Sonoma County, CA Accessory Structures: ADU Rules (2026)

Permit Sonoma processes ADUs ministerially (no zoning permit, no discretionary review) under Sonoma County Code Sec. 26-88-060, consistent with California's state ADU framework (Gov. Code 66310-66342) that applies to counties for their unincorporated territory. ADUs are established secondary to an existing or proposed single-family or multi-family primary dwelling. Per Permit Sonoma's Development Standards, an ADU is generally allowed up to 1,200 square feet of floor area, and the County must allow at least an 800-square-foot ADU even where lot coverage, floor area ratio, open space, or minimum-lot-size rules would otherwise restrict it.

Detached ADUs may be built to 18 feet (or the base zoning limit for accessory structures); attached ADUs and those above accessory structures follow base zoning height. Minimum side and rear setbacks are 4 feet. One parking space is required unless an exemption applies (within one-half mile of transit, within an architecturally/historically significant district, part of the primary residence, a studio, or where the ADU is built with a new primary dwelling, among others). Owner-occupancy is not required for an ADU. ADUs cannot be used as vacation or other transient rentals and may only be rented for terms of 30 days or longer.

Most residential and agricultural zones allow an ADU unless the parcel carries the 'Z' (Accessory Dwelling Unit Exclusion) combining district. The 2024 housing-element ordinance (No. 6458) and later updates expanded ADU entitlements.