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Napa Agricultural Preserve (AP) and Watershed (AW) Zoning Explained

By Matthew Martinez, Founder, CEO & Managing Broker, Diamond Real Estate Group. California DRE #01869679.

Most farm and hillside land in unincorporated Napa County is zoned either Agricultural Preserve (AP), for the valley floor and foothills, or Agricultural Watershed (AW), for the hills, watersheds and reservoir areas. The county’s zoning schedule sets a 40-acre minimum lot area in AP and 160 acres in AW. Both allow one single-family home per legal lot by right, while new wineries need a use permit. Since 1990, voter-approved Measure J, extended by Measure P in 2008, has made the agricultural land designations behind these zones hard to change without a countywide vote.

This page explains what each zone allows, the development standards in Title 18 of the Napa County Code, the winery rules that sit on top of the zoning, and how to check a specific parcel. It is written for buyers of rural parcels, estates and vineyards, and for sellers who need to explain what their land can and can’t be used for. For the broader buying process, start with our vineyard and winery real estate hub.

How the Agricultural Preserve began

Napa County’s Board of Supervisors created the Agricultural Preserve on April 9, 1968, on a 4-0 vote with one abstention, according to the Napa Valley Register’s history. The Register puts the original area at about 26,000 acres from Calistoga to Oak Knoll. The Napa Valley Vintners describe it as the nation’s first agricultural preserve and cite 23,000 acres at the start and more than 32,000 acres today.

The minimum parcel size for land divisions was not 40 acres from the start. The Register reports an original 20-acre subdivision limit, an expansion between Napa and Yountville in 1977, and the increase to a 40-acre minimum in 1979. Where older summaries say “40 acres since 1968,” treat that as shorthand.

Two layers: General Plan designation and zoning

Every rural parcel has a General Plan land use designation and a zoning district, and both matter. The General Plan sets density and minimum sizes for new parcels. The zoning code lists permitted uses and building standards.

Item	Agricultural Preserve (AP)	Agricultural Watershed (AW)
Code chapter	18.16	18.20
Intended for	Fertile valley and foothill land where agriculture is and should remain the main use	Agriculturally oriented areas that include watersheds, reservoirs and floodplain tributaries
Usual General Plan designation	Agricultural Resource (AR), Policy AG/LU-21	Agriculture, Watershed and Open Space (AWOS), Policy AG/LU-20
Minimum lot area (zoning schedule, 18.104.010)	40 acres	160 acres
Minimum new parcel (General Plan)	40 acres	160 acres
Yards: front, side, rear	20 feet each	20 feet each
Maximum height	35 feet	35 feet
Homes by right	One single-family dwelling per legal lot, plus ADU and junior ADU	One single-family dwelling per legal lot, plus ADU, junior ADU and one guest cottage
New winery	Use permit	Use permit

On legal parcels of two acres or less in either zone, the side yards follow the single-family residential (RS) standards instead. The zoning schedule also sets a two-acre minimum parcel for farmworker centers in AP and AW.

Many existing lots are far smaller than 40 or 160 acres. The minimums govern new divisions, not whether an existing legal lot can be used. The General Plan’s density rule is one dwelling per parcel, except as the Housing Element provides.

What AP allows

Chapter 18.16 lists the uses allowed without a use permit. They include agriculture, one single-family dwelling per legal lot, and accessory and junior accessory dwelling units that meet section 18.104.180. Small residential care facilities and family day care homes are listed too.

Three kinds of existing wineries may continue without a new permit: wineries that existed before July 31, 1974 and haven’t been abandoned, small wineries operating under an earlier certificate of exemption, and wineries operating under a use permit. Expanding any of them beyond what was established or permitted needs a use permit. Farmworker housing for six or fewer employees, or up to 36 beds in group quarters or 12 units, is also allowed by right under state Health and Safety Code limits.

With a use permit, AP also allows:

New wineries, and winery uses such as crushing, aging, bottling and incidental office, lab, marketing and retail sales.
Uses accessory to a winery: tours and tastings, art or historical and viticultural displays, and sales of wine-related products.
Processing of non-grape crops grown on the same or contiguous parcels, and small-scale processing of county-grown products on parcels of 10 acres or more, up to 5,000 square feet.
Larger farmworker housing and seasonal farmworker centers, kennels, veterinary facilities and feed lots.

Tasting and marketing rights are therefore permit-specific. A winery’s use permit conditions, not the zoning label, tell you how many visitors and events it can host. Our guide to the Napa winery use permit process covers applications and modifications.

