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.
Check Your Parcel's Zoning
State law does not allow private covenants such as CC&Rs to prohibit the construction or use of an ADU or JADU on a lot zoned for single-family residential use. The County does not enforce those private covenants.
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 a multifamily development, the number of ADUs permitted within the footprint of the existing structure may be up to 25 percent of the number of existing units, or one unit, whichever is greater. Up to two detached ADUs may be permitted on a lot with an existing, legal multifamily development.
Maximum and Minimum Sizes
The minimum size of an ADU is 225 square feet. The maximum size of an ADU is 1,200 square feet. ADUs must have permanent provisions for for living, sleeping, eating, cooking, and sanitation.
ADUs must meet at least four foot setbacks from rear and side property lines. For units on 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. Front yard setbacks are determined by the base zoning district ( check your parcel's zoning). In addition, the Fire Code may require additional setbacks from property lines or between buildings.
Minimum Lot Size
There is no minimum lot size for an ADU, but adequate water and sewer or septic must be demonstrated.
Septic System Suitability
Water availability and soil suitability for a septic system are significant factors that determine whether a permit for an accessory dwelling unit can be issued. Property owners are advised to investigate water availability and septic suitability prior to proceeding with design plans. The construction of an accessory dwelling unit represents a new use on the property which must meet current codes with respect to the septic system for the unit. If one septic system is to serve both the main dwelling and accessory 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 the two units).
Water Availability and Water-Scarce Areas
Accessory dwelling units served by a well in areas with low groundwater availability must meet additional requirements. 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.
Water-scarce areas are identified on the Sonoma County Groundwater Availability Map and each parcel’s Groundwater Availability Area classification can be found in its parcel report.