How to Appeal Property Taxes in Contra Costa County, California (2026 Guide)

Direct answer: For the 2026-27 regular assessment roll, Contra Costa County homeowners must file a formal assessment appeal between July 2 and November 30, 2026; because November 30, 2026 is a Monday, that is the concrete deadline. File the county’s Assessment Appeal Application / BOE-305-AH with the Contra Costa County Assessment Appeals Board, 1025 Escobar Street, 1st Floor, Martinez, CA 94553, with the $40 per-parcel, per-tax-year fee. (contracosta.ca.gov)

How assessments work in Contra Costa County

Contra Costa’s 2026 appeal window is tied to the regular assessment roll for the fiscal year that began July 1, 2026. The Assessor reported delivering the 2026-2027 Contra Costa County Assessment Roll to the Auditor-Controller on June 30, 2026; the roll totaled about $300.80 billion in net tax base value and 383,191 assessed parcels. (contracosta.ca.gov)

For an owner-occupied home, the starting point is usually Proposition 13: your base-year value is generally set when you buy the property or complete new construction, then it may rise by no more than the annual inflation factor, capped at 2%, unless another reassessable event occurs. Contra Costa’s own “Helpful Terms” page explains that post-1975 acquired or constructed property is assessed at the value when you took title or completed construction, plus up to 2% per year. (contracosta.ca.gov)

A 2026 appeal usually argues one of two things. First, a decline-in-value / Proposition 8 appeal says your January 1, 2026 market value was lower than the taxable assessed value on the roll. Contra Costa says Proposition 8 reductions must reflect market value on the January 1 lien date, and the Assessor analyzes similar sales in your neighborhood, including sales up to 90 days after lien date. (contracosta.ca.gov) Second, a base-year or supplemental appeal challenges a reassessment caused by a purchase, transfer, or new construction; supplemental and escape assessments have a separate 60-day deadline from the Assessor’s notice. (contracosta.ca.gov)

Your tax bill is not just “1%.” Contra Costa’s Auditor-Controller says the tax rate consists of the basic 1% general tax plus voter-approved general obligation bond rates, and the Treasurer-Tax Collector says the bill is computed by multiplying assessed value by the tax rate, then adding special assessments. (contracosta.ca.gov) Direct charges, parcel taxes, sewer charges, and some special assessments may not fall when your assessed value falls.

Whether you should appeal

Appeal if you can prove a lower value as of January 1, 2026, not just that your payment feels high. Strong evidence is usually three to five comparable sales near your property that closed before April 1, 2026, adjusted for size, condition, location, lot, views, and date. A Zillow estimate alone is not enough; use closed MLS sales, an appraisal with a January 1 valuation date, or public-record comparable sales.

Before paying $40, use Contra Costa’s free informal route. The Assessor’s “Review Your Value” page offers a free Request for Value Review for residential/multi-family properties, and the Assessor has warned that fee-based solicitations are not official county documents. (contracosta.ca.gov) The informal review does not extend the formal appeal deadline, so if November 30 is approaching, file the formal appeal to preserve rights.

There are no current county-published residential success rates or median reductions on the official appeal pages I found. The State Board of Equalization’s latest public reporting points users to its open data portal for appeals statistics, but Contra Costa’s local pages do not publish a homeowner “win rate,” so do not rely on a consultant’s advertised average unless they show parcel-level evidence. (boe.ca.gov)

Also check exemptions before appealing. The Contra Costa Assessor lists the Claim for Homeowners’ Property Tax Exemption and Spanish version among common forms; California’s homeowners’ exemption can reduce taxable value by up to $7,000 for a principal residence. (contracosta.ca.gov) Disabled veterans should check the Disabled Veterans’ Exemption; seniors should read the tax bill carefully because Contra Costa notes some parcel-tax senior exemptions appear with an asterisk next to special-tax codes, and those are handled by the levying agency, not by an assessment appeal. (boe.ca.gov)

