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

Napa County's accessory dwelling unit and junior accessory dwelling unit rules are found in Napa County Code Title 18 (Zoning), Chapter 18.104 (General Regulations), Section 18.104.180, which implements California Government Code Section 65852.2 (ADUs) and Section 65852.22 (JADUs). The current ordinance was rewritten to comply with the wave of state ADU legislation - principally AB 68 (2019), SB 13 (2019), AB 881 (2019), AB 670 (2019, HOA pre-emption), SB 897 (2022, height and non-conforming standards), AB 2221 (2022, denial limits), and AB 1033 (2023, separate-condominium sale option). Napa is distinctive because of its 1968 Agricultural Preserve - the first in California, created under the Williamson Act to protect wine-grape farmland from suburban conversion. The AP (Agricultural Preserve) zone covers the floor of the Napa Valley between the Mayacamas and Vaca ranges; the AW (Agricultural Watershed) zone covers the hillsides. In the AP zone, only JADUs are allowed (no detached or new-construction ADUs), reflecting the policy of keeping the valley floor in vineyard rather than residential use. In the AW zone and in residential districts (RC, RS, RM, RR, etc.), full ADU development is permitted ministerially. Detached new-construction ADUs are capped at 1,200 square feet of floor area, attached ADUs at 50 percent of the primary dwelling or 1,200 square feet (whichever is less, with state law guaranteeing at least 800 square feet), and JADUs at 500 square feet contained within the walls of a proposed or existing single-family dwelling, with an efficiency kitchen and a separate exterior entrance. Building height for ADUs is 16 feet by right, with administrative permits available for taller structures up to the underlying zoning height limit (state law guarantees a minimum of 18 feet for a detached ADU on a single-family lot under SB 897). Side and rear yard setbacks are 4 feet; front setbacks follow the underlying zone. There is no minimum lot size for an ADU under state pre-emption, but most rural Napa parcels rely on private wells and on-site septic systems, so adequate water yield and septic capacity must be demonstrated through Environmental Health review before a building permit can issue. One off-street parking space is required per ADU unless waived under the standard state-law exceptions (within half a mile of transit, in a historic district, conversion of existing structure, on-street permit unavailable, studio unit, or within one block of car-share). JADUs require no additional parking. ADUs cannot be sold separately from the primary dwelling - Napa County has not adopted a local AB 1033 condominium-conversion ordinance, so the parcel remains a single fee-simple lot. JADUs require the owner to live in either the primary dwelling or the JADU plus a recorded deed restriction. Because most of unincorporated Napa County is in a CAL FIRE State Responsibility Area or designated Very High Fire Hazard Severity Zone after the 2017 Atlas Fire, the 2017 Tubbs Fire (which spilled across the Sonoma-Napa border into Calistoga and Mount Veeder), the 2020 LNU Lightning Complex (Hennessey Fire), and the 2020 Glass Fire (which burned thousands of acres on the east side of the valley including parts of Deer Park, Angwin, and the St. Helena hills), ADUs in these areas must comply with California Building Code Chapter 7A ignition-resistant construction (ember-resistant vents, Class A roofing, multi-pane tempered windows) and Public Resources Code Section 4291 defensible space (0-30 foot 'lean, clean, and green' zone and 30-100 foot 'reduced fuels' zone). Hillside parcels in AW are also subject to viewshed and grading review under Napa County's hillside development standards. The County's Affordable ADU Program is run jointly with the Napa Sonoma ADU Center and provides design assistance and forgivable loans for income-restricted ADUs.