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Portola Valley, California: ADU Regulations

City or County:

Portola Valley, California

Date Updated:


Portola Valley has not updated its regulations in 2020, suggesting the Planning Department sees itself in accordance with statewide regulations.


Accessory dwelling units shall be permitted on all parcels in all zoning districts, where a main building is in existence or is proposed concurrently.

Exception: ADUs are prohibited on parcels smaller than one acre whose direct vehicular access is from a road or cul-de-sac which (1) has a single point of ingress/egress and (2) has a width of less than eighteen feet.

Types of ADUs:

External, Attached, Internal, Junior and Garage Conversion



Lot Size:


Maximum ADU Floor Area:

AMFA applies.

External (Parcels smaller than 3.5 acres): 1,200 sq.ft.
External (Parcels 3.5 acres or larger): 1,500 sq.ft.
Internal ADUs: 1,700 sq.ft.
External ADUs: also limited to fifty percent of the floor area of the existing or concurrently proposed main building.

Junior: 500 sq.ft.

Lot Coverage:



No min/max


No min/max


ADUs must have separate kitchen facilities.


ADUs require a full bathroom. Junior ADUs may share a bathroom with primary residence.

Note: Drainage and sewer connections should be assessed on a project by project basis.


No parking required for internal ADU. 1 parking space required for external ADU. On 1 acre or more, ADUs < 1,200 sq. ft. can overlap guest & ADU parking. Separate driveway permitted rarely. Spaces do not have to be covered and can be in tandem. No additional parking space is required if the ADU is created by conversion of existing space within main dwelling.

Covered Parking Conversion. When covered parking which is required by this code is demolished in conjunction with the construction of an ADU or converted to an ADU, the required parking spaces must be provided elsewhere on site. The replacement parking may be covered or uncovered, in tandem, or in mechanical lifts.


Front, side and rear yard setbacks vary depending on both parcel size and combining district. Refer to Table 1 in Section 18.48.010 of the municipal code.

No setback required for existing garage conversion and setback of no more than 5' from side and rear lot lines required for ADU constructed above garage.

Note: Fire Department regulations should be assessed on a project by project basis.


Max: 18' to 24'.


Cannot be visible from a local scenic corridor without ASCC approval.

Rear Yard Coverage:



Must comply with site development and design standards of applicable district, additionally, all ADU projects must meet the 2013 Cal Green Code requirements. Colors, materials, and residential design must match main dwelling unless approved by ASCC. 

Materials: Color reflectivity values shall not exceed forty percent, except that trim colors and roofs shall not exceed fifty percent reflectivity.

ADU applications submitted to Town Planner must be reviewed by Town Geologist, Director of Public Works, Fire Chief, and County Health Department.

Note: Historic Neighborhoods or Districts should be assessed on a project by project basis.


A separate driveway for the ADU is subject to ASCC review.

Source Code:

Important Note: Rules and regulations are updated and changed frequently. It’s one of our jobs to be up to speed and navigate those regulations. Let us know if our website isn’t up to date or if you have any information that could help. Cheers!