Accessory Dwelling Unit 

ADUWhat is an Accessory Dwelling Unit (ADU)?

An “ADU” means a building or portion of a building designed for use and occupancy by people living independently of the occupants of the main residence building and containing separate kitchen, bath, sleeping, or living  facilities. 

    Attached Accessory Dwelling Unit: means an accessory dwelling unit that is structurally attached to the main residence but which has independent, direct access from the exterior. 

    Detached Accessory Dwelling Unit: means an accessory dwelling unit that is not structurally attached to the main residence. 

There are three key types of ADUs outlined below:

o    Conversion ADU: Space within an existing or proposed home (like an attached garage or attic) or within the existing footprint of a permitted accessory structure (like a detached garage or outbuilding), can be converted into an ADU, as long as the space has side and rear setbacks sufficient for safety (per the building and fire codes) and it has its own front door, independent of the main home.  

ADU Image 1o    Attached ADU: A newly constructed unit that is connected to the primary main residence by at least one wall.

o    Detached ADU: A newly constructed unit that is freestanding or separate from the primary main residence, like a cottage, pool house, or guesthouse. 

What is an Junior Accessory Dwelling Unit (JADU)?

A “JADU” means a unit that is no more than five hundred square feet in size and contained entirely within a single-family residence. A junior accessory dwelling unit shall include an efficiency kitchen, which shall include all of the following: a cooking facility with appliances, a food preparation counter and storage cabinets that are of reasonable size in relation to the size of the junior accessory dwelling unit. A junior accessory dwelling unit may include separate sanitation facilities or may share sanitation facilities with the existing structure. 

General Town ADU Requirements and Guidelines

ADUs and JADUs are allowed in all residential zoning districts in Town including the R1-A, and R1-B residential zoning districts. The following are general regulations and guidelines pertaining to ADU’s and JADUs, however it is strongly recommended you confer with Planning staff to confirm applicability to your property, as some lots may have unique or special circumstances.  

Please refer to Chapter 17.52 “Accessory Dwelling Units”  of the Atherton Municipal Code for the full Ordinance. 

Floor Area:

Floor area calculations are summarized in the tables below.  Every ADU, regardless of attached or detached, new or converted, is allowed up to 800 square feet of floor area which is exempt from the counting towards the sites overall maximum, allowable square footage.   

Up to 1,200 square feet of ADU floor area may be exempt, for ADUs that meet certain setback requirements. 

DETACHED ADUs

Maximum allowed square footage 

 

Floor area exemption for detached ADUs built up to the 4-foot setback 

 

Floor area exemption for detached ADUs utilizing a setback consistent with Accessory Buildings     (AMC 17.40.040) 

 

1,200 sq.ft.

Up to 800 SF (any additional square footage in excess of the 800 SF counts toward the maximum allowed floor area for the site) 

1,200 sq.ft.

ATTACHED ADUs

Maximum allowed square footage 

 

Floor area exemption for attached ADUs built up to the 4-foot setback 

 

Floor area exemption attached ADUs located in the main buildable area and encroaching no more than 20% of the required main building setback 

1,200 sq.ft.

Up to 800 SF (any additional square footage in excess of the 800 SF counts toward the maximum allowed floor area for the site)

1,200 sq.ft.

JADUs

Maximum allowed square footage 

 

Floor area exemption

 

500 sq.ft. 

500 sq.ft.

 

 

Setbacks:

General setbacks for ADUs are summarized in the tables below:

DETACHED  ADU SETBACKS

Front Setback (minimum)

120’ from the front property line or 30 feet from the main residence, whichever is less. 

*Rear Setback (minimum)

4’

*Side Setback (minimum)

4’

Setback From Main Building

8’

*Detached ADUs that propose side and rear yard setbacks greater than 4 feet and that meet the minimum setbacks for detached Accessory Buildings/Structures are allowed additional floor area exemptions. See Section 17.52.040.5.B. of ADU Ordinance.

ATTACHED ADU SETBACKS

Front Setback (minimum)

Consistent with the main building setback

*Rear Setback (minimum)

4’

*Side Setback (minimum)

4’

*Attached ADUs that propose side and rear yard setbacks that meet the minimum setbacks for main building with up to a 20% encroachment into these required setbacks are allowed  additional floor area exemptions. See Section 17.52.040.5.B. of ADU Ordinance. 

