Bolthole Index

San Mateo County, California

Zoning, building permits and septic rules in the Town of Portola Valley, California

The Town of Portola Valley, California, in San Mateo County, adopts its own building code and enforces it through the town's building official1. Zoning is the town's too, and RVs are allowed only as temporary dwellings while an owner's home is remodeled2. Septic permits for any address in San Mateo County come from the county's Environmental Health office3.

At a glance

Building code
Town adopts 2025 California Title 24 with local amendments1
Building permits and inspection
Town Planning & Building Department; final inspection signed by the town's building official before sale or transfer45
Zoning
Town zoning code applies inside town limits6
Septic
San Mateo County Environmental Health issues septic permits3
RV living
Temporary only, owner-occupied while the main home is remodeled; Architectural and Site Control Commission approval, up to one year2
Accessory dwelling units
Allowed on lots with a house; new detached units up to 1,200 sq ft7
Owner-builder
State contractor-license exemption exists but does not waive permits or inspections8
Wells
Drilling permit from county Environmental Health9

Who to call

Building permits

Town of Portola Valley Planning & Building Department

Building permits, plan review and inspections, and planning and zoning applications inside the town

(650) 851-1700

planbuild@portolavalley.net

765 Portola Road, Portola Valley, CA 94028

Septic permits

San Mateo County Environmental Health Services

Septic (OWTS) permits, inspections and 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 Portola Valley means bringing your own water and power. The kit we’d start with:

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Does Portola Valley, California have zoning, and what does it mean for RVs and tiny homes?

Portola Valley applies its own zoning code inside town limits, including a list of temporary uses permitted in every district6. Zoning for land outside the town belongs to San Mateo County, not the town; see the San Mateo County page.

RVs: motor homes and travel trailers are allowed as temporary dwellings on residentially zoned parcels, but only for owners of the main house while it is being remodeled and unfit to live in. Approval comes from the Architectural and Site Control Commission, which sets a time limit of no more than one year, and the town can require a $10,000 security2. The unit must stay under 320 sq ft inside and hook up to sewer or septic and a power supply2. Living in an RV on a vacant lot while you plan a build does not fit that rule.

Tiny homes: we could not confirm a town rule for tiny homes as primary dwellings. Ask the Planning & Building Department4. Accessory dwelling units are allowed on lots with a house, with 4-foot side and rear setbacks for new detached units and a 1,200 sq ft cap on new floor area7.

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

Inside Portola Valley, the town, not San Mateo County, runs building permits. The town adopts the California building codes with its own amendments1, and the town's Planning & Building Department staffs a building permit counter open Monday through Thursday from 8:30 am to 12:00 pm4. A building cannot be sold or transferred until the town's building official signs the final inspection5. Temporary certificates of occupancy can be issued before all permitted work is complete10.

Owner-builders: California's contractor-license exemption lets owners build on their own property, but the state treats it as separate from local permits and inspections8. We could not confirm the town's owner-builder process or fee schedule; call the department.

How do septic permits work inside Portola Valley?

Septic permits are not issued by the town's building counter. San Mateo County Environmental Health requires a permit from anyone who installs an individual sewage disposal system in the county3, and its staff inspect installs, repairs and alterations11. Call Environmental Health at (650) 372-620012. We could not confirm whether the town adds its own septic rules.

For new homes created under SB 9, the town requires either a public sewer connection or a private wastewater system fully contained on the parcel13. Well drilling needs a separate county permit9.

Building a home in Portola Valley: the steps, in order

  1. Call the town's Planning & Building Department to confirm which approvals your parcel needs before design4.
  2. Check whether the parcel is in the town's floodplain district; development there needs a permit from the floodplain administrator14.
  3. Settle sewer or septic with San Mateo County Environmental Health12.
  4. Apply for the building permit; the town's amended California codes apply1.
  5. For any driveway, expect Woodside Fire Protection District driveway and turnaround standards on SB 9 units15; confirm the standard for other homes with the town.
  6. Get the final inspection signed by the town's building official before any sale or transfer5.

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

Septic permits, inspections and well-drilling permits stay with San Mateo County Environmental Health for every address in the county39. Outside town limits, unincorporated San Mateo County requires a building permit from the county instead16; the county page covers it.

