Building permits, zoning and land rules in Napa County, California

Yes. A building permit is required when a structure in unincorporated Napa County is erected, constructed, enlarged, altered, repaired, moved, improved, removed, converted or demolished¹. A utility-free shed no larger than 120 square feet and a fence no higher than 7 feet are listed exceptions, but zoning, floodplain or other approvals may still affect the project¹. Applications are submitted digitally through the Online Permit Center; plan-review fees are collected before multi-division review, with permit and inspection fees invoiced after approval¹¹. Permitted construction must receive inspections before work proceeds beyond each required inspection point¹². Inside Napa, the city handles building permits; the county construction rules here apply to unincorporated land^1,13. Napa County Environmental Health still regulates septic systems on city parcels without public sewer⁴.

The Building & Development Division within Napa County Planning, Building and Environmental Services reviews building applications and plans, inspects approved construction and issues certificates of occupancy⁵. The Planning Division reviews development proposals against the General Plan, County Code and other applicable law, and maintains the county's zoning rules⁵. Environmental Health handles on-site sewage systems, wells and groundwater permits⁵. The PBES Engineering Division handles private-development permits involving grading, drainage, roads, stormwater and floodplain requirements⁵. These PBES divisions share the county office at 1195 Third Street, 2nd Floor, Napa, CA 94559 and the main phone number 707-253-4417⁵.

Napa County enforces zoning on unincorporated land and publishes a zoning map^2,14. Napa and St. Helena have their own zoning; Napa County Environmental Health retains septic oversight on city parcels without public sewer^3,4,15. District tables specify yard setbacks, with separate road and stream rules¹⁶. Minimum parcel sizes include 40 acres in AP, 160 acres in AW with a conditional 40-acre lot-line-adjustment floor, and 10 acres in RC; the RC source distinguishes parcelization standards from minimum developable lot size^17,18,19. Special standards identify a 3,500-square-foot site minimum for qualifying single-family construction using the :AH Affordable Housing Combination District and 1,200-square-foot parcels for individual farmworker homes within qualifying AP or AW farmworker-center parcels²⁰. Entry structures at primary vehicular entrances may not exceed 16.5 feet and need the applicable encroachment or fence permit when occupying a road right-of-way or setback²¹. Ask Planning for parcel-specific standards; we could not confirm a general minimum dwelling size, general building-height schedule or countywide lot-coverage limit⁵.

Napa County Environmental Health regulates wastewater treatment and disposal systems in unincorporated areas and on parcels inside cities that are not served by public sewer⁴. Every new onsite wastewater treatment system requires plan review, an installation permit and construction inspections; new alternative treatment and private sewage-disposal systems also require annual operating permits and routine inspections²². Environmental Health also permits and inspects alternatives such as treatment systems and holding tanks, so off-grid sanitation is not permit-free⁴. Within NapaSan's service area, construction, extension or connection to public sewer requires a written NapaSan permit, paid charges and any required bonds; property outside the service area also needs annexation²³. Only 4.7% of county soils are rated without severe limitations for a conventional drain field, making early site evaluation useful¹⁰. Napa County requires a permit before well construction, modification or destruction, and a licensed well contractor must perform the work⁶.

Since October 1, 2025, living in an RV, trailer or similar vehicle on unincorporated Napa County property outside mobile home park zoning is prohibited; an active building permit may support eligibility for a Watchmen’s Trailer Permit during construction⁸. The same restriction applies to a wheeled tiny home if Napa County classifies the unit as a trailer, RV or similar vehicle⁸. We could not confirm a general private-land camping rule or a separate occupancy path for a permanently founded tiny home; ask Planning and Building to classify the proposed use⁵. Manufactured homes outside mobile home parks require permanent foundations in AW, AP, RS or PD districts, a building permit, conversion to real property and Environmental Health clearances for onsite septic and wells; private restrictions may also prohibit installation⁹. Seasonal farm-labor use of mobile or manufactured homes is limited to 120 days per year and must meet zoning and building-code requirements⁹.

An ADU in unincorporated Napa County must accompany a proposed or existing single-family or multifamily primary residence, provide independent living facilities and be no larger than 1,200 square feet²⁴. The county’s AB 2533 pathway can legalize an unpermitted ADU or JADU built before January 1, 2020 on a property with an existing permitted single-family residence, provided the unit has no serious health or safety hazards²⁴. Qualifying impact fees are waived, while standard permitting, plan-check, inspection and code-investigation fees apply²⁴. A qualifying JADU must be no larger than 500 square feet within the existing home, include an efficiency kitchen and have a recorded owner-occupancy covenant covering the home or JADU²⁴.

1. Confirm that the parcel is unincorporated, then use the county map and ask Planning for its zoning district, allowed residential use, parcel-size rules and setbacks^5,14,16.

1. Have Environmental Health evaluate septic feasibility and the county well-permit path before fixing the house site^6,22.

1. Ask Engineering whether floodplain, grading or drainage approvals affect the site; private development within the 100-year flood zone is regulated through Engineering’s Floodplain Management Permit program^5,25.

1. Obtain a Public Works/Roads Division encroachment permit before driveway work in a county road right-of-way; the listed driveway fee is $591.25, an Underground Service Alert tag is required, and the Roads Inspector needs 72 hours’ advance notice²⁶.

1. Submit the required documents through the Online Permit Center and pay invoiced fees after plan approval¹¹. An owner-builder must return the signed disclosure and provide acceptable identity verification before permit issuance⁷.

1. Schedule required inspections and obtain final approvals before occupancy^5,12.

Check septic feasibility first: only 4.7% of county soils are rated without severe limitations for a conventional drain field, and only 25.3% of the county is at a slope of 15% or less¹⁰. Check county mapping because all private development in the 100-year flood zone is regulated, and floodway development must be designed and built for zero impact to floodwaters^25,27. In the Napa River area from Highway 29 at Butler Bridge south to the county line, revised flood maps require higher elevation for new construction and substantial improvements in affected locations; ask Engineering for the site’s elevation requirements^5,28. Work in a state highway right-of-way requires Caltrans approval²⁹. Proposed subdivisions can be required to connect to feasible public water within 1,000 feet or public sewer within 1,500 feet³⁰. Vineyard grading, vegetation removal or land disturbance on slopes over 5% requires an approved Erosion Control Plan and environmental review³¹. Unincorporated Napa County also generally prohibits dwelling rentals for stays under 30 days³².