STATEWIDE UPDATE: New California ADU Bills SB13, AB68, AB881, AB670 have passed. These laws will create more options for building an ADU in Atherton in 2020. To prepare for the upcoming changes use our ADU Services to learn how these laws will effect your property!
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Atherton ADUs

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Atherton ADU Guidelines Last Updated 08/11/19


The city government of Atherton will approve ADUs under California's statewide mandated regulations.
The California Building Code (CBC) 2016 is adopted with the following amendment(s): 622 and 626
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:

Only one accessory dwelling unit shall be allowed on each lot or parcel.

AB68 & AB881 Update: Properties which convert existing space to create ADUs may be eligible for one ADU -AND- one JADU.

Types of ADUs allowed:

Attached, detached, and junior ADUs allowed.

Lot size:

Per state standards.

AB68 & AB881 Update: Properties will not be subject to minimum lot standards by ordinance or of the applicable zoning district.

ADU size:

Maximum square feet. The size of each such accessory dwelling unit shall not exceed one thousand two hundred square feet. Floor area. The increased floor area of an attached accessory dwelling unit shall not exceed fifty percent of the existing living area of the main residence, with a maximum increase in floor area of one thousand two hundred square feet.

AB68 & AB881 Update: Properties will be allowed up to 850 square feet for a one bedroom or up to 1,000 square feet for a two bedroom ADU.


Each newly constructed attached or detached accessory dwelling unit shall be located in the main building area. In addition, a newly constructed accessory dwelling unit may encroach into the side and rear yard setbacks by not more than twenty percent of the required setback. Newly constructed, detached accessory dwelling units shall meet all other applicable development standards as set forth in chapter 17.40 (Accessory Buildings and Structures). Separation Requirement. A separation of at least eight feet shall be maintained between any detached accessory dwelling unit and its main residence structure.

AB68 & AB881 Update: The maximum required setback for a detached new construction ADU shall be no more than 4 feet from the rear or side property lines.


1. Unless subject to one of the exceptions immediately below, each accessory dwelling unit shall have assigned to it one parking space for each bedroom in the accessory dwelling unit. Parking shall be accessible from a public street and screened from view of the public street. These spaces may be provided as tandem parking on an existing driveway.

2. No additional parking required. No additional parking shall be required where: (a) the unit is located within one-half mile of public transit, or (b) the unit is part of the existing primary dwelling or an existing accessory structure.

AB68 & AB881 Update: Properties which convert an existing garage will not be subject to replacement parking requirements.

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.


Owner Occupancy. Either the main residence or the accessory dwelling unit, or both, shall be occupied by the owner of the property.

Rental Only. The accessory dwelling unit shall not be sold separate from the main residence and may be rented. Rentals must be for terms of thirty days or longer.

SB13 Update: Owner-occupancy requirements for ADUs will be voided statewide for 5 years.

Utility connections:

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

Permit Fees

SB13 Update: 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.

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) 2016 is adopted with the following amendment(s): 622 and 626

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

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


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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How does it work? Get started on your project with expert assistance and without commitment using our customized ADU Property and Design Reports as your first step. Then, get on-site visits from local contractors who can provide you with guidance and estimates for your ADU project.

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Use our local contractor database to get on-site visits and cost estimates to compare services for your ADU project.

Getting your designs ready for city permitting and construction can speed up your ADU project and reduce unforseen costs later on in the process.

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