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County of San Mateo, California: ADU Regulations

City or County:

County of San Mateo, California

Date Updated:

11.30.2020

Zoning:

Non-Coastal Zones - R-1, R-2, R-3, PC, CMU-1, CMU-2, CMU-3, NMU, NMU-ECR, R-E, RH, RM and TPZ District, and all other districts in which residential uses are permitted, regardless of 22.5.3 any regulations that might otherwise prohibit accessory dwelling units in any specific district.

Coastal Zone - R-1, except for non-conforming R-1 parcels of less than 5,000 square feet within the Coastal Zone.

Types of ADUs:

Attached Detached, Junior

Stories:

N/A

Lot Size/Density:

Accessory dwelling units shall be exempt from the minimum lot area per dwelling unit provisions in the applicable district.

Accessory dwelling units shall be exempt from all minimum lot size requirements

Maximum ADU Floor Area:

Attached: 800 sq. ft. (750 for Coastal Area)or 50% of the livable floor area of the existing or proposed primary residence, whichever is larger, up to a maximum of 1,500 sq.ft. 

Detached: 800 sq. ft. (750 for Coastal Area) or 35% of the livable floor area of the existing or proposed primary residence, whichever is larger, up to a maximum of 1,500 sq.ft.
Junior: maximum 500 s.f. and minimum 150 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 parcel, shall be provided. State parking exceptions apply (public transit, historic district, no permits for ADU occupants, car share, attached/conversion/junior), as well as when a garage, carport, or covered parking structure is demolished to build ADU.

If the parking already existing on the parcel exceeds that required for existing development on the parcel, excess parking spaces shall be counted toward the new parking required for the accessory dwelling unit.

Parking spaces shall be provided in the following manner: 22.5.7
(1) Pervious Surfaces. All new parking spaces created for the accessory dwelling unit must be provided on pervious surfaces. The maximum amount of impervious surfaces designated to satisfy the accessory dwelling unit parking requirement shall be no greater than the amount of impervious surfaces existing at time of application. Existing impervious surface area may be used for parking and need not be converted to pervious surface.
(2) Uncovered Parking. All parking required for the accessory dwelling unit may be uncovered.
(3) Front or Side Yard Parking. Up to three parking spaces may be provided in the front or side yard. Not more than six hundred (600) sq. ft. of the front yard area shall be used for parking.
(4) Tandem Parking. Required parking spaces for the primary residence and the accessory dwelling unit may be provided in tandem on a driveway. A tandem parking arrangement consists of one car behind the other. No more than three total cars in tandem may be counted toward meeting the parking requirement.
(5) Compact Spaces. All parking required for the accessory dwelling unit may be provided by compact parking spaces, as defined in Section 6118.a.

Setbacks:

Required setbacks of applicable district notwithstanding, ADU setbacks are:

Detached ADU of 16’ or less height - Side setback 4’ | Rear setback 4’ (5' and 5' in Coastal Zone)

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

Chimneys, pipes, mechanical equipment, antennae, and other similar structures may extend up to eight feet beyond the building height, as required for safety or efficient operation. Accessory dwelling units built entirely within an existing building shall be subject to the greater of the height limit applicable to that building in the relevant district, or the maximum height of the existing primary residence, measured in the manner described in the Zoning Regulations of the relevant district.

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, except in the case that clearance and/or landing requirements preclude door placement on the side or rear, in which case the required entrance may face the front of the parcel.

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!