Bolthole Index

San Mateo County, California

Building permits, zoning and septic rules in the Town of Hillsborough, California

The Town of Hillsborough, California issues its own building permits through the Town Building Division and applies its own zoning code, including size limits on new homes12. New buildings and additions need a building permit1. Septic permits are a San Mateo County matter, not a Town one3. Land outside town limits follows the county's rules instead.

At a glance

Building permits
Issued by the Town of Hillsborough Building Division; new buildings and additions need one1
Zoning
The Town has its own zoning code, including a home-size and lot-coverage chapter24
Building code
2025 California Residential Code with local amendments, including fire sprinklers in new one- and two-family homes5
Accessory dwelling units
Allowed; ADU permit and building permit required, decision within 60 days of a complete application6
Septic
County septic permits and inspections; the Town also runs a public sewer system78
Wells
San Mateo County requires a permit to drill a water well9

Who to call

Building permits

Town of Hillsborough Building Division

Building permits, plan check and permit records for projects inside Hillsborough town limits; planning questions go to the Planning Division at the same address

(650) 375-7411

plancheck@hillsborough.net

1600 Floribunda Ave. Hillsborough, CA 94010

Also useful

Town of Hillsborough Water & Sewer Services

Town water billing and sewer service questions; main Town Hall line

650-375-7400

1600 Floribunda Avenue Hillsborough, CA 94010

Septic permits

San Mateo County Environmental Health Services

Septic (OWTS) permits, site evaluations and water-well drilling permits for San Mateo County

(650) 372-6200

Contact details are copied from each office’s official page and were checked against it on October 16, 2026.

Off-grid gear

Going off-grid in Town of Hillsborough means bringing your own water and power. The kit we’d start with:

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Does Hillsborough, California have zoning, and what does it mean for homes and ADUs?

Yes. The Town of Hillsborough applies its own zoning rules to land inside town limits2. Hillsborough's residential chapter exists to regulate the size of single-family homes and accessory structures and to send very large homes to the town council for review2.

Two limits matter most when you plan a house. Total floor area is capped at a ratio of 25% of net lot area for the first acre plus 15% beyond one acre, and a new primary dwelling over 8,000 square feet needs town council review10. Structural coverage is capped on a sliding scale by lot area, and structures plus hardscape may not exceed 40% within street-line setback areas4.

Accessory dwelling units are allowed. A limited detached new-construction ADU needs side and rear setbacks of at least 4 feet and must be 800 square feet or smaller; a junior ADU is capped at 500 square feet inside the existing single-family structure6. We could not confirm district-by-district minimum lot sizes, setbacks or height limits, so ask the Town Planning Division before you buy.

Do you need a building permit in Hillsborough, and who issues it?

Yes, for construction work. The Town of Hillsborough Building Division issues building permits to homeowners and contractors inside town limits1. Hillsborough has adopted the 2025 California Residential Code with local amendments, such as automatic fire sprinklers in new one- and two-family dwellings5.

Every ADU and junior ADU requires both an ADU permit and a building permit11. The Town completes a completeness review within 15 business days and approves or denies within 60 days of a complete application; a denial can be appealed to the town council12.

The Town's Building Division says the best way to learn whether a given project needs a permit is to call1. Call the Building Division at (650) 375-74111. The county's Planning and Building Department handles building permits only in unincorporated San Mateo County, not inside Hillsborough13.

How do septic permits work in Hillsborough?

San Mateo County, not the Town, is the septic authority for the county3. The county requires a permit to install an individual sewage disposal system and inspects systems that are installed, repaired or altered714. Contact San Mateo County Environmental Health Services at (650) 372-620015.

Hillsborough also operates a public water and sewer system, and residential sewer accounts are billed through the county property tax bill8. We could not confirm whether any Hillsborough parcel is allowed to use a septic system or whether the Town requires connection to sewer, so ask Environmental Health and the Town before relying on a septic plan.

Can you live in an RV or tiny home in Hillsborough?

We could not confirm a Hillsborough rule for living in an RV, camping or placing a tiny home. The Town's own code sections we reviewed do not address them. California law allows occupancy of an RV only conditionally and temporarily16. Ask the Town Building and Planning staff at (650) 375-7411 before parking or placing anything to live in1.

Building a home in Hillsborough: the steps, in order

  1. Confirm the parcel is inside Hillsborough town limits, not unincorporated San Mateo County.
  2. Ask the Town Planning Division about zoning, floor-area and lot-coverage limits for the lot104.
  3. Apply for a building permit through the Town Building Division; plan for fire sprinklers in a new home5.
  4. If a septic system or well is involved, contact San Mateo County Environmental Health before design79.
  5. For an ADU, file both the ADU permit and the building permit11.
  6. Ask the Building Division how inspections and final sign-off work; we could not confirm those steps from the Town code.

What stays with San Mateo County, and what to check before you buy?

San Mateo County keeps septic permits, septic inspections and water-well permits for the whole county39. For land outside town limits, see the San Mateo County page, where the county's own building and zoning rules apply.

Before you buy, check floodplain status with the Town, because we could not confirm Hillsborough's floodplain permit rules. Also check floor-area limits against your plan, since the Town sends very large homes to the council10. San Mateo County's typical home value is about $1,595,76417 and its median property tax bill is $9,167 a year18.

Watch-outs

  • Hillsborough's size and lot-coverage limits can cap a house well below what the lot seems to allow, and homes over 8,000 square feet need town council review10.
  • County building and zoning rules do not apply inside Hillsborough; they cover unincorporated San Mateo County only13.
  • We could not confirm Hillsborough's rules on septic eligibility, wells, tiny homes, minimum lot sizes, setbacks or building height, so call the Town Building Division at (650) 375-74111 and County Environmental Health before you commit.

