ADU rules in Napa County, CA — also called accessory dwelling unit regulations or granny flat ordinances — cover setbacks, owner-occupancy, parking, and permit requirements.
Napa County Code Section 18.104.180 ministerially permits Accessory Dwelling Units (ADUs, called 'second units') and Junior ADUs (JADUs) on legal lots zoned RS, RC, AW, or PD, with a 1,200 sq ft maximum for ADUs (500 sq ft for JADUs) and four-foot side/rear setbacks for new detached units up to 16 feet tall.
Per Napa County Code § 18.104.180(A) (as amended by Ordinance 1456, adopted February 11, 2020), implementing California Government Code §§ 65852.2 and 65852.22, second units and JADUs are permitted ministerially in the RS, RC, AW, and PD zones (JADUs are also allowed in AP per § 18.104.180(D)(1)). The total floor space cap is 1,200 sq ft for an ADU and 500 sq ft for a JADU (measured from inside the exterior walls). Lot coverage, FAR, open-space and setback standards 'must permit, or shall be waived to allow, an 800 square foot detached or attached second unit 16 feet high with four-foot side and rear yards' if the unit meets all other development standards including front-yard setbacks (§ 18.104.180(A)(14)). No setback is required for a second unit created by converting an existing living area or existing accessory structure, or for a new unit constructed in the same location and dimensions as an existing structure (§ 18.104.180(A)(16)(a)). For all other detached second units, a four-foot rear/side setback is required (§ 18.104.180(A)(16)(b)). Second units require one parking space per unit or per bedroom (whichever is less), which may be tandem on the existing driveway, with multiple exemptions including transit proximity (within 1/2 mile), historic districts, conversions, and car-share locations (§ 18.104.180(A)(11)). No impact fees apply to second units under 750 sq ft (§ 18.104.180(A)(6)(c)). Fire sprinklers are not required if not required for the primary residence (§ 18.104.180(A)(10)). The unit cannot be sold separately from the primary dwelling and neither unit may be used for short-term rentals (§ 18.104.180(A)(13)).
Building or occupying an ADU without ministerial permit approval is a code-enforcement violation under Title 18. Napa County operates an ADU Delayed Enforcement Program (per AB 2533) under which unpermitted ADUs may qualify for delayed enforcement if a Building Inspector confirms no inherent health and safety risk and the unit meets minimum setback, size, and zoning standards listed on the Code Compliance ADU Delayed Enforcement Program Inspection Checklist.
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