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Buying Land and Farmland in Contra Costa County

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

Most farmland and rural acreage for sale in Contra Costa County sits in the east county, around Brentwood, Knightsen, Byron and the Delta, outside the county’s voter-approved Urban Limit Line. Out there, county zoning and general plan designations set how small a parcel can be and how many homes it can carry: one home per 40 acres in the Agricultural Core near Brentwood, and five-acre minimum lots in the A-2 district. Water, levees, habitat fees and any Williamson Act contract shape value as much as soil and location.

This guide is for buyers of farm parcels, ranchettes, orchards and estate lots in unincorporated Contra Costa. It explains the Urban Limit Line after the June 2026 vote, the agricultural designations and zoning districts, the Delta rules, and the water, septic and fee questions to settle before you write an offer. Rules are described as of October 2026. For the county as a whole, start with our Contra Costa County real estate guide.

The Urban Limit Line and the 65/35 standard

Contra Costa’s Urban Limit Line (ULL) is the boundary beyond which the county will not designate urban land uses. Its 2045 General Plan pairs it with the 65/35 Land Preservation Standard, which limits urban designations to no more than 35% of the county’s land and keeps at least 65% non-urban.

The line has a long voter history. According to Save Mount Diablo, a land conservation group that campaigned for it, voters first approved the line in 1990, the 2004 Measure J transportation tax required the county and its cities to adopt voter-approved lines, and Measure L in 2006 extended the county line by 20 years. That extension was due to run out in 2026.

Measure A (June 2026)

On June 2, 2026, county voters considered Measure A, which proposed renewing the line with a new map. DanvilleSanRamon.com’s preview of the measure described it as extending the 65/35 policy for 25 more years, and Save Mount Diablo says it renews the line through 2051. According to the same preview, supporters said nearly 9,400 acres would move outside the line, largely land with government or environmental restrictions, buffer land and land with fire, flood or terrain hazards. Save Mount Diablo, which campaigned for the measure, reported that it passed with about 70% of the vote. Check the certified results from the county elections office and the adopted map before you rely on a parcel’s status.

What the line means for a buyer

Urban-style development, such as large subdivisions or industrial areas, isn’t allowed outside the line.
Save Mount Diablo describes a limited exception under the 2006 measure: the Board of Supervisors could move the line by up to 30 acres with a four-fifths vote and specified findings, with larger changes going to the voters. Ask the county whether Measure A kept that rule.
Each city also has its own line, so a parcel next to Brentwood or Oakley may be governed by that city’s boundary and annexation plans, not only the county’s.

If you’re paying a premium because you expect future development potential, get a written opinion from a land use attorney or planner first.

Agricultural designations in the 2045 General Plan

The general plan’s land use map sets the density ceiling, and zoning has to be consistent with it. Two agricultural designations cover most rural parcels in the east county:

Designation	Where it applies	Density	Notes
Agricultural Core (AC)	About 11,900 acres between Brentwood, Discovery Bay and Byron, mostly prime farmland	1 home per 40 acres	Agricultural production takes precedence; the plan calls ranchette or estate-style development inconsistent with the designation
Agricultural Lands (AL)	Land with lower-rated soils (Class 3 or lower)	1 home per 10 acres; 1 per 20 acres in the Delta Primary Zone	The density is lower inside the Primary Zone

The Agricultural Core matters most for buyers looking at farmland around Brentwood. A 20-acre orchard there can’t be split into two home sites, and the plan’s stated policy is to keep it in production rather than rural estates. Read the designation on the county’s map for each parcel; a property listed as near Brentwood may be inside the city, in the Agricultural Core, or in another designation entirely.