Sources

  1. Town of Hillsborough Building Division (hillsborough.net)
  2. Hillsborough Code ch. 17.32 Residential structure size and lot coverage (Town of Hillsborough), quoted and re-checked
  3. Land Use, Septic & Wells, quoted and re-checked
  4. Hillsborough Code ch. 17.32 Residential structure size and lot coverage (Town of Hillsborough), quoted and re-checked
  5. Hillsborough Code 15.03.250 Updated editions of the California Residential Code (Town of Hillsborough), quoted and re-checked
  6. Hillsborough Code 17.52.020 ADU general provisions (Town of Hillsborough), quoted and re-checked
  7. Land Use, Septic & Wells, quoted and re-checked
  8. Town of Hillsborough Water & Sewer Services (hillsborough.net)
  9. Land Use, Septic & Wells, quoted and re-checked
  10. Hillsborough Code ch. 17.32 Residential structure size and lot coverage (Town of Hillsborough), quoted and re-checked
  11. Hillsborough Code 17.52.020 ADU general provisions (Town of Hillsborough), quoted and re-checked
  12. Hillsborough Code 17.52.020 ADU general provisions (Town of Hillsborough), quoted and re-checked
  13. Apply for a Permit, quoted and re-checked
  14. Land Use, Septic & Wells, quoted and re-checked
  15. San Mateo County Environmental Health Services (smcgov.org)
  16. hcd.ca.gov (California statewide rule), quoted and re-checked
  17. Typical home value (Zillow): Bolthole Index county data
  18. Median property tax bill: Bolthole Index county data

Written October 16, 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.

Answers from the rulebook for Town of Hillsborough

Town of Hillsborough’s own rules

  • Building permitRequired

    ADUs and JADUs inside Hillsborough town

    • Permit needed before construction proceeds
    “every adu and jadu requires an adu permit and a building permit.”
  • Building code adoptedAdopted

    All buildings and construction inside Hillsborough town limits (California Residential Code with local amendments)

    • Local amendments apply, e.g. automatic fire sprinklers required in new one- and two-family dwellings (15.03.110)
    “the rules, regulations and requirements published in the 2025 california residential code, including appendix chapters bf, bo, cj, as contained in the california building standards code, are hereby adopted as and for the rules, regulations, and standards for the town as to all matters contained therein except as herein otherwise provided.”
  • Local zoningPresent

    Land inside Hillsborough town limits

    “the purpose of this chapter is to regulate the size of single-family dwellings and accessory structures and to provide for review by the city council of very large homes.”
  • Minimum dwelling sizeSpecified

    Single-family dwellings inside Hillsborough town (chapter 17.32 is not tied to a named district)

    • Measured as total covered floor area
    • Maximum is a floor area ratio of 25% of net lot area for the first acre plus 15% over one acre
    • New primary dwellings over 8,000 sq ft need city council review

    minimum total covered floor area: >= 2500 sq ft

    “the total covered floor area shall be at least twenty-five hundred square feet.”
  • Lot coverageSpecified

    Residential lots inside Hillsborough town (chapter 17.32)

    • Structural coverage capped by a sliding table by net lot area (e.g. 22 sq ft per 100 sq ft up to 17,500 sq ft)
    • Within street-line setback areas, structural plus hardscape may not exceed 40%
    • Softscape limits are in 17.56.040

    structural plus hardscape coverage: <= 50 percent of net lot area (40% in setback area adjacent to street line)

    “the combined total of structural plus hardscape coverage shall not exceed fifty percent of the net lot area; provided, however, that in any setback area adjacent to the street line, the combined total of structural plus hardscape coverage shall not exceed forty percent of such area.”
  • Accessory dwellingsAllowed (with conditions)

    ADUs and JADUs on residential lots inside Hillsborough town

    • ADU permit and building permit required
    • Ministerial approval within 60 days of a complete application
    • Limited detached new-construction ADU: side/rear setbacks at least 4 feet, 800 sq ft or smaller
    • JADU: no more than 500 sq ft within the single-family structure
    • Fire sprinklers required in some cases (15.03.110)

    approval deadline: <= 60 days (tolled for applicant-requested delay)

    “the purpose of this section is to allow and regulate adus and jadus in compliance with chapter 13 of division 1 of title 7 of the california government code.”
  • Permit processPartial

    ADU/JADU permit process inside Hillsborough town

    • Completeness determination within 15 business days
    • Approve or deny within 60 days of a complete application
    • Appeal of denial to the city council
    “the town must approve or deny an application to create an adu or jadu within sixty days from the date that the town receives a complete application.”

Still unanswered

We looked and could not find an official answer yet. Silence is not permission; ask Town of Hillsborough or San Mateo County directly.

  • Owner-builder exemption — Owner-builder exemption inside Hillsborough town
  • Inspections — Building inspections inside Hillsborough town
  • Certificate of occupancy — Certificate of occupancy inside Hillsborough town
  • Zoning districts — Zoning districts inside Hillsborough town
  • Minimum lot size — Minimum lot size inside Hillsborough town
17 more unanswered
  • Setbacks
  • Building height
  • Camping on your land
  • Tiny homes
  • Manufactured homes
  • Who permits septic
  • Septic permit
  • Septic inspection
  • Off-grid sanitation
  • Well permit
  • Mandatory utility hookup
  • Agricultural exemption
  • Subdividing land
  • Driveway access
  • Building in a floodplain
  • Short-term rentals
  • Permit fees

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.

Unanswered questions are usually a sign that Town of Hillsborough publishes no rule on the point, not that none exists. San Mateo County’s page lists the county offices to call.

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