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The Williamson Act for Vineyard Buyers and Owners in Napa and Sonoma

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

The Williamson Act, formally the California Land Conservation Act of 1965, lets a county contract with a landowner to keep land in farming or open space. In return, the county assesses the land on what it earns as farmland rather than on its market value. Contracts renew automatically every year, bind every later owner, and end only through a nine-year nonrenewal or a cancellation that costs 12.5% of the land’s unrestricted value.

For vineyard land in Napa and Sonoma counties, a contract affects the property tax bill, what you can build, whether you can split or adjust lot lines, and what a buyer can do with the parcel later. The contract is recorded against the parcel, so a buyer takes it on at closing. This guide covers the state rules, how each county runs its program, and what to check before you buy, sell or file to leave a contract.

How a Williamson Act contract works

The contract is voluntary. The landowner agrees to keep the parcel in agricultural or related open-space use, plus whatever the county treats as compatible uses, and the county assesses the land on the income it produces rather than its potential market value. The Department of Conservation says enrollment statewide has hovered at about 16 million acres since the early 1980s.

Term: the minimum term is ten years, and the contract renews on each anniversary date, so in practice it has no end date until someone starts nonrenewal or cancellation.
Agricultural preserve: contracted land must sit inside a preserve, which should generally be at least 100 acres. Contiguous or commonly owned parcels can be combined to reach that size, and a county can approve smaller preserves if it makes certain findings.
Local rules: each participating county sets its own enrollment rules, acreage minimums, compatible uses and enforcement. That is why a Napa contract and a Sonoma contract can read quite differently.
Successors: the Department of Conservation states that contracts bind all later purchasers and stay on the property until a nonrenewal or cancellation is filed.

Farmland Security Zone contracts

A Farmland Security Zone (FSZ) contract is a 20-year rolling version for higher-quality farmland. FSZ land is valued at 65% of its Williamson Act value or 65% of its Proposition 13 value, whichever is lower, so the tax cut is larger. The trade-offs are a longer exit, a cancellation fee of 25% of the land’s value instead of 12.5%, and possibly stricter use limits. Sonoma County offers FSZ contracts under its Uniform Rules. Napa County’s program page lists Type A, C, H and E contracts and doesn’t mention FSZs, so ask the county’s Conservation Division before assuming one is available there.

Napa and Sonoma program rules compared

Both counties apply the state’s 10-acre and 40-acre thresholds, but they use different contract names, different application steps and different compatible-use rules. The table summarizes what each county publishes. Confirm the details for a specific parcel, because the recorded contract and the county rules in force are what control it.

Item	Napa County	Sonoma County
Who runs it	Conservation Division, 1195 Third Street, Napa	Permit Sonoma, 2550 Ventura Avenue, Santa Rosa
Zoning required	Agricultural Preserve (AP) or Agricultural Watershed (AW)	Zoning compatible with agricultural or open-space use
Minimum parcel	10 acres of prime land; 40 acres of nonprime land	10 acres (Type I contracts) or 40 acres (Type II); the first digit of the preserve number shows the type
Smaller parcels	Type C contracts for 5 to 10 acres if at least 75% is in commercial agriculture and other conditions, including crop diversity beyond wine grapes, are met	No small-parcel option is described on the program pages
Farming test	A bona fide agricultural use in at least 3 of the preceding 5 years, plus proof of agricultural income	An income statement (form PJR-065) decides which preserve type, if any, the land qualifies for; minimum income in 3 of the last 5 years
Contract types	Types A and C (AP zoning); Types H and E (AW zoning)	10-year Land Conservation Contracts; 20-year FSZ contracts
When to apply	September through the first Friday in October; contracts are recorded by December 31	Electronic applications only; establishing a preserve is combined with a new contract application
Compatible uses	Listed in Section IV of the county’s approved contract form	Listed in the Uniform Rules; combined area generally no more than 15% of the contracted land or 5 acres, whichever is less

For the zoning that sits underneath the contract, see our guides to Napa’s AP and AW zoning and Sonoma County agricultural zoning.

Homes, wineries and other uses on contracted land

Farming is allowed on any contracted parcel. Everything else depends on the county’s list of compatible uses and on the recorded contract. State law treats gas, electric, water and communication facilities and agricultural laborer housing as compatible unless the county decides otherwise.

Sonoma County

Permit Sonoma’s Procedure 8-1-8 (effective July 2007) says compatible uses must be incidental to the farming. Their combined footprint, including structures, parking and landscaping, should generally be no more than 15% of the contracted land or 5 acres, whichever is less. Agricultural processing, farmworker housing and ponds don’t count toward that limit.

