3 county-level rules, plus city-specific rules for 1 city in Napa County, California.
Verified from official government sources
Napa County's ADU and JADU ordinance is codified at Napa County Code Title 18 (Zoning), Chapter 18.104, Section 18.104.180. Detached ADUs are capped at 1,200 square feet and 16 feet in height (taller allowed by administrative permit up to underlying zoning), with 4-foot side and rear setbacks. JADUs are capped at 500 square feet within an existing single-family dwelling. Critically, in the AP (Agricultural Preserve) zone - the 1968 Williamson Act preserve that protects Napa's wine industry - only JADUs are allowed, not ADUs. The less restrictive AW (Agricultural Watershed) zone and all residential districts permit full ADUs. State law (California Government Code Sections 65852.2 and 65852.22) sets the baseline ministerial standards.
In unincorporated Napa County, detached one-story accessory structures (sheds, pump houses, small workshops) that do not exceed 120 square feet of floor area and are not used for habitation, sleeping, or sanitation do not require a building permit under California Building Code Section 105.2 (Item 1), as adopted by Napa County. Larger sheds and any structure with electrical, plumbing, or mechanical work require a building permit from Napa County Planning, Building and Environmental Services (PBES). Zoning setbacks, height limits, and lot coverage standards in Napa County Code Title 18 still apply even to permit-exempt sheds.
In unincorporated Napa County, converting an attached or detached garage to habitable living space is treated as a conversion ADU or JADU under Napa County Code Title 18, Chapter 18.104, Section 18.104.180. Conversion ADUs are exempt from minimum setback requirements (they retain the existing structure's setbacks) and from replacement parking under California Government Code Section 65852.2(a)(1)(D)(xi). JADU conversions within an attached garage are capped at 500 square feet and require owner occupancy. A building permit is required from Napa County PBES, and the conversion must meet California Building Code requirements for habitable space (ceiling height, light, ventilation, egress, energy code).
1 cities in Napa County have their own accessory structures rules. Each link goes to that city's dedicated page with code citations.
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