What AW allows

Chapter 18.20 follows a similar pattern for the hills. By right, AW allows agriculture, one single-family home per legal lot, accessory and junior accessory dwelling units, and one guest cottage. Existing pre-1974 and permitted wineries may continue, and several legacy uses are listed, such as specified campgrounds and small hunting clubs.

With a use permit, AW adds parks and rural recreation, wineries and winery uses, non-winery agricultural processing, kennels and stables, and small-scale processing on parcels of 10 acres or more. Because AW land often sits in watersheds, projects there tend to meet the county’s hillside, stream and water rules sooner. If you are looking in hill communities such as Angwin on Howell Mountain or Pope Valley, check whether each parcel is AP or AW, because the minimum lot size differs fourfold.

Homes, ADUs and guest cottages on ag land

For an estate buyer, the residential limits matter most. Both zones allow one main home per legal lot, so two homes on one parcel is usually a main home plus an ADU, not two primary residences.

ADUs (section 18.104.180). A detached ADU may be up to 1,200 square feet, measured inside the exterior walls, with 4-foot side and rear setbacks. A detached unit that isn’t otherwise exempt must sit within 500 feet of the main home, measured in a straight horizontal line. Exceptions exist for agricultural, water, sewer or environmental constraints.
Guest cottages (section 18.104.080). Living area may not exceed 1,000 square feet, and the cottage must be within 500 feet of the main or secondary residence unless the planning director waives it. Chapter 18.20 lists a guest cottage as a by-right use in AW. Chapter 18.16 does not list one for AP, so ask Planning before you count on adding one there.

Whether an existing guest house is a permitted ADU, a guest cottage or an unpermitted structure changes what you can do with it. Pull the permit history before you write an offer. Our guide to building a home on Napa agricultural land covers siting, water and septic for a new house.

Short-term rentals

Don’t buy rural Napa land expecting vacation-rental income. A December 2019 Board of Supervisors report on enforcement says stays of less than 30 days are generally not allowed in the unincorporated county under County Code section 18.104.410, which bars advertising, negotiating or using a dwelling that way. The report lists two narrow exceptions: bed and breakfasts that already hold a use permit (the county no longer issues new ones) and some designated Silverado Country Club rentals under an approved development agreement.

Rentals of 30 days or more remain allowed, as the 2018-2019 Grand Jury report also notes. The cities set their own rules. See our post on Napa County vacation rental rules.

Winery standards that apply in AP and AW

The Winery Definition Ordinance of 1990 added standards that apply on top of the zoning. Chapter 18.104 of the code now holds several of them:

Rule	Code section	What it says
Minimum parcel	18.104.240	10 acres for a winery established after the ordinance took effect; 1 acre for a winery established before
Grape sourcing	18.104.250	At least 75% of the grapes for a new winery’s still wine must come from Napa County; the same applies to expansions beyond a winery’s development area
Small winery production	18.104.250	Small wineries established after July 31, 1974 without a use permit are capped at the lesser of their certificate limit or 20,000 gallons a year
Road setbacks	18.104.230	600 feet from the centerline of a state highway, Silverado Trail or an arterial county road; 300 feet from other qualifying roads
Historic building exception	18.104.235	Qualifying wineries may sit as close as 300 feet to those major roads if required findings are made
Accessory space	18.104.200	Winery accessory structures may not exceed 40% of the area of the production facility
Pre-Prohibition wineries	18.104.245	Two-acre minimum parcel and a 20,000-gallon annual cap for qualifying historic wineries

A buyer planning a new winery should test the parcel against these rules before anything else: acreage, frontage and setbacks, fruit sourcing and water. Our guide to the Winery Definition Ordinance explains permit vintages and visitation, and our Napa groundwater rules guide covers the Water Availability Analysis that most winery permits require.

Measures J and P: why the map rarely changes

Napa County voters approved Measure J on November 6, 1990, with about 63% of the vote, according to the Napa Valley Register. It requires voter approval to redesignate land the General Plan designates for agriculture. Before it, the Board of Supervisors could change the preserve on its own. The county’s General Plan element is internally inconsistent here: its timeline lists Measure J as enacted in 1991, while its policy text says 1990.

Measure P, passed in 2008 with about 62% of the vote, extended those protections from a 2020 sunset to 2058. The General Plan’s Agricultural Preservation and Land Use Element now says policies derived from Measures J and P may not be amended or deleted without voter approval until after December 31, 2058.