The same procedure treats a residence as part of the agricultural use only when it’s occupied by the owner, the farm family, agricultural employees or others working the operation. It says second units, income rentals and vacation rentals aren’t permitted on contracted land. Building permits that add a structure, expand a footprint or change a use need a Williamson Act compliance statement and a zoning clearance, and some need a consistency determination. Permit Sonoma won’t act on those applications until it has evidence the project is consistent with the contract. Winery projects also go through the county’s own permit process; see Sonoma County winery use permits.

Napa County

Napa’s rules point to Section IV of the approved contract form for the list of compatible uses, so get a copy of the recorded contract for the parcel you’re considering. The county’s brochure says subdivision is generally prohibited while a contract is in force. The exceptions are divisions for immediate family members (Gov. Code §51230.1) and for agricultural laborer housing (§51230.2). A lot line adjustment requires rescinding the contract and replacing it, with county findings under §51257. Wineries are permitted under the county’s zoning rules; see the Napa winery use permit guide.

Material breach

Under state law, a material breach is a commercial, industrial or residential building of more than 2,500 square feet, built or permitted after January 1, 2004, that is neither agricultural nor compatible. Permit Sonoma’s procedure cites a penalty of 25% of the unrestricted value of the land and buildings involved. It also limits new permits to agricultural-exempt and like-for-like repair work until the breach is cured.

How contracted vineyard land is assessed

For contracted land, the assessor values the land by capitalizing its agricultural income under Revenue and Taxation Code §423. BOE annotations say the assessor then compares that restricted value with the factored Proposition 13 base year value and the current market value and enrolls the lowest of the three. Napa County’s brochure describes the same three-way test. A county may also elect under §423.3 to cap restricted values at 70%, 80% or 90% of the factored base year value.

When contracted land is sold, BOE annotation 620.0005 says the new base year value of the restricted land isn’t enrolled. The parts that aren’t restricted, such as the house, barns and other structures, are enrolled at their new values, and supplemental assessments run from the date of the change in ownership. Expect a supplemental tax bill on the improvements even if the land assessment stays low.

The Department of Conservation cites a 1989 study that estimated savings of 20% to 75% a year for contracted landowners. That figure is decades old and varies widely, so get an estimate for the actual parcel. Napa’s Assessor Division offers one for a fee of $75 to $225. Because a buyer’s land tax depends on income rather than the price paid, contract status belongs in any vineyard valuation.

Leaving a contract: nonrenewal versus cancellation

There are two main ways out of a contract, and they differ sharply in time and cost. A third route, easement exchange, applies in narrower cases.

Route	Who starts it	How long	Cost	What happens to taxes and use
Nonrenewal (Gov. Code §51245)	Landowner or county	The contract runs out over the remaining 9 years (19 for an FSZ)	No fee	All restrictions stay in force until expiration; the assessment rises gradually toward full value
Cancellation (Gov. Code §§51281–51283)	Landowner only	No fixed timeline; it requires an assessor valuation, a public hearing and board findings	12.5% of the land’s fair market value as if unrestricted (25% for an FSZ), plus local fees	Ends the contract once conditions are met and the final certificate is recorded
Easement exchange (since January 1, 1998)	Landowner, with county approval	Case by case	A permanent agricultural conservation easement on other land	The easement land must be at least as large, and the easement worth at least the cancellation fee

Nonrenewal

A landowner must file the notice at least 90 days before the annual renewal date; a county must give at least 60 days’ notice. The clerk records the notice within 20 days of receiving it. Napa’s brochure says that once the notice is recorded, “the annual tax assessment gradually increases over nine years,” and the contract then ends. Under Revenue and Taxation Code §426, the assessor keeps valuing the land on its income during the phase-out, adjusted each year as the expiration date gets closer.

Cancellation

The county assessor values the land as if it had no contract, and the fee is 12.5% of that value, or 25% for an FSZ contract. The Department of Conservation notes that cancellation is almost always part of a larger development project, not a stand-alone exit. The board of supervisors may tentatively approve cancellation only if it finds that cancellation is consistent with the Act or in the public interest (Gov. Code §51282). The consistency findings include that a notice of nonrenewal has been filed and that cancellation won’t likely push neighboring land out of farming. The board then holds a hearing, records a certificate of tentative cancellation, and records a final certificate once the landowner meets the conditions and pays. FSZ cancellations require additional findings (§51297), and the FSZ fee must be paid before tentative approval. Public agencies with eminent domain power have a separate route through public acquisition.

What a buyer inherits: a due-diligence checklist

Treat contract status as a core item in vineyard due diligence, alongside water and zoning. Work through these steps before you remove contingencies.

