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Atherton, California: ADU Regulations

City or County:

Atherton, California

Date Updated:




Types of ADUs:

Attached, Detached, Conversion


Must meet all development standards of underlying zone

Lot Size:

Must follow zoning minimum

Maximum ADU Floor Area:

Attached: no more than 50% of primary dwelling floor area with max. increase of 1,200 sq.ft.

Detached: shall not exceed 1,200 sq.ft.

*Exemption from Floor Area Limitations. The first 1,200 sq.ft. of floor area of each newly constructed accessory dwelling unit shall be excluded from maximum floor area calculations; provided, that the floor area exclusion shall not to exceed 10% of the allowed floor area for the lot.

Lot Coverage:

Must maintain consistency with zoning requirements designated for lot.


No min/max, but must not exceed allowable density for lot upon which it is located.


No min/max


Must provide separate kitchen facilities for unit (permanent range, counters, refrigerator, sink).


Must provide separate bathroom facilities for unit.

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


One assigned parking space for each bedroom in unit. Parking must be accessible from and screened from view of public street. May be provided as tandem parking on existing driveway.

Parking Exceptions:

No additional parking required if unit:

  1. Is located within one-half mile of public transit.
  2. Is part of existing primary residence or existing accessory structure.


Each newly constructed attached or detached ADU to be located in main building area and may encroach into side and rear yard setbacks by not more than 20% of required setback. Detached must maintain 8' of separation from main dwelling. No setback required for existing, legal attached or detached garage converted ADU. Setback of 5' required from side and rear lot lines for ADU constructed above a garage.

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


Must meet all development standards of the underlying zone.


To be attached to existing main residence or located within habitable area of the existing main residence or detached from the main residence and located on the same lot as the main residence.

Rear Yard Coverage:

Must maintain consistency with zoning requirements designated for lot.


The accessory dwelling unit to be consistent with existing general plan and zoning land use designation for lot.

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


Any new ADU incorporated into an existing main residence must have a separate entrance facing side or rear yard.

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!