County agricultural zoning districts

Zoning sets lot sizes and allowed uses. The county code (Title 8, Division 84) has six agricultural districts:

District	Name	Code chapter	Lot size notes
A-2	General Agricultural	84-38	Five acres, 250 feet average width and 200 feet depth; a single lot legally created before November 29, 1973 may qualify at 40,000 square feet
A-3	Heavy Agricultural	84-40	Check the chapter’s lot article
A-4	Agricultural Preserve	84-42	Used for Williamson Act land
A-20	Exclusive Agricultural	84-80	Check the chapter
A-40	Exclusive Agricultural	84-82	Follows the A-20 rules, but no building may be placed on a lot smaller than 40 acres
A-80	Exclusive Agricultural	84-84	Check the chapter

We read the A-2 and A-40 provisions on GoCodebook’s copy of the county code in October 2026; we haven’t stated lot sizes we couldn’t read. The county code also allows accessory dwelling units in these agricultural districts, subject to any Williamson Act contract. It has separate divisions on agricultural land conservation, right to farm and agritourism, which matter if you plan a farm stand, events or a winery.

A lot that is smaller than its district’s minimum may still be a legal lot if it was created lawfully before the current rules. Ask the seller for the parcel’s history and confirm it with the county before you count on building.

Williamson Act land

Many east county parcels have been under Williamson Act contracts, which restrict land to agricultural and compatible uses in exchange for lower property tax assessments. The county’s 2045 General Plan includes a policy to rezone parcels that have left the program out of the A-4 Agricultural Preserve District into an appropriate agricultural district, so the zoning label alone may not tell you whether a contract is still in force.

Ask the seller and the county for the contract and any notice of nonrenewal, and look at the property tax bill. The contract is recorded against the parcel and binds later owners, so a buyer takes on its limits along with its lower assessment. Our Williamson Act guide explains contract terms, nonrenewal and cancellation.

Delta parcels: Bethel Island, Knightsen and Discovery Bay

The Sacramento-San Joaquin Delta covers 738,000 acres across six counties, including a substantial part of eastern Contra Costa, according to the Delta Protection Commission. State law sets aside a Primary Zone, defined in Public Resources Code section 29728 as the Delta land and water area of primary state concern, excluding land inside urban limit lines or spheres of influence as of January 1, 1992.

Consistency rule. The county’s general plan requires its decisions in the Legal Delta to be consistent with the Commission’s Land Use and Resource Management Plan for the Primary Zone and with the Delta Plan.
Appeals. Under Public Resources Code section 29770, an aggrieved person can appeal a local action on land entirely in the Primary Zone to the Commission as inconsistent with that plan; the Commission must hear the appeal within 60 days of filing.
Lower density. On Agricultural Lands in the Primary Zone, the county plan allows one home per 20 acres instead of 10.

Delta parcels also raise levee and flood questions. Check the FEMA flood zone for the exact parcel, compare it with the Natural Hazard Disclosure report, and ask who maintains the levee and what the landowner pays toward it. Our pages on Bethel Island, Knightsen and Discovery Bay cover each community.

Water, wells and septic

Rural parcels live or die on water. Three sources show up in east county sales:

Irrigation district water. East Contra Costa Irrigation District, an independent special district formed in 1926 under the state Irrigation District Law, covers about 40 square miles with more than 8,000 irrigated acres around Brentwood. Ask whether the parcel is inside the district, what delivery it receives and what it pays.
Wells. Get the well log, pump test and water quality results. The county’s Environmental Health Division oversees well construction and destruction, according to a presentation by its director to the county’s Local Agency Formation Commission.
Surface water rights. A riparian or appropriative claim needs documents, not assumptions; our guide to water rights for farm and vineyard land explains what to ask for.

Where sewer isn’t available, homes rely on onsite wastewater (septic) systems, which Environmental Health reviews and inspects. The same presentation says the division also oversees small water systems serving two or more connections, a point to check if several homes share one well. Ask for permits and service records, and budget for an inspection.

