Vallejo

Napa County is a North Bay California wine-country county (population ~138,000) with five incorporated cities (Napa, American Canyon, Yountville, St. Helena, Calistoga) and the unincorporated town of Angwin (the Pacific Union College corner). Most of the famous Napa Valley wine-growing region itself is unincorporated land between the cities, governed directly by the county. The county Board of Supervisors administers ADUs in unincorporated areas under California Government Code Sec. 65852.2 and 65852.22 (state ADU preemption, as amended by AB 68/881, SB 13, AB 670/3182, AB 2221/SB 897, AB 976, and AB 1033). Napa County's ADU permitting environment is uniquely shaped by the 1968 Napa County Agricultural Preserve (the first agricultural preserve in the United States, predating Williamson Act use in the county) and the 1990 Measure J (later renewed as Measure P in 2008, extending to 2058) which requires voter approval to remove agricultural-preserve protections from any parcel. The Agricultural Preserve and Watershed District zoning categories cover the bulk of the rural valley floor and surrounding hillsides. Within these zones, residential structures (including ADUs) are subordinate to the principal agricultural use, and multiple AVA (American Viticultural Area) designations carry county-level vineyard-protection rules. Napa County is also the only California county with a Hillside Erosion Control / Watershed Protection ordinance that specifically restricts new development on slopes greater than 30%.

• Napa County Code Title 18 - Zoning, ADU/JADU provisions, Agricultural Preserve and Watershed District regulations
• Napa County Measure J (1990) / Measure P (2008) - Agricultural Lands Preservation Initiative
• Cal. Gov't Code Sec. 65852.2 (Accessory Dwelling Units)
• Cal. Gov't Code Sec. 65852.22 (Junior Accessory Dwelling Units)
• Napa County Hillside / Conservation Regulations (Watershed Protection)

State-floor overlay: California state ADU preemption applies in full and supersedes any local restrictions on ADU floor area, parking, or owner occupancy that conflict with state law. However, agricultural-preserve and watershed-district base zoning is not preempted - the underlying parcel use (winery / vineyard / agricultural) governs whether an ADU can be sited at all, and on AP / AW parcels ADUs must be subordinate to the principal agricultural use. AB 1033 condo-conversion election is at the county's option and Napa County has not adopted it as of last check. AB 976 prohibits owner-occupancy mandates on detached ADUs through 2025 expiry. HCD oversight applies to ordinance amendments per Sec. 65852.2(h).

Napa County overlays of consequence: Napa County Agricultural Preserve and Agricultural Watershed (the 1968 county-level preserve, the first in the US, augmented by 1990 Measure J / 2008 Measure P voter protection); Napa Valley AVA with multiple sub-AVAs (Stags Leap District, Yountville, Oakville, Rutherford, St. Helena, Calistoga, Spring Mountain District, Mt. Veeder, Diamond Mountain District, Howell Mountain, Atlas Peak, Chiles Valley, Wild Horse Valley, Coombsville, Oak Knoll District, Los Carneros - Napa portion); CAL FIRE State Responsibility Area Very High FHSZ across the Mayacamas and Vaca Mountain ridges; Hillside / Conservation Regulations on slopes greater than 30%; FEMA SFHA along the Napa River, Conn Creek, Sulphur Creek, Dry Creek; oak woodland and tree-protection ordinances; STR-overlay zones; and parcels affected by the 2017 Atlas Peak / Tubbs / Nuns / Pocket / Adobe Fires and the 2020 LNU Lightning Complex / Glass Fire (significant hillside burn footprints).

• Napa County Agricultural Preserve / Agricultural Watershed (Measure J / Measure P)
• Napa Valley AVA + sub-AVAs (federal viticultural area designations)
• CAL FIRE State Responsibility Area / Mayacamas and Vaca Mountains FHSZ
• Napa County Hillside / Conservation Regulations (slope-based)
• FEMA SFHA - Napa River, Conn Creek, Sulphur Creek, Dry Creek
• Napa County STR-zone overlay (restrictive)

The Napa County Planning, Building, and Environmental Services (PBES) Department issues ADU permits in unincorporated areas (the bulk of Napa Valley wine country and the surrounding watershed). Practical permitting frictions: Agricultural Preserve / Agricultural Watershed (AP / AW) base-zoning subordination - the parcel must remain in active agricultural use and the ADU must be subordinate to the agricultural operation; Measure J / Measure P preserve-protection rules; Hillside / Conservation Regulations restricting new construction on slopes greater than 30%; very high fire severity zones across the surrounding Mayacamas and Vaca Mountain ridges; oak woodland and tree-protection ordinances on parcels with significant heritage-tree canopy; Napa Valley AVA and sub-AVA scenic-view considerations; multiple FEMA SFHA areas along the Napa River and tributaries; STR-overlay zoning that significantly limits short-term-rental use of ADUs in the wine-country areas; and well-water / septic considerations on rural parcels.