Step-by-step how to file

1. Find the value you are challenging. Use your 2026-27 assessed value from the notice or secured tax bill. Contra Costa tax bills show ownership, assessed value, exemptions, assessments, and taxes. (contracosta.ca.gov)

2. Download the correct form. Use Contra Costa’s Assessment Appeal Application, the county version of BOE-305-AH. The form itself says it is the required application for changed assessment and warns that incomplete applications can be rejected. (contracosta.ca.gov)

3. Fill in your opinion of value. For a 2026 decline-in-value appeal, check Regular Assessment – Value as of January 1 of the current year and Decline in Value. Put the Assessor’s roll value in column A and your opinion of value in column B. Do not attach all hearing evidence to the application; the form says evidence should be presented later if requested or at hearing. (contracosta.ca.gov)

4. Pay the fee. Contra Costa charges a non-refundable $40 application fee for each application; a separate application is required for each parcel and tax year. Payment can be by cash, check, or money order payable to Contra Costa County Treasurer. (contracosta.ca.gov)

5. File by an accepted method. The official county page instructs applicants to mail the completed application and $40 check to: Assessment Appeals Board, 1025 Escobar Street, 1st Floor, Martinez, CA 94553. The same address is the Assessment Appeals Clerk’s physical address, open Monday-Friday, 8 a.m.-5 p.m., so in-person filing/payment is the practical walk-in method. (contracosta.ca.gov) I found no official Contra Costa online portal or email filing method for assessment appeal applications; email is mentioned for withdrawal/contact, not filing. (contracosta.ca.gov)

What happens after

The appeal is heard by the Contra Costa County Assessment Appeals Board, not by the Assessor. The county says Assessment Appeals is separate from the Assessor’s Office, staffed through the Clerk of the Board, and disputes are heard by Board-appointed Assessment Appeals Board members. (contracosta.ca.gov)

Hearings are scheduled on Thursdays at 9:00 a.m.; the county says you will receive a written notice about 45 days before the hearing. You must return the notice confirming your intent to appear, or the hearing will be cancelled. (contracosta.ca.gov) Continue paying your tax bill on time while the appeal is pending; Contra Costa warns that taxes must be paid by the delinquent dates regardless of review status, and refunds are issued later if the value is reduced. (contracosta.ca.gov)

At the hearing, you and the Assessor present evidence. The Board can lower, leave unchanged, or increase the assessment based on the evidence. (contracosta.ca.gov) If you and the Assessor settle before hearing, Contra Costa’s local rules allow a written stipulation to be submitted; the Board may accept it, waive your appearance, and change the value, or it may ask for more information or reject the stipulation and set a hearing. (contracosta.ca.gov)

Decision timing has two layers. Locally, the clerk generally schedules applications for hearing within 22 months of filing, and California law generally requires the hearing and final determination within two years unless extended. (contracosta.ca.gov) If the Board takes a matter under submission and no written findings are requested, Contra Costa’s rules call for written notice of decision no later than 120 days after the hearing concludes; if findings are requested, the notice is due within 120 days and findings within 45 days after the notice. (contracosta.ca.gov)

Local tips

Use the county’s own math when estimating savings. Example: suppose a Concord-area homeowner in a tax rate area listed at 1.1023% for 2025-26 proves the January 1, 2026 value should be $850,000 instead of the roll value of $900,000. The reduction is $50,000. Estimated annual ad-valorem savings: $50,000 × 1.1023% = $551.15, before any refund interest and excluding fixed direct charges that do not change with assessed value. Contra Costa’s refund FAQ states refunds are usually calculated as the reduction in assessed value multiplied by the applicable tax rate. (contracosta.ca.gov)

Do not miss the distinction between an informal review and a formal appeal. The Assessor can correct a value for free, but only the Assessment Appeals Board filing protects your right to a formal decision if the Assessor disagrees. Also, if you bought recently and received a supplemental notice, mark your calendar for the 60-day supplemental deadline; waiting for the regular November 30 deadline can forfeit the supplemental bill issue. (contracosta.ca.gov)