local Government Article ↗Re-checked Sep 21, 2026
county building authorityLimited
Maryland counties requiring building permits and adopting or enforcing building codes for residential construction in unincorporated areas.
- Local Government Article § 13-901 applies to all counties except Anne Arundel County, Baltimore City, Baltimore County, Cecil County, Charles County, Howard County, Prince George’s County, Queen Anne’s County, and Worcester County.
- Under § 13-901(c), covered county governing bodies may regulate building construction, adopt and enforce building codes, require building permits, impose permit and inspection fees, and inspect buildings and structures.
- Section 13-902 applies only to Calvert, Caroline, Dorchester, Frederick, Harford, Kent, and Worcester Counties.
- Under § 13-902(c), a building or housing code generally does not apply to a building on a farm or premises devoted solely to agricultural uses, except that it applies in Calvert County to dwellings on which construction began after July 1, 1979 and in Frederick County to residential buildings or buildings constructed for human habitation regardless of location or auxiliary use.
- Kent County must adopt a building code and housing code; Calvert, Caroline, Dorchester, Frederick, Harford, and Worcester Counties may adopt them.
- A building code adopted under § 13-902 must require a building permit before a building is built or improved and include standards for structural safety, fire prevention, lighting, ventilation, and proper access.
- Later amendments to codes incorporated by reference are not effective in Caroline, Frederick, Harford, Kent, and Worcester Counties until incorporated into the county code.
- Separate county-specific provisions apply to Cecil, Charles, Queen Anne’s, and St. Mary’s Counties, including the Charles County exemption for farm buildings and other outbuildings costing less than $2,500.
- The Maryland Building Performance Standards apply statewide to each building or structure for which a building permit application is received by a local jurisdiction on or after August 1, 1995.
building permit before building is built or improved: true (Required in a building code adopted under Local Government Article § 13-902.) · Charles County farm-building or outbuilding cost exemption: < 2500 USD (The Charles County building code does not apply to farm buildings or other outbuildings costing less than $2,500.) · Maryland Building Performance Standards application date: >= 1995-08-01 (Applies to building permit applications received on or after this date.)
“This section applies to all counties except: [...] The governing body of a county may: (1) regulate the construction of buildings and signs; (2) adopt a building code; (3) provide for enforcement of the code; (4) require permits for the construction of buildings and signs; (5) impose permit and inspection fees; (6) provide for inspection of buildings and structures;”
Maryland Code, Local Government Article, §§ 13-901 through 13-906 ↗Re-checked Sep 21, 2026

Inside cities and towns in St. Mary's County

Municipal rules found while researching the county. They apply inside that city or town, not to unincorporated land.

Local zoningPresent (with conditions)
Planning and zoning within the incorporated Town of Leonardtown, St. Mary's County.
- The Town's planning and zoning authority applies within the Town's boundaries.
- The Planning and Zoning Commission has approval authority for new houses and commercial developments.
“Leonardtown is also the only incorporated town in St. Mary's County. This grants the Town unique status and allows control over planning and zoning functions within it's boundaries. The Planning and Zoning Commission has approval authority for new houses and commercial developments.”
Town of Leonardtown - Planning & Zoning Commission ↗Re-checked Oct 5, 2026

Still unanswered

We looked and could not find an official answer yet. Silence is not permission — ask the county directly.

Lot coverage — Maximum lot coverage in St. Mary's County
Building height — Building height limits in St. Mary's County
Tiny homes — Occupancy of a tiny home on private land in St. Mary's County.
Agricultural exemption — Agricultural exemptions from the requested county land-use or construction requirements
Short-term rentals — Short-term rental use in St. Mary's County.

Each answer was extracted from an official source and independently re-checked against that source by a second reviewer. That is an evidence check, not legal advice. Rules change; confirm with the authority before you buy or build.

Living off-grid in St. Mary's County, Maryland

Is St. Mary's County a good place to live off-grid or homestead?

St. Mary's County scores 55/100 on the Bolthole Index, a solid county profile. Its strengths: mixed durability signals and strong self-sufficiency basics: 45.3″/yr rainfall plus 57.5% wooded land. Watch-outs: about 58,850 people within a 30-minute drive.

Do you need a building permit in St. Mary's County, Maryland?

St. Mary's County generally requires a residential building permit, and it enforces county zoning. Onsite septic is permitted by St. Mary's County Health Department - Environmental Health Division. Owner-builders may act as their own contractor to build their home. These answers are quoted from official sources and independently re-checked. Rules change and cities within the county differ — always confirm with the county before you buy or build.

Can you live in an RV or tiny home on your land in St. Mary's County?

Living in an RV on your own land in St. Mary's County is limited to specific cases: Occupancy of one camping or recreational trailer on an owner's lot in an allowable St. Mary's County zoning district. Recreational vehicles may not be occupied as permanent residences. The parcel must be at least 1 acre. Confirm with the county before relying on it.

How much does land cost in St. Mary's County, Maryland?

Land in St. Mary's County runs about $8,652 per acre, based on the latest county data.

What is the water situation in St. Mary's County?

St. Mary's County gets about 45.3" of rain a year, with severe drought in roughly 1.6% of years.

How remote is St. Mary's County?

A representative private-land location has about 58,850 people within a 30-minute drive and 6,612,918 within two hours. The road-network catchments count population across county lines and clusters of smaller cities; straight-line radius values remain available as a comparison.

Building permits, zoning and land rules in St. Mary's County, Maryland

At a glance

Who to call

Do you need a building permit in St. Mary's County, Maryland?

Who issues building permits in St. Mary's County, Maryland, and how do you reach them?

Does St. Mary's County have zoning?

How do septic permits work in St. Mary's County?

Can you live in an RV or tiny home on your land in St. Mary's County?

What are the steps to build a home in St. Mary's County, Maryland?

What should you check before buying land in St. Mary's County, Maryland?

Watch-outs

Sources

Compared with neighboring counties

St. Mary's County in Maryland

How St. Mary's County scores

Find land. Bring the promising ones back.

Parcel due diligence

Cities and towns in St. Mary's County

Answers from the rulebook for St. Mary's County

St. Mary's County rules

Maryland rules that apply here

Inside cities and towns in St. Mary's County

Still unanswered

Living off-grid in St. Mary's County, Maryland

Is St. Mary's County a good place to live off-grid or homestead?

Do you need a building permit in St. Mary's County, Maryland?

Can you live in an RV or tiny home on your land in St. Mary's County?

How much does land cost in St. Mary's County, Maryland?

What is the water situation in St. Mary's County?

How remote is St. Mary's County?

How St. Mary's compares in Maryland