Portola Valley - San Mateo County ADU Resource Center

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Local ADU Rules

Portola Valley

Last Updated May 12, 2026

ADU Contact:

planbuild@portolavalley.net

Standards	Details
Zones allowed	All residential zones. Non-residential zones require discretionary approval.
Lot Restrictions	Floor area ratio (FAR), impervious surface requirements for underlying zone
Minimum Lot Size	None
Min/Max ADU Size	External ADU: 800 to 1,500 square feet depending on proposed setbacks/FAR JADU: 500 square feet Internal ADU: 1700 square feet *Not more than 50% of primary dwelling square footage, unless 50% of primary dwelling is less than 800 square feet.
ADU Building Height	Setbacks less than base zoning district: 16 feet vertical/maximum height Compliant setbacks: 18 feet vertical height/24 feet maximum height *Higher builds require discretionary approval unless preempted by State Law
Setbacks	Less than base zoning district: Subject to additional standards (height/size) Attached: Same as zoning for primary dwelling Detached: rear and side – 4 feet, front – same as underlying zoning unless 800 square foot unit not feasible
Parking Requirements	JADUs/Internal ADUs: None External ADUs: One parking spot required but may be shared with primary unit on parcels 1 acre or greater. Tandem parking/parking in setbacks allowed
Owner Occupancy	JADU: Required, ADU: not required for ADUs built recently (2020-2024)
Sample fees from recent permits	800 square feet detached ADU (2022): Town fees $11,800 (including geologic review). Fire and Environmental Health fees additional. Sewer Connection Charges include Connection Fee ($430.30 per fixture unit) and a permit fee ($510). *No town impact fees. No impact fees for ADUs under 750 square feet.
Other requirements	Second Addresses allowed ADUs must comply with all requirements of the underlying zoning district If the primary dwelling has sprinklers, the ADU must have sprinklers Single family lots with 3.5+ acres can have two ADUs, otherwise only one ADU & one JADU is allowed Depending on amount of grading, topography of the site, geologic conditions, and fire safety considerations, additional reports may be required.
Process	General process: Preliminary meeting with planner to review proposal. Submittal for building permit. Some exceptions if there are geologic or fire safety concerns. ASCC Review: Special review needed, such as historic, second driveway or non-residential zone. Must obtain a sewer permit from the West Bay Sanitary District before starting construction (www.westbaysanitary.org or call 650-321-0384). *Making an appointment to meet with a planner in the initial stages of the project can help avoid challenges and is highly recommended.
Exceptions	ADUs smaller than 800 square feet are exempt from floor area ratio (FAR) and impervious surface requirements per State Law Some parcels under 1 acre may not qualify for ADUs based on location and fire/life safety requirements.

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