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Atherton ADUs

Everything about building Accessory Dwelling Units in Atherton, CA

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Atherton ADU Guidelines Last Updated 04/05/20

The city government of Atherton will approve ADUs under California's statewide mandated regulations.
The California Building Code (CBC) 2019 is adopted.
Relevant state bill updates for SB13, AB68, AB881, AB670 are shown in green below.


City Hint: The city permitted a total of 13 ADU project(s) in the past year of 2018, which makes it above average for ADU production compared to other cities statewide.

Number of ADUs allowed:

Under state law, each single-family home may now add at least one Accessory Dwelling Unit (ADU). Owner occupied single-family parcels may add one Accessory Dwelling Unit (ADU) and one Junior ADU. Multi-family parcels, per every four existing residential units, may add one Accessory Dwelling Unit (ADU), so long as the existing units were legally established.

As of January 1st, 2020

Types of ADUs allowed:

Detached ADU, Attached ADU, Junior ADU are allowed. Accessory Dwelling Unit (ADU)s may be new construction or converted from existing space. JADUs must be converted from existing space.

Lot size:

Accessory Dwelling Unit projects on single family or multi-family residential parcels will not be subject to the minimum lot size of the underlying local zoning code or by special ordinance. ~ AB68

As of January 1st, 2020

ADU size:

Residential lots will be permitted to add up to 850 square feet for a one bedroom ADU or up to 1,000 square feet for a two bedroom ADU, regardless of the underlying zoning standards of the property. A Junior Accessory Dwelling Unit is permitted up to 500 square feet. According to Califronia Residential Code, the minimum size of any living unit is 150 square feet (including Accessory Dwelling Unit (ADU)s and Junior Accessory Dwelling Units). ~ AB-68

As of January 1st, 2020


Setbacks for a detached new construction Accessory Dwelling Unit shall be no more than 4 ft. from the rear and side lot lines. No setbacks shall be required for Accessory Dwelling Units and Junior ADUs converted from legally eastablished existing space. Attached new construction Accessory Dwelling Units and conversions from existing space to be expanded must comply with the setbacks of the underlying zone. ~ According To AB68

As of January 1st, 2020


Parcels which convert an existing garage to an Accessory Dwelling Unit or Junior ADU will not be subject to replacement parking requirements. ~ AB68

As of January 1st, 2020

Fire Safety:

Fire regulations and shall comply with local building codes. Fire sprinklers shall not be required for the accessory dwelling unit if they are not required for the primary dwelling.

Shape, materials and style:

Letting Restrictions. Any lighting associated with the accessory dwelling unit shall be shielded or download and shall not shine onto adjoining properties.


Height is restricted to the requirements of the underlying zoning district.

Design review:

A completed application for an accessory dwelling unit that complies with the provisions of this chapter shall be considered ministerial, without discretionary review or a hearing, within one hundred twenty days after receiving the application.

Number of Bedrooms:

Not specified by State Standards.


Between between 2020 and 2025, owner occupancy requirements will not apply to new Accessory Dwelling Unit (ADU)s. (applicable to new construction and conversion units) ~ According To SB13

As of January 1st, 2020

Utility connections:

Local health officer approval where a private sewage disposal system is being used, if required. (State requirements)

Permit Fees

SB13 Update: As of January 1st, 2020, local impact fees for ADUs will be reduced substantially statewide.


“Accessory dwelling unit” 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.


Entrance Requirements. Any new accessory dwelling unit that is incorporated into an existing main residence shall have a separate entrance facing the side or rear yard.
Passageway. No passageway, as defined in California Government Code Section 65852.2, shall be required in conjunction with the construction of an accessory dwelling unit.

Allowed Zones:

*** Beginning in 2020, according to state law, all residential zoning districts, including single-family and multi-family, are allowed to build Accessory Dwelling Units. ***

As of January 1st, 2020

Lot Coverage:

Consistency with General Plan. The accessory dwelling unit shall be consistent with the existing general plan and zoning land use designation for the lot. (Ord. 624 § 1 (part), 2017: Ord. 591 § 1, 2010: Ord. 582 § 1 (Exh. A) (part), 2009. Formerly 17.52.030)

Existing Garage

No setback shall be required for an existing, legal attached or detached garage that is converted to an accessory dwelling unit. A setback of five feet is required from the side and rear lot lines for an accessory dwelling unit that is constructed above a garage.

SB13 Update: Amnesty program to permit unpermitted units will be administered statewide, allowing non-health and safety code violations to be deferred.


The accessory dwelling unit shall be attached to the existing main residence or located within the habitable area of the existing main residence or detached from the main residence and located on the same lot as the main residence.


Exemption from Floor Area Limitations. The first one thousand two hundred square feet 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 ten percent of the allowed floor area for the lot.

Landscaping Requirements

Landscape Screening. Landscape screening shall be provided in compliance with the provisions of chapter 17.50 (Landscape Screening).

Residential Building Code:

The California Building Code (CBC) 2019 is adopted.

Atherton City Planning Offices

Atherton Planning Department

Hours Tuesday and Thursday 8 - 11 a.m. and 1 - 4 p.m.

150 Watkins Avenue Atherton, CA 94027

Call/Text ADU Operator (650) 730-2387
Call Atherton ext: 975 |
'Text to Check' Your Property sms: "123 Main St, Atherton"

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ADU Example Designs

ADU Model 625 sqft

W 25 ft L 12 ft

ADU Model 640 sqft

W 32 ft L 20 ft

Atherton ADU Marketplace

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Featured Financing Program

ADU Financing

CA Statewide

Tailored to ADU construction.
Based on project size & appreciation.
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Karyn Jones

Program Representative

Call Now (760) 496-5959

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Costs / ROI



* upon completion of ADU


* based on local average price per sqft


* estimated by location


* 30 years / 5% interest rate / 20% down payment


Size: 500 sqft


Appreciation Estimate = (Base Value: $) / (Home's Living Space: sqft) * (ADU's Living Space: sqft)

Rental Estimate = (Rent Per Sqft: $13.04) * (ADU's Living Space: sqft)

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