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Local ADU Rules

County of Sonoma

Last Updated September 13, 2024

ADU Contact:

Planning Department, planner@sonoma-county.org
Katrina Braehmer, Katrina.Braehmer@sonoma-county.org

Standards	Details
Zones allowed	ADUs shall be ministerially permitted in zoning districts that allow single-family or multifamily dwellings, in compliance with Government Code § 65852.2 ADUs are not allowed on lots within the Z (Accessory Dwelling Unit Exclusion) Combining District or lots subject to a Williamson Act contract.
Lot Restrictions
Minimum Lot Size	There is no minimum lot size for an ADU, but adequate water and sewer service, or septic capacity and well yield, must be demonstrated.
Min/Max ADU Size	The minimum size for an ADU is as determined by the building code. Maximum size: Detached: Newly constructed detached ADUs are limited to a maximum of 1,200 square feet, regardless of the number of bedrooms. Attached: ADUs attached to a primary residence or accessory structure are limited to 1,200 square feet. Conversions: ADUs created from existing legally established space in a primary residence or accessory structure are limited to the dimensions of the existing structure with an addition of no more than 150 square feet if needed to accommodate ingress/egress only. Additions beyond 150 square feet may be allowed for a maximum unit size of up to 1,200 square feet.
ADU Building Height	Detached: Newly constructed detached ADUs are generally subject to the maximum height for accessory structures specified in the applicable zoning district, but shall not be limited to less than 16 feet. On lots with multifamily dwellings, new detached ADUs are limited to 16 feet. Attached to or converted from space in the primary residence: ADUs attached to the primary residence or converted from existing space are limited to the height limit of the lot’s base zoning district. Attached to or converted from an accessory structure: ADUs attached to or converted from an accessory structure are limited to the height limit for accessory structures, but not less than 16 feet. In General Plan-designated Urban Service Areas, and where the unit is proposed to be located above an accessory structure, the maximum height shall be that established for the primary residence in the base zoning district.
Setbacks	Side and rear yard setbacks: 4 feet Exception(s): Side and rear yard setbacks may be reduced to 0 feet if the ADU is converted from an existing legally established structure. Greater setbacks may be required by the Building or Fire Code. Please have your plans reviewed by those divisions for compliance with those codes. Front setbacks: established by a lot’s base zoning district
Parking Requirements	One parking space for the ADU is required in addition to the parking required for the primary residence, unless the ADU qualifies for any of the following exemptions: Property is located within ½ mile walking distance to transit stop Lot is zoned HD (Historic District) Combining Zone The ADU is part of the proposed or existing primary residence or existing accessory structure On-street parking permits are required, but not offered for the ADU Property is located within one block of a car share program The ADU is a studio. If a project does not qualify for an exemption, the required parking space may be located in setback areas or in tandem. No replacement parking may be required if parking is eliminated due to construction of the ADU (i.e. if a garage is converted to an ADU).
Owner Occupancy	Not required.
Sample fees from recent permits	840 SF detached with a 240 SF Carport. Total: $15,887 – $21,725, Including impact fees *Fees are specific to each project and will vary. Fees change regularly. These are provided as examples only.
Other requirements
Process	Ministerial approval shall be granted for ADUs through the building permit process. In addition to obtaining a building permit, other site development permits may be required (e.g. grading, septic, sewer, well). Disclaimer: standards and process may vary. Consultation with the contact is recommended before starting any project. For more information, visit the Permit Sonoma website: https://sonomacounty.ca.gov/PRMD/Regulations/Housing/Housing-Types/Accessory-Units-and-Junior-Units/Accessory-Dwelling-Units/
Exceptions	This is a summary of standards that apply to ADUs in the unincorporated areas of Sonoma County in which Permit Sonoma is the permitting agency. This summary should serve as early guidance for homeowners and should not be used in lieu of reviewing applicable regulations directly from Permit Sonoma. ADU regulations in the Coastal Zone are not the same as the rest of Sonoma County. For the Sonoma Coast region, see the Coastal Zoning Code https://library.municode.com/ca/sonoma_county/codes/code_of_ordinances?nodeId=CH26CCOZOREDI_ARTXXXIIGEUSBUEXBULI_S26C-325.1SEDWUN ? Recent changes in the California Government Code that allow streamlining of ADU and JADU permits do not apply to implementation of the California Coastal Act (Section 65852.2.(l): Nothing in this section shall be construed to supersede or in any way alter or lessen the effect or application of the California Coastal Act of 1976 (Division 20 (commencing with Section 30000) of the Public Resources Code), except that the local government shall not be required to hold public hearings for coastal development permit applications for accessory dwelling units.)

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