Confirm the status. Check the Department of Conservation’s Williamson Act Enrollment Finder, then ask the county and the title officer for the recorded contract, the preserve number and any recorded notice of nonrenewal.
Read the contract type. In Sonoma, Type I or Type II (or FSZ) sets the parcel minimum and the rules. In Napa, Type A, C, H or E does the same, and the contract form lists the compatible uses.
Check for a pending nonrenewal. If one was filed, find out what year it was recorded. That date tells you how far into the nine-year tax phase-up the parcel is and when the restrictions end.
Test your plans against the rules. A second home, guest rental, event space or tasting room may not be allowed. In Sonoma, the residence rules and the 15% or 5-acre cap on compatible uses come first.
Confirm you can keep qualifying. Both counties require ongoing commercial agriculture. If you plan to pull vines, change crops or lease the land out, ask how the county applies its income and use tests.
Get a tax estimate. Ask the assessor how the land and the improvements will be enrolled after your purchase, and budget for the supplemental bill.
Plan any lot line or split work early. In Napa, a lot line adjustment means rescinding and replacing the contract, so leave time for county findings.

For the full buying sequence, see how to buy a vineyard. If the parcel also carries a land trust easement, the two restrictions stack; see buying property with a conservation easement.

Selling contracted land: disclosure and pricing

Because the contract binds every later owner, buyers and their lenders will ask about it. Assemble the recorded contract, the preserve number, any notice of nonrenewal, recent agricultural income and use records, and any county compliance determinations for structures built while the land was under contract. A seller who has already filed nonrenewal should give the buyer the recording date, because it fixes the timeline for the restrictions and the tax increase.

Pricing should compare the parcel with sales that were also under contract, or adjust for the difference, since the restrictions and the tax treatment both affect what a buyer will pay. Our guide to selling a vineyard covers the rest of the listing process, and our Sonoma County ranch and acreage guide covers grazing land. The Department of Conservation notes that roughly two-thirds of contracted land statewide is nonprime, and range and grazing land usually doesn’t qualify as prime, so the 40-acre rules often apply there.

Questions people ask

What is the Williamson Act cancellation fee?

The fee is 12.5% of the land’s fair market value as if it had no contract restrictions, as determined by the county assessor, or 25% for a Farmland Security Zone contract. Local processing fees are on top. A waiver or deferral is possible only in narrow cases, such as an involuntary transfer, and needs approval from the Secretary of the Natural Resources Agency; the Department of Conservation says its records show no approved requests.

How long does it take to get out of a Williamson Act contract?

Nonrenewal takes nine years from the next renewal date, or 19 years for a Farmland Security Zone contract, and it costs nothing. Cancellation can be faster, but it requires the cancellation fee, a public hearing and specific findings by the board of supervisors, so approval isn’t guaranteed.

Does a Williamson Act contract transfer when I buy the property?

Yes. The contract binds all later owners and stays in effect until a nonrenewal runs out or a cancellation is completed. The land generally keeps its restricted valuation after a sale, while the improvements are reassessed.

Can I build a house on Williamson Act land?

Often, within limits. Permit Sonoma’s permit procedure treats a residence as part of the agricultural use when it’s occupied by the owner, the farm family or people working the operation, and says second units, income rentals and vacation rentals aren’t permitted on contracted land; confirm the current rules with Permit Sonoma. In Napa, the compatible uses are listed in the contract form. Any building that isn’t allowed and is larger than 2,500 square feet can be a material breach.

What is the minimum acreage for a Williamson Act contract in Napa and Sonoma?

Ten acres of prime land or 40 acres of nonprime land in both counties. Napa also offers Type C contracts for parcels of 5 to 10 acres that meet extra conditions, including at least 75% commercial agricultural use.

Is the Williamson Act worth it for a vineyard owner?

It depends on how long you plan to keep the land in farming and what else you might want to do with it. The tax savings can be large, but the contract limits building, splitting and nonfarm uses, and getting out takes years or a substantial fee. Ask the assessor for a parcel-specific estimate and weigh it against your plans for the land.

What is a Farmland Security Zone?

It’s a 20-year rolling contract for higher-quality farmland. The land is valued at 65% of its Williamson Act or Proposition 13 value, whichever is lower, in exchange for a longer exit and stricter terms. Sonoma County offers FSZ contracts.

This is general information, not legal or tax advice; talk to a California land-use attorney, the county assessor or a tax adviser about your situation.

If you’re weighing a contracted parcel in Napa County or Sonoma County, we can pull the contract, preserve and nonrenewal records with you and set them against your plans for the land before you commit. Start with our vineyards and wineries hub, or contact us to talk through a specific property.

Sources & notes

Last reviewed October 2026 by Matthew Martinez.