Habitat conservation fees in the east county

The East Contra Costa County Habitat Conservation Plan and Natural Community Conservation Plan, administered by the East Contra Costa Habitat Conservancy, streamlines endangered species permits for development in its permit area in exchange for fees. Brentwood’s implementing ordinance shows how it works there:

Brentwood approved the plan on January 23, 2007, with state and federal permits issued that year.
A development fee is charged for each acre permanently disturbed, at a rate set by fee zone (I, II or III), and is due by the grading or first construction permit.
Projects that fill wetlands or remove riparian habitat pay a separate wetland mitigation fee, and fees adjust for inflation.
Projects disturbing less than one acre are among the listed exemptions.

The Conservancy notes that its mitigation fees were adjusted on March 10, 2022. If you plan to build, ask the Conservancy and the county or city planning department whether the parcel is in the permit area, which fee zone applies and the current per-acre amount.

Buying or selling rural land in Contra Costa

Confirm the jurisdiction. Brentwood and Oakley addresses often cover unincorporated parcels; see our guide to unincorporated Contra Costa County.
Pull the designation and zoning for each parcel, and check its position relative to the ULL and the Delta Primary Zone.
Verify legal lot status and whether a home site is allowed.
Collect water, well and septic records and any irrigation district bills.
Check contracts and fees: Williamson Act status, habitat fees and any levee or district assessments on the tax bill.
Plan the transfer tax. Outside Richmond and El Cerrito, the county’s $1.10 per $1,000 is the only transfer tax; who pays is negotiable. Our California transfer tax guide covers exemptions.

Sellers can save weeks by assembling those records before listing. Our Contra Costa property responsibility ledger keeps them in one place, and the Brentwood and Byron pages cover those areas.

Diamond may refer you to partners we work with, such as well and septic inspectors, surveyors and land use consultants; you’re free to choose any provider.

This is general information, not legal advice. Zoning, Williamson Act and Delta rules turn on the specific parcel, so confirm them with Contra Costa County planning staff and a California land use attorney before you rely on a use or a lot split.

Questions people ask

What is the Contra Costa County Urban Limit Line?

It is the voter-approved boundary beyond which the county won’t designate urban land uses. It works with the 65/35 standard, which limits urban designations to 35% of the county’s land.

Did Contra Costa voters renew the Urban Limit Line in 2026?

Measure A, on the June 2, 2026 ballot, proposed renewing the line with a new map. Save Mount Diablo, which campaigned for it, reported it passed with about 70% of the vote, and Local News Matters’ June 2, 2026 election-night report also described strong support; check the county’s certified results and the adopted map.

What is the Agricultural Core near Brentwood?

A general plan designation covering about 11,900 acres of mostly prime farmland between Brentwood, Discovery Bay and Byron. It allows one home per 40 acres and treats ranchette or estate development as inconsistent.

What is the minimum lot size in the A-2 zone?

Five acres, with a 250-foot average width and 200-foot depth. A single lot legally created before November 29, 1973 can qualify at 40,000 square feet.

Can I build a home on a 20-acre parcel in the A-40 district?

Not under the district’s standard rule, which bars placing a building on a lot smaller than 40 acres. A lawfully created older lot may be treated differently, so check with the county.

Can I add an ADU on agricultural land in Contra Costa?

The county code allows accessory dwelling units in its agricultural districts, subject to any Williamson Act contract. Water, septic and the parcel’s permits still have to support it.

Is there a transfer tax on rural land in Contra Costa?

Yes, the county tax of $1.10 per $1,000 of value. Richmond and El Cerrito add city taxes; Brentwood, Oakley and unincorporated areas do not.

Does the Delta Protection Commission approve land sales?

Not directly. Its role is the consistency of local land use actions in the Primary Zone, including appeals of them. A later permit or lot split on Primary Zone land is the kind of action it can review.

If you’re looking at farmland, a ranchette or a Delta parcel in the east county, we can pull the parcel records and work through the zoning, water and Delta questions with you before you commit. See buying in Contra Costa County or contact Diamond.

Sources & notes

The county code was read on GoCodebook because the county’s official code host was not reachable from our research tools; confirm sections against the adopted code. Transfer tax figures match our Contra Costa County guide, which cites the title companies’ rate sheets.

Last reviewed October 2026 by Matthew Martinez.