Below are some of the more significant standards property owners need to know to plan an ADU project. For a complete list of development standards, please review Zoning Code and the Permit Sonoma ADU Checklist.
Note: The standards below apply to inland zones. For accessory dwelling unit regulations in the Coastal Zone, see Section 26C-325.1 of the the County's Coastal Zoning Ordinance.
ADUs are generally allowed in residential, mixed-use, and agricultural and resource zones but are prohibited in the ADU Exclusion (Z) Combining Zone. the Z Accessory Dwelling Unit Exclusion Combining District and on properties subject to a Williamson Act contract in accordance with the current Uniform Rules for Agricultural Preserves and Farmland Security Zones.
ADU Exclusion District
The Z Accessory Dwelling Unit Exclusion Combining District prohibits ADUs where there are natural hazards, public safety issues, or water availability or water quality issues. In 2019, the County reevaluated the application of this district and ultimately rezoned over 1,900 parcels to remove the Z Combining District and allow ADUs as a permitted use. Parcels that were not rezoned and still contain this Z Combining District may apply for a zone change. A zone change is a discretionary permit, considered on a case-by-case basis, and approval is not guaranteed. The proposal must be reviewed by the Planning Commission and ultimately approved by the Board of Supervisors.
More information about applying for a zone change
Check your parcel's zoning
Number of Units Permitted
A maximum of one ADU and one JADU may be permitted on parcels with a single-family dwelling. On parcels with multi-family buildings, the maximum number of ADUs will vary. Please review the ADU Checklist and the Zoning Code.
Maximum and Minimum Sizes
ADUs must meet minimum building code standards for living space. ADUs must have permanent provisions for for living, sleeping, eating, cooking, and sanitation. The maximum size of an ADU is generally 1,200 square feet.
ADUs must meet of at least four feet from rear and side property lines, provided that applicable fire-resistive construction standards are met. Front yard setbacks are determined by the base zoning district (check your parcel's zoning). Check with the Fire Prevention division for information about applicable fire safe standards.
For properties with septic systems, setbacks will be required from septic areas and required septic reserve areas. Setbacks from waterways are also required where the Riparian Corridor Combining Zone applies. In addition, the Fire Code may require additional setbacks from property lines or between buildings.
Minimum Lot Size
There is no minimum lot size to establish an ADU.
Septic System Suitability
In areas where public sewer service is not available, the ability to dispose of wastewater on site is a significant factor in determining whether a permit for an ADU can be issued. Property owners are advised to investigate septic capacity on the property and/or in the existing septic system prior to proceeding with design plans. The construction of an ADU represents a new use on the property which must meet current codes with respect to the septic system for the unit. The septic system must meet current code requirements and be adequately sized for the proposed sewage discharge (based upon total number of bedrooms in all the units to be served by that system).
Water Availability and Water-Scarce Areas
In areas where public water service is not available, the availability of sufficient well water is a significant factor in determining whether a parcel can can accommodate an ADU. Parcels in areas with low groundwater availability ("water-scarce areas") must meet additional requirements. Water-scarce areas are identified on the Sonoma County Groundwater Availability Map and the County's Zoning and Land Use GIS Viewer, and each parcel’s Groundwater Availability Area classification can be found in its parcel report.
Well Yield Test
If the well is located in a Class 3 or Class 4 Groundwater Availability Area, the applicant must provide a well test conducted between July 15 and October 1 demonstrating the well meets minimum water yield requirements according to the County’s Dry Weather Well Testing policies. Review Section 7-12.2 of the Sonoma County Building Code for additional requirements if the property is supplied by a well on another parcel with a water supply easement.
Zero Net Use Requirements
If the well is located in a Class 4 Groundwater Availability Area or a Critical Habitat Area, the applicant must demonstrate compliance with Permit Sonoma Policy and Procedure 8-2-2 Guidelines for Net Zero Groundwater Use.