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Atherton CA ADU Guidelines

Accessory Dwelling Unit (ADU) City Regulations



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.

Number of Bedrooms:

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


No min/max


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


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.


Must meet all development standards of 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.


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

For more ADU information:

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