Contra Costa County Property Tax: Exemptions & Appeals (2026)

The 2026 regular assessment appeal window in Contra Costa County is open from July 2 through November 30, 2026, so the deadline for most homeowners appealing the 2026-2027 regular roll is November 30, 2026. File the county’s Assessment Appeal Application with the Contra Costa County Assessment Appeals Board/Clerk of the Board, 1025 Escobar Street, 1st Floor, Martinez, CA 94553, and include the $40 per-application fee.

How assessments work in Contra Costa County

Contra Costa County’s 2026-2027 assessment cycle is based on the January 1, 2026 lien date. The Assessor delivered the 2026-2027 roll to the Auditor-Controller on June 30, 2026; the Assessor reported a net countywide assessment roll of nearly $300.80 billion, up 3.48% from the prior year. That roll is what feeds the tax bills that come later in the year.

The Assessor’s job is to determine taxable assessed value, maintain parcel and ownership records, apply exemptions and exclusions, and deliver the roll. The Auditor-Controller then calculates tax by multiplying assessed value by the applicable tax rate and adding special assessments; the Treasurer-Tax Collector sends and collects the bills; and the Clerk of the Board manages the independent Assessment Appeals Board.

For a typical owner-occupied home, the key California rules are:

• Prop. 13 base year value: a purchase or other reassessable change in ownership generally creates a new base-year value, usually close to market value at that time.
• Annual inflation factor: the factored base value generally can rise, but not more than 2% per year unless there is a reassessable event.
• New construction: additions or major improvements can trigger a supplemental assessment for the added value, not usually a full reassessment of the entire property.
• Prop. 8 decline-in-value relief: if your January 1, 2026 market value is lower than the 2026 assessed value, the Assessor can temporarily reduce the value for that tax year. Contra Costa’s 2026-2027 Residential/Multi-Family Request for Value Review specifically asks for your opinion of value as of January 1, 2026 and comparable sales before March 31, 2026.

The county’s own 2026 Prop. 8 review form is an informal route through the Assessor. It is free, due November 30, 2026, and goes to the Assessor’s Office, not the appeals board. A formal appeal is different: it preserves your right to a hearing before the Assessment Appeals Board and requires the $40 filing fee.

Whether you should appeal

Appeal if you can prove value, not just because the bill feels high. The Assessment Appeals Board can lower or raise assessed value, but it cannot change tax rates, remove late-payment penalties, grant exemptions, or reduce taxes because of hardship.

The strongest Contra Costa homeowner appeal in 2026 is usually one of these:

1. Decline in value: homes bought near a market peak where January 1, 2026 comparable sales support a lower value than the enrolled value.
2. Supplemental assessment after purchase: the Assessor set a new base value higher than what the market evidence supports.
3. New construction issue: the added value from a remodel, ADU, pool, or other improvement is overstated, or you believe the reassessment treated too much of the property as new.
4. Factual error: wrong square footage, condition, unit count, classification, or ownership/exclusion treatment.

Use comparable sales that closed close to January 1, 2026. Contra Costa’s Prop. 8 form says residential comps must be before March 31, 2026. BOE guidance also warns that sales more than 90 days after the valuation date are commonly rejected in assessment appeals.

No official Contra Costa County or California BOE source I found publishes a homeowner-only appeal success rate, median reduction, or average tax savings for Contra Costa assessment appeals. Be skeptical of mailers or firms quoting county-specific win rates without a public source.

A realistic savings example: suppose your 2026 assessed value is $925,000, but three nearby January 2026-era sales support $850,000. The disputed reduction is $75,000. Contra Costa’s latest official Detail of Tax Rates book includes actual 2025-2026 tax-rate areas at 1.1023%; using that real county rate as an example, $75,000 × 1.1023% = about $827 in annual ad valorem tax savings. Subtract the $40 appeal fee and the first-year net is about $787, before considering any refund interest. Flat parcel taxes and fixed special charges usually do not drop just because assessed value drops.

Also check exemptions. Contra Costa’s Assessor handles the Homeowners’ Exemption, Disabled Veterans’ Exemption, Prop. 19 transfers, and other exclusions. The statewide Homeowners’ Exemption reduces taxable value by $7,000 for a qualifying owner-occupied home, usually saving at least about $70 per year. Seniors should also check the county’s senior parcel-tax exemption page because some school or district parcel taxes have separate age-65 exemption rules and deadlines.

Step-by-step how to file

1. Start with the free Assessor review. For a 2026 decline-in-value issue, use the 2026-2027 Request for Value Review (Prop. 8) for residential/multi-family property. Return it to the Assessor’s Office by November 30, 2026, with your APN, property address, opinion of value as of January 1, 2026, and three comparable sales. Assessor contact: 2530 Arnold Drive, Suite 100, Martinez, CA 94553; phone (925) 313-7400.

