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San Mateo CA ADU Guidelines

Accessory Dwelling Unit (ADU) City Regulations


R-1/R-2/R-E/RH/RM/TPZ (within the Coastal Zone, only R-1 is permitted)

Lot Size:

Exempt from minimum lot and density provisions of underlying zoning district.

ADU Floor Area:

Attached: shall not exceed 750 sq.ft. or 50% of the floor area of the primary residence up to 1,200 sq.ft.

Detached: shall not exceed 750 sq.ft. or 35% of the floor area of the primary residence up to 1,200 sq.ft.

Note: floor area of an ADU counts against total floor area allowed on a parcel. Total floor area of both ADU and primary dwelling must not exceed max. allowed for underlying zoning district.

No. of Bedrooms:


Storage / Closet:


Kitchen / Bath:  

Require separate bathroom and kitchen facilities (permanent range, counters, refrigerator, and sink).


One new space, in addition to the already existing spaces on the parcel, shall be provided on-site for ADUs with up to two bedrooms. Two new spaces must be provided on-site for ADUs with three or more bedrooms.

Up to two spaces may be provided in front or side yard provided not more than 600 sq. ft. of the front yard is used for parking. Tandem parking is permitted provided no more than three cars are parked in such a way. Spaces must be on pervious surfacing. ADU parking may be uncovered.


Detached of 16' or less in height: side = 5', rear = 5'

Detached greater than 16' in height: side = 5', rear = 10'

Attached are subject to same requirements of primary residence, except when constructed entirely within existing garage.

ADUs constructed above a garage are subject to same setbacks as detached ADU of 16' or less in height, regardless of garage height.

Detached ADUs with any portion located in front of primary dwelling must follow setbacks required of primary residence.

Must maintain a separation of 5' from any other building on parcel.


26' max


Must be located on a parcel which contains or will contain a primary residence.

Rear Yard Coverage:

Must adhere to requirements of underlying zoning district.

Materials / Design:  

Not subject to design review unless located within Coastal Zone.


Must have an independently accessible entrance that does not require passage through primary dwelling. For attached, any new entrance must face the side or rear of lot.

For more ADU Information:

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