The practical effect is scarcity. Converting AR or AWOS land to residential or commercial designations needs a countywide vote, so new rural homesites mostly come from existing legal lots. That is one reason a legal, buildable lot can price very differently from a larger parcel without a usable site. Our page on subdividing land in Sonoma and Napa explains what splitting can and can’t achieve.

Other overlays that shape a parcel

Viewshed Protection (Chapter 18.106). The county’s 2016 program packet says review is needed for structures on slopes of 15% or more, on major or minor ridgelines, and for related grading. The trigger is visibility from designated roads, including Silverado Trail and Highway 29. Administrative approval requires, among other things, a main structure of 4,000 square feet or less, a height of 24 feet or less and less than 1.5 acres of grading.
Conservation Regulations (Chapter 18.108). Hillside, erosion and stream setback rules for earthmoving. Our guide to Napa County land for sale covers slope thresholds and stream setbacks.
Williamson Act contracts. The county’s Type C agricultural preserve rules apply to land zoned AP that is mainly prime agricultural land. A parcel under 10 acres can qualify only if it is over five acres, at least 75% in commercial agricultural use, and meets other findings. See our Williamson Act guide for the tax effects.

How to check a specific parcel

Look up the APN. Napa County’s GIS Map Viewer searches by parcel number or address and produces a parcel report with boundary and jurisdiction data. Confirm zoning and the General Plan designation in its layers or with Planning (707-253-4417).
Confirm legal lot status. Ask whether the county has recognized the parcel as a legal lot. A lot created by an old deed may need a certificate of compliance.
Pull permits. Request use permits, building permits and any certificate of exemption for wineries, homes, ADUs and guest structures.
Check overlays. Viewshed visibility, slopes, streams, fire hazard zone and any Williamson Act contract.
Test water and septic early, since they often limit a second unit or a winery more than zoning does.

A note on sources: the code sections cited on this page were read on GoCodebook’s reproduction of the Napa County Code (text dated October 4, 2026), because the county’s official online code host could not be reached when we reviewed it. Confirm the current wording with Planning before you rely on a specific section.

Our Napa estate and vineyard evidence worksheet helps organize these records by question. Diamond may refer you to partners we work with, such as land-use attorneys, civil engineers and surveyors; you’re free to choose any provider.

This is general information, not legal advice; talk to a California land-use attorney or Napa County Planning about your parcel.

Questions people ask

What is the minimum parcel size in Napa’s Agricultural Preserve?

The county’s zoning schedule sets a 40-acre minimum lot area in the AP district, matching the 40-acre minimum for new parcels in the General Plan’s Agricultural Resource designation. Existing smaller legal lots can still be used.

What is the minimum parcel size in AW zoning?

160 acres for the Agricultural Watershed district, matching the General Plan’s AWOS designation.

Can I build a house on AP or AW land?

Both zones allow one single-family dwelling per legal lot without a use permit, plus an accessory dwelling unit and junior ADU that meet section 18.104.180. Site, water, septic, fire and hillside rules still apply.

Can I add a guest cottage in Napa County?

Chapter 18.20 lists one guest cottage as a by-right use in AW, limited to 1,000 square feet of living area and generally within 500 feet of a residence. AP’s use list doesn’t include one, so confirm with Planning.

How many acres do I need for a new winery in Napa County?

Section 18.104.240 requires a 10-acre minimum parcel for wineries established after the Winery Definition Ordinance took effect, and new wineries need a use permit in both AP and AW.

What did Measure J and Measure P do?

Measure J (1990) requires a countywide vote to redesignate agricultural land in the General Plan. Measure P (2008) extended that protection, and the General Plan says the related policies can’t be amended without voter approval until after December 31, 2058.

Can I use a Napa County farm property as a vacation rental?

Generally not for stays under 30 days. County Code section 18.104.410 bars short-term rentals in the unincorporated county, with narrow exceptions the county lists for existing permitted bed and breakfasts and some Silverado Country Club units. Rentals of 30 days or more are allowed.

How do I find a parcel’s zoning in Napa County?

Search the APN or address in the county’s GIS Map Viewer and confirm with Napa County Planning, Building & Environmental Services. Ask about legal lot status at the same time.

Zoning is usually the first question on any rural Napa purchase or sale, and the answer is parcel-specific. If you are weighing a property, or preparing land for sale, contact Diamond Real Estate Group or read more about Napa County on our Napa County guide.

Sources & notes

Last reviewed October 2026 by Matthew Martinez.