Global Announcement Text

Atherton, California: ADU Regulations

City or County:

Atherton, California

Date Updated:

11.25.2020

Zoning:

All single-family residential zones

Types of ADUs:

Attached, Detached, Conversion

Stories:

Attached: Must meet all development standards of underlying zone
Detached: maximum one story

Lot Size:

N/A

Maximum ADU Floor Area:

a. No ADU or JADU may be smaller than the size required to allow an efficiency unit (Health and Safety Code Section 17958.1.)

b. The maximum size of a detached or attached ADU is 1200 ft.

c. The increased floor area of an attached ADU can not exceed 50% of the existing floor area of the main residence, unless necessary to accommodate at least an 800 sf ADU.

d. No floor area limitation shall be required for any conversion of an existing legal nonconforming building to an ADU.

Exemption from Maximum Floor Area Limitations.

a. The first 800 sf of floor area of each newly constructed ADU or JADU shall be excluded from maximum floor area calculations.

b. Additional floor area for newly constructed ADUs or JADUs may be excluded from maximum floor area calculations where staff determines the following conditions are met:

i. Newly constructed detached ADUs which meet and/or exceed the minimum side and rear setbacks of chapter 17.40, Accessory Buildings and Structures, will be allowed additional floor area exemptions in addition to the first 800 sf of floor area up to a total of one thousand two hundred square feet.

ii. Newly constructed attached ADUs which are located in the main buildable area and encroach into the side and rear yard setbacks by not more than 20% of the required setback shall be allowed additional floor area exemptions in addition to the first 800 sf of floor area up to a total of 1200 sf.

Lot Coverage:

Must maintain consistency with zoning requirements designated for lot. Any ADU or JADU does not count towards the allowable density for the lot upon which it is located.

Bedroom:

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

Storage:

No min/max. JADU must be reasonable size relative to JADU.

Kitchen:

Must provide separate kitchen facilities for detached / attached ADUs or and efficiency kitchen for JADU.

Bath:

Must provide separate bathroom facilities for unit.

Note: A junior accessory dwelling unit may include separate sanitation facilities or may share sanitation facilities with the existing structure.

Parking:

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:

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.

Setbacks:

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 unless necessary to accommodate at least an eight-hundred-square-foot ADU. 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. No setback shall be required for an existing, legal, attached or detached garage and/or other existing structure that is converted to an ADU or JADU.

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


Height:

Must meet all development standards of the underlying zone. Maximum detached: 16’. Maximum sidewall height: 11’.

Location:

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.

Design:

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.

Heritage Trees:

In all instances, the location of any newly constructed ADU or JADU shall meet the minimum requirements in accordance with chapter 8.10, Removal of and Damage to Heritage Trees, for any existing heritage trees.


Entryways:

Any new ADU incorporated into an existing main residence must have a separate entrance facing side or rear yard. Any new detached ADU or JADU shall have a separate entrance facing towards the interior of the lot.

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!