County-level numbers: the effective property tax rate is 0.6%17, and the typical home value is about $1.6 million18. About 16.3% of county soils rate as not limited for a conventional septic drain field, so expect a site evaluation19.

Watch-outs

  • Rentals of SB 9 dwelling units in Portola Valley must be for a term longer than thirty days20.
  • SB 9 lot splits in Portola Valley create at most two parcels, require an owner-occupancy affidavit for three years and allow no further subdivision21.
  • We could not confirm the town's minimum lot size, setbacks for main homes, permit fees or owner-builder process; ask the Planning & Building Department at (650) 851-17004.

Sources

  1. Portola Valley Code 15.04.010 Building Code (Town of Portola Valley), quoted and re-checked
  2. Portola Valley Code 18.36.030 Temporary buildings and uses (Town of Portola Valley), quoted and re-checked
  3. Land Use, Septic & Wells, quoted and re-checked
  4. Town of Portola Valley Planning & Building Department (portolavalley.net)
  5. Portola Valley Code 15.04.030 Adoption of and Amendments to the California Residential Code (Town of Portola Valley), quoted and re-checked
  6. Portola Valley Code 18.36.030 Temporary buildings and uses (Town of Portola Valley), quoted and re-checked
  7. Portola Valley Code 18.25.060 Accessory dwelling unit (ADU) regulations (Town of Portola Valley), quoted and re-checked
  8. California Business and Professions Code Section 7044 (California statewide rule), quoted and re-checked
  9. Land Use, Septic & Wells, quoted and re-checked
  10. Portola Valley Code 15.04.030 Adoption of and Amendments to the California Residential Code (Town of Portola Valley), quoted and re-checked
  11. Land Use, Septic & Wells, quoted and re-checked
  12. San Mateo County Environmental Health Services (smcgov.org)
  13. Portola Valley Code 18.27.060 SB 9 Development Standards (Town of Portola Valley), quoted and re-checked
  14. Portola Valley Code 18.32.020 Floodplain definitions (Town of Portola Valley), quoted and re-checked
  15. Portola Valley Code 18.27.060 SB 9 Development Standards (Town of Portola Valley), quoted and re-checked
  16. Apply for a Permit, quoted and re-checked
  17. Effective property tax rate: Bolthole Index county data
  18. Typical home value (Zillow): Bolthole Index county data
  19. Share of soils USDA rates "not limited" for a conventional septic drain field (SSURGO). A strict rating: the median county is about 6% and a tenth of counties are 0%, so a low figure means most lots need a site evaluation or an engineered system, not that septic is impossible. Cite it only with that caveat: Bolthole Index county data
  20. Portola Valley Code 18.27.060 SB 9 Development Standards (Town of Portola Valley), quoted and re-checked
  21. Portola Valley Code 17.13.040 Objective standards for SB 9 lot split (Town of Portola Valley), quoted and re-checked

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 Portola Valley

Town of Portola Valley’s own rules

  • Building code adoptedAdopted

    Building construction inside Portola Valley town

    • Adopts 2025 California Title 24 parts with local modifications
    • Town amends Residential Code sections (e.g., R105.2, R108.5, R110.3)
    “the town's building code is comprised of the following uniform codes:”
  • InspectionsRequired

    Buildings under permit inside Portola Valley town

    • Final inspection signed by town building official before sale or transfer
    “in no event shall a building be sold or transferred until the final inspection has been signed by the town's building official.”
  • Certificate of occupancyLimited

    Temporary certificates of occupancy inside Portola Valley town

    • Building official sets validity period
    • Surety deposit may be required
    • Only the temporary certificate provision was reviewed; final CO text not located
    “the building official is authorized to issue a temporary certificate of occupancy before the completion of the entire work covered by the permit”
  • Local zoningPresent

    Land inside Portola Valley town limits

    “The following uses or facilities are permitted in all districts as temporary uses:”
  • Building heightSpecified

    Detached ADUs from new floor area inside Portola Valley town (base-district height not located)

    • 18 ft if within half mile of major transit stop or high-quality transit corridor, or multi-family multi-story lot
    • Attached ADUs: 25 ft or district maximum, whichever is lower, max two stories

    detached ADU height: <= 16 ft (18 ft with transit proximity)

