3 county-level rules, plus city-specific rules for 2 cities in Sonoma County, California.
Verified from official government sources
Unincorporated Sonoma County allows accessory dwelling units (ADUs) and junior accessory dwelling units (JADUs) ministerially in any zoning district that permits a single-family or multi-family primary dwelling, under Sonoma County Code Section 26-88-060 (ADUs) and Section 26-88-061 (JADUs). The current inland ADU ordinance was adopted by the Board of Supervisors on December 5, 2023 and took effect January 4, 2024, implementing California Government Code Sections 65852.2 and 65852.22 along with the suite of state ADU bills (AB 68, SB 13, AB 881, AB 670, AB 671, SB 897, AB 2221, AB 1033, SB 1211, AB 2533). Detached new-construction ADUs are capped at 1,200 square feet, attached ADUs at 50% of the primary dwelling or 1,200 square feet (whichever is less, but at least 800 sq ft must be allowed), and JADUs at 500 square feet contained within an existing or proposed single-family dwelling. State law guarantees an 18-foot height for detached ADUs (16 feet plus 2 feet for roof pitch) on most lots, 4-foot side and rear setbacks, no minimum lot size, and no owner-occupancy requirement for ADUs permitted between January 1, 2020 and January 1, 2025. JADUs require the owner to live in either the primary dwelling or the JADU and a recorded deed restriction. ADUs and JADUs in unincorporated Sonoma County may not be rented for terms shorter than 30 days, and properties in the State Responsibility Area (SRA), Local Responsibility Area Very High Fire Hazard Severity Zone, or Wildland-Urban Interface (WUI) must meet California Building Code Chapter 7A ignition-resistant construction and PRC 4291 / Sonoma County defensible space requirements - heightened standards adopted in response to the 2017 Tubbs Fire, 2019 Kincade Fire, and 2020 Glass Fire that destroyed thousands of homes countywide. The County has not adopted an AB 1033 condominium-conversion ordinance, so ADUs in unincorporated Sonoma County cannot currently be sold separately from the primary dwelling.
Sonoma County allows one-story detached accessory structures used as tool or storage sheds without a building permit when located on a parcel with an existing single-family dwelling, provided the structure does not exceed 120 square feet of floor area and 12 feet in height above grade, under Sonoma County Permit Sonoma Administrative Manual 1-4-6 and California Building Code Sec. R105.2 as adopted in Chapter 7 of the County Code. No more than one such permit-exempt structure is allowed unless separated from another by more than 50 feet. Sheds and other detached accessory structures exceeding 120 square feet require a building permit, must not exceed 15 feet in height (Sec. 26-88-040 and Permit Sonoma 1-4-6), are prohibited in required front and street-side setbacks and in designated creek setback areas, and may not cover more than 50% of the required rear yard. In CAL FIRE State Responsibility Areas or Very High Fire Hazard Severity Zones, sheds within 30 feet of any structure must meet Public Resources Code Sec. 4291 defensible-space rules, and sheds within 5 feet of a property line may require fire-resistive construction under California Residential Code Sec. R302. Sheds in Scenic Resources (SR), Coastal (CC), and Floodway combining districts have additional siting and material requirements.
Sonoma County permits garage conversions to accessory dwelling units (ADUs) ministerially under Sec. 26-88-060 of the Zoning Code and California Government Code Sec. 65852.2, with no replacement off-street parking required when the converted garage previously satisfied a parking requirement. Conversion ADUs are limited to the existing dimensions of the structure plus an addition of up to 150 square feet for ingress and egress. Side and rear setbacks for converted garages are 0 feet (legal-existing) for the existing footprint and 4 feet for any new construction. Garage conversions to non-ADU living space (a den, family room, or bedroom that's still part of the main dwelling) are a separate building-permit pathway under Chapter 7 of the County Code, do require the original off-street parking to be replaced elsewhere on the parcel under Sec. 26-86-010, and must meet California Residential Code habitability standards (minimum ceiling heights, egress windows, natural light, ventilation, smoke and carbon monoxide alarms). Conversion of a garage to a Junior ADU (JADU) under Sec. 26-88-061 is allowed when the garage is attached to the single-family dwelling and the JADU is capped at 500 square feet. Garage conversions in SRA, Very High Fire Hazard Severity Zones, or WUI must meet California Building Code Chapter 7A ignition-resistant standards if exterior elements are altered, and defensible space under PRC Sec. 4291 must be maintained.
2 cities in Sonoma County have their own accessory structures rules. Each link goes to that city's dedicated page with code citations.
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