No setback requirement shall apply to any ADU or JADU constructed in an existing living area, a converted existing accessory structure or building, or for a new ADU or JADU constructed in the same location and built to the same dimensions as an existing structure. 

Height:

ADU height requirements are summarized in the below table. AMC 17.42 “Building Height and Measurement” describes how to measure building height. AMC 17.60 “Definitions” defines the AMC 17.60 “Definitions” defines the terms “sidewall” and “endwalls”.

ADU HEIGHT REQUIREMENTS

Detached ADU

Attached ADU (R1-A)

Attached ADU (R1-B)

Sidewall Height

n/a

Sidewall Height

22’

Sidewall Height

18’

Endwall Height

16’

Endwall Height

30’

Endwall Height

28’

 

Parking:

One (1) on-site parking space (can be opened and uncovered) is required for a new ADU, with the following exceptions:

  • The parking space for the ADU can be tandem and/or located within a setback area;
  • No additional onsite parking is required for a new JADU;
  • No additional onsite parking is required when an ADU is located within one-half mile of a public transit stop or when an ADU is established by converting floor area in an existing primary residence or an existing accessory structure (no new floor area is added); and
  • When a garage or carport is demolished or converted in relation to the establishment of an ADU, the onsite parking spaces do not need to be replaced.

Heritage Trees:

Newly constructed attached or detached ADUs shall meet the Town’s minimum Heritage Tree regulations, standards, and protection measures, including maintaining minimum distances, or Tree Protection Zone (TPZ) distances, for all surrounding heritage trees.

Information on the Town’s heritage tree standards can be found in Chapter 8.10 “Removal of and Damage to Heritage Trees” of the AMC, and in the Town’s adopted Tree Preservation Guidelines Standards and Specifications. 

Questions on the Town’s Heritage Tree regulations and guidelines can be directed to the Town’s Arborist, at sallybentz-dalton@ci.atherton.ca.us, or at 650-752-0526.

For more information on ADUs, and list of FAQs, please visit the Town ADU Handout and FAQ.

Permitting Process

ADU applications that meet all zoning ordinance requirements may be reviewed and approved by staff.  In Atherton, any new ADU permit has the option of submitting simultaneously to both the Building Department (for permit) and to the Planning Department (for zoning clearance) for concurrent review. Alternatively, you may chose to first submit directly to the Planning Department for zoning clearance, and then subsequently submit to the Building Department for permit. 

If you are to ready to apply for your ADU permit for a concurrent Building and Planning Department review, please note the following:

  • Application and fees are submitted directly to the Building Division.  Building Department staff will then route the materials submitted to Planning Staff who will review the plans for a determination of compliance with the regulations of Chapter 17.52 “Accessory Dwelling Units” of the Atherton Municipal Code. 
  • General Building Department permit submittal requirements can be found here
  • Building permit applications can be uploaded online here
  • Building permit fees and application forms can found here
  • Any questions on a Building Permit Submittal process, requirements, or fees can be directed to: buildingdept@ci.atherton.ca.us, or at 650-752-0560.Ph

If you are ready to apply for your ADU permit and would like to first submit only to the Planning Department for zoning clearance approval prior to submitting to the Building Department for permit, please note the following:

  • For construction of a new ADU submit a Planning Department application, (Note submittal materials on the back of the application, and Planning Department fees in the  amount of $1,105.41 (payable by check made out to the Town of Atherton, or cash. No credit cards accepted.)
  • For conversion of existing space to a new ADU, submit the ADU Conversion application.  (Note submittal materials on the back of the application, and Planning Department fees in the  amount of $1,504.03 (payable by check made out to the Town of Atherton, or cash. No credit cards accepted.)
  • Planning Department applications can be uploaded online here
  • Planning staff will review your application for compliance with Chapter 17.52 “Accessory Dwelling Units” of the Atherton Municipal Code and issue you a determination letter, typically within 10 business days of the submittal date. 

Outside Agency Housing Resources – Renting your ADU or Finding an ADU to Rent

These organizations can help match individuals seeking housing with people who have an ADU to rent and/or can assist homeowners in renting out their ADUs or JADUs. 

Other Resources

Visit Atherton Municipal Code to learn more about ADUs and Junior ADUs in the Town of Atherton. To access additional information about ADUs in San Mateo County please visit the Second Unit Resource Center - click here