    “detached adus created from new floor area, on a lot with an existing or proposed single-family or multi-family dwelling unit, shall not exceed a height of sixteen feet, or a height of eighteen feet if either of the following conditions are met:”
  • Living in an RVTemporary only (with conditions)

    Motor homes and travel trailers as temporary dwellings on residentially zoned parcels inside Portola Valley town

    • Occupied by owners of the principal dwelling while it is being remodeled and unsuitable for occupancy
    • Connected to sanitary sewer or on-site septic system and to an established power supply (generator only in emergency)
    • ASCC approval; time limit set by ASCC, no more than one year
    • $10,000 security and $250/day fine for late removal
    • Under 320 sq ft internal living area, under 8 ft wide and 40 ft long

    maximum duration: <= 1 year · removal security: 10000 USD

    “e. motor homes and travel trailers designed for human habitation are permitted as temporary dwelling units on residentially zoned parcels, provided each one complies with the following provisions:”
  • Accessory dwellingsAllowed (with conditions)

    ADUs on lots with a single-family or multi-family dwelling inside Portola Valley town

    • ADUs count toward AMFA, lot coverage, FAR and CAR except as state law allows
    • Detached new ADU: 4-ft side and rear setbacks, 16 ft height (18 ft in some cases)
    • No parking required
    • New floor area ADU maximum 1,200 sq ft

    max new-construction ADU size: <= 1200 sq ft (Gov. Code 66323 ADUs exempt)

    “adus are permitted on lots with an existing or proposed single-family or multi-family dwelling in areas zoned to allow single-family or multi-family residential use.”
  • Mandatory utility hookupConditional

    New dwelling units created under SB 9 inside Portola Valley town (general rule for other homes not located)

    • Public sewer connection OR a fully contained private wastewater system
    • Utilities placed underground per 18.36.010.B
    “(j) utilities. (1) all newly created dwelling units shall be connected to public sewer or provide a private wastewater system that is fully contained within the parcel boundaries. (2) utilities shall be required to be placed underground”
  • Subdividing landRegulated (with conditions)

    SB 9 lot splits inside Portola Valley town (Title 17 Subdivisions)

    • No more than two new parcels, neither smaller than 40% of original lot and at least 1,200 sq ft
    • Owner-occupancy affidavit for three years
    • No further subdivision permitted
    • Parcels connect to public sewer or have contained private wastewater system
    “the subdivision shall create no more than two new parcels of approximately equal area provided that neither parcel shall be smaller than forty percent of the lot area of the original parcel proposed for subdivision. in no instance shall any resulting parcel be smaller than one thousand two hundred square feet in area.”
  • Driveway accessLimited

    New SB 9 units inside Portola Valley town

    • Woodside Fire Protection District standards apply
    • General driveway rules for other homes not located
    “Driveway access to all new units shall be compliant with the Woodside Fire Protection District standard details and specifications for driveways and turnarounds.”
  • Building in a floodplainRequired

    Development in F-P floodplain combining district / FIRM-mapped properties inside Portola Valley town

    • Development permits granted, denied or conditioned by the floodplain administrator (Public Works Director)
    • Properties on Flood Insurance Rate Maps must comply with Chapter 18.32
    • Manufactured homes elevated to or above base flood elevation on permanent foundation
    “d. properties identified on the flood insurance rate maps shall comply with the provisions of chapter 18.32.”
  • Short-term rentalsLimited

    SB 9 dwelling units inside Portola Valley town (general STR rule not located)

    • Rentals must be for a term longer than thirty days

    minimum rental term: > 30 days

    “(k) rental restrictions. (1) any rental of a sb 9 dwelling unit shall be for a term longer than thirty days.”

Still unanswered

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

  • Owner-builder exemption — Owner-builder exemption inside Portola Valley town
  • Minimum dwelling size — Single-family residential district inside Portola Valley town
  • Minimum lot size — Main single-family residential district inside Portola Valley town
  • Lot coverage — Residential districts inside Portola Valley town
  • Camping on your land — Temporary camping inside Portola Valley town
10 more unanswered
  • Tiny homes
  • Manufactured homes
  • Who permits septic
  • Septic permit
  • Septic inspection
  • Off-grid sanitation
  • Well permit
  • Agricultural exemption
  • Permit fees
  • Permit process

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 Portola Valley 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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