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County of San Mateo, California: ADU Regulations

City or County:

County of San Mateo, California

Date Updated:

12.9.2018

Zoning:

Non-Coastal Zones - R-1 | R-2 | R-E | RH | RM | TPZ

Coastal Zone - R-1

Types of ADUs:

Attached and Detached

Stories:

N/A

Lot Size:

Second units are EXEMPT from minimum lot area restrictions per Sec. 6429.

Maximum ADU Floor Area:

Attached: 750 sq.ft. or 50% of primary dwelling floor area, whichever is larger, up to 1,200 sq.ft.

Detached: 750 sq.ft. or 35% of the floor area of the existing or proposed primary dwelling, whichever is larger, up to 1,200 sq.ft.

Note: Floor Area of ADU shall count against the total allowed floor area of a parcel, such that the total floor area of the ADU and primary dwelling shall not exceed the maximum floor area allowed within the zoning district.

Lot Coverage:

Must maintain consistency with zoning requirements designated for lot.

Bedroom:

No min/max

Storage:

No min/max

Kitchen:

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

Bath:

Require a separate bathroom.

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

Parking:

One new parking space, in addition to those already existing on the lot, shall be provided on-site for each studio, one bedroom, and two bedroom ADU. Two new parking spaces shall be provided on-site for each ADU with three or more bedrooms.

Parking exceptions apply.

Setbacks:

Required setbacks of applicable district notwithstanding, ADU setbacks are:

Detached ADU of 16’ or less height - Side setback 5’ | Rear setback 5’

Detached ADU greater than 16’ - Side setback 5’ | Rear setback 10’

Detached ADU and Other Buildings - Setback from foundation to foundation is 5’

Detached ADU in Front of Primary Residence - If any portion of an ADU is located in front of the primary dwelling, then the front and side setbacks applicable to the ADU shall be those required of the primary dwelling in the same zoning district.

Detached ADU and Property Lines - If different setbacks to property lines are required by any other section of the Zoning Regulations, those requirements shall be disregarded, and the standards listed above shall govern.

Attached ADU and Garage ADU - Attached ADUs must comply with the same setbacks as the primary dwelling. ADUs constructed entirely within an existing garage are exempt from setback requirements. ADUs constructed above an existing garage, regardless of height, are subject to the same setbacks as a Detached ADU of 16’ or less height.

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

Height:

26’ maximum

Location:

N/A

Rear Yard Coverage:

Must maintain consistency with zoning requirements designated for lot.

Design:

ADUs are not subject to design review, except for ADUs located in the Coastal Zone which are subject to the design review requirements incorporated in the County’s Local Coastal Program.

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

Entryways:

ADU entrance must be independently accessible and must not require passage through the primary residence. For Attached ADUs, any new entrances and exits shall face the side and rear of the lot only.

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!