2. Do not rely on the informal review to preserve your formal appeal rights. If November is approaching and the Assessor has not resolved the value in writing, file the formal appeal too.

3. Use the correct formal form. The county form is BOE-305-AH, Assessment Appeal Application. Contra Costa’s form page also describes it as the Assessment Appeal Application. A separate application is required for each parcel and each tax year.

4. Fill out the value section carefully. Put the roll value in the “Value on Roll” column and your requested value in the “Applicant’s Opinion of Value” column. For a regular 2026 decline-in-value appeal, your value date is January 1, 2026. Do not attach your full hearing evidence packet to the application unless the form specifically asks for it; the county form warns applicants not to attach hearing evidence to the application.

5. Pay the fee. Contra Costa charges a non-refundable $40 application fee for each application. The county says payment may be by cash, check, or money order payable to Contra Costa County Treasurer. If mailing, a check is the practical method.

6. File by the right method.

• Mail: Assessment Appeals Board, 1025 Escobar Street, 1st Floor, Martinez, CA 94553. Include the signed application and $40 payment.
• In person: Assessment Appeals Clerk, 1025 Escobar Street, 1st Floor, Martinez, CA 94553. County hours are Monday-Friday, 8 a.m.-5 p.m. Call (925) 655-2008 if you are filing close to the deadline.
• Email: Contra Costa’s FAQ mentions email for withdrawals/postponement communications, but the official filing instructions I found do not publish email filing for new Assessment Appeal Applications.
• Portal: I found no official Contra Costa online portal for initial residential assessment appeal filing. Use mail or in-person filing unless the Clerk confirms a newer method directly.

For regular roll appeals, file between July 2 and November 30, 2026. For supplemental or escape assessments, Contra Costa says the appeal must be filed within 60 days of the Assessor’s notice.

What happens after

The appeal authority is the Contra Costa County Assessment Appeals Board. It is separate from the Assessor’s Office, staffed through the Clerk of the Board, and its members are appointed by the Board of Supervisors.

After your application is accepted, the Assessor’s Office may contact you to discuss value. If you reach a written stipulation with the Assessor, Contra Costa’s local rules allow the Board at a hearing to accept the stipulation, waive your appearance, and change the value; request more information; or reject the stipulation and set the case for hearing. That is the closest thing to a non-oral option published in the local rules—there is not a guaranteed paper-only appeal simply because you ask for one.

If the case goes to hearing, Contra Costa schedules assessment appeal hearings on Thursdays at 9:00 a.m. The county FAQ says you should receive written notice about 45 days before the hearing. You must return the notice indicating your intent to appear; the FAQ says failure to return it can cancel the hearing. Local rules also require you, within 15 days after the Notice of Hearing, to tell the Clerk whether you will appear, withdraw, or request a different hearing date.

At the hearing, expect an informal but evidence-based proceeding. You and the Assessor present facts, comparable sales, photos, appraisal analysis, and testimony. The Board makes an independent decision based on the evidence. If no written findings are requested, the Board may announce a decision at the end of the hearing or take the matter under submission; Contra Costa’s local rules say that if a matter is taken under submission, written notice of decision is due no later than 120 days after the hearing concludes. Written findings cost extra: $150 per hour, with a $150 deposit for single-family residential properties.

Keep paying your property tax bills while the appeal is pending. Contra Costa’s tax-bill guidance and FAQ both warn that taxes remain due even if you are in informal review or formal appeal. If you win, the county issues the adjustment/refund through the tax system.

Local tips

• File the formal appeal before Thanksgiving, not on November 30. Contra Costa has a hard November 30 regular deadline, and 2026’s deadline is a Monday. Mailing delays are your problem; if it is close, file in person.
• Use Contra Costa’s parcel tools and your bill. Your APN, roll value, exemptions, Tax Rate Area, and fixed assessments are on the bill. The tax-rate area matters because rates vary across Antioch, Concord, Walnut Creek, Martinez, San Ramon, unincorporated areas, and school districts.
• Do not compare only to Zillow. Bring closed comparable sales near January 1, 2026, with similar size, location, condition, lot, view, ADU/pool status, and school district. Adjust obvious differences.
• Watch the 90-day comp window. Contra Costa’s Prop. 8 form asks for comps before March 31, 2026. Sales after that may be weak or inadmissible for a January 1 value dispute.
• Separate value appeals from exemptions. The Assessment Appeals Board will not grant the Homeowners’ Exemption, disabled-veterans exemption, senior parcel-tax exemptions, or Prop. 19 transfer. Those go through the Assessor or the relevant district program.
• If you get a supplemental notice, count 60 days from the notice. Do not wait for the annual July-November window if the issue is a supplemental or escape assessment notice.
• Ask for written findings only if you may litigate. They are useful for further court review but add cost. Most first-time homeowner value disputes do not need them unless there is a serious legal issue or large dollars at stake.