How to Appeal Your Property Tax in Contra Costa County
If your Contra Costa County home is assessed for more than it's worth, you have two ways to fix it: a free informal value review with the Contra Costa County Office of the Assessor, and a formal appeal to the county's Assessment Appeals Board. Both are due by November 30 — and most homeowners can handle both without a lawyer.
What are the two ways to appeal my property tax?
Contra Costa County offers two paths: a free informal review through the Contra Costa County Office of the Assessor, or a formal appeal through the Contra Costa County Assessment Appeals Board. Both file by mail — there's no online submission for either — and both close on November 30.
What is an informal review?
Through the Contra Costa County Office of the Assessor
The informal review is a free request you mail to the Assessor's office — no hearing, no fee, no lawyer. You download the Request for Value Review (Prop 8) form, fill in your property details and comparable sales, and mail it in by November 30. An appraiser reviews your evidence and either lowers your assessed value or confirms it. Your value cannot go up as a result of this request.
- Free to file — no fee
- No hearing required — an appraiser reviews your evidence and the county mails you a decision
- Mail-in only: send the form to Office of Assessor, 2530 Arnold Drive, Suite 100, Martinez, CA 94553-4359
- The form holds three comparable sales — and invites you to attach anything extra
Mail your informal review request by November 30
A postmark by the deadline counts. There's no online submission — build in postal time.
What is a formal property tax appeal?
Through the Contra Costa County Assessment Appeals Board
The formal appeal is filed with the Contra Costa County Assessment Appeals Board using form BOE-305-AH, mailed or delivered in person with a $40 nonrefundable fee — there's no online filing in Contra Costa. It leads to a scheduled hearing where the Board weighs your evidence against the Assessor's and can lower, confirm, or in rare cases raise the value. Sign the form in blue ink; photocopied signatures aren't accepted.
- Filing window: July 2 – November 30 each year
- $40 nonrefundable filing fee (Cash, check, or money order payable to 'Contra Costa County Treasurer')
- In-person before the Board (Martinez); hearings Thursdays at 9:00 AM
- Mail-in or in-person filing only — no online submission portal exists
No online filing — paper only
Contra Costa County has no online filing portal for formal appeals. You must print the BOE-305-AH, sign it in blue ink, include the $40 fee, and mail or hand-deliver it to: Assessment Appeals Board, 1025 Escobar Street, 1st Floor, Martinez, CA 94553
Pro tip: Filing the informal review does not extend or preserve your formal appeal deadline. File the BOE-305-AH by November 30 as a backstop even while your informal review is pending — you can withdraw it if the informal review succeeds.
Contra Costa County filing dates and cost, at a glance
- Informal review
- By November 30Free
- Formal appeal
- July 2 – November 30
- Formal filing fee
- $40 per parcel
Want every date for Contra Costa County in one place — bill due dates, supplemental assessment clocks, and what to do if you miss a window? See the Contra Costa County property tax deadlines page.
What evidence do I need for a property tax appeal?
Comparable sales are the heart of a Contra Costa appeal: recent sales of similar homes near yours that show the market was below your assessed value on January 1. Only sales that closed on or before March 31 count for that year's case. The county's free CCMAP tool gives you square footage, lot size, and parcel details to match comps properly. The informal form holds three comps — attach anything extra, per the form's own instruction.
“Similar” means comparable in:
- Square footage and lot size
- Bedrooms and bathrooms
- Condition and age
- Proximity to your home
Don't attach evidence to the formal application
The BOE-305-AH says it itself: “Do not attach hearing evidence to this application.” Your evidence is presented at the hearing. The informal review form is the opposite — it has three comp rows and invites attachments.
Free data source: The county's CCMAP parcel tool provides building square footage, lot size, parcel maps, and aerials at ccmap.us.
This is what Overassessed does. We find comparable sales from county records, calculate professional-grade adjustments for differences in size, features, and condition, and package the evidence you need — ready to mail.
The Overassessed Filing Guide includes comparable sales data, dollar adjustments, and a ready-to-submit value argument — everything listed above.
See what's includedHow do I file a property tax appeal step by step?
The process runs on one calendar: comps by March 31, filings by November 30. Here's the whole thing, start to finish.
Check if you qualify
Look up your assessed value on CCMAP and compare it to what your home was actually worth on January 1 — if the market value is lower, you likely qualify.
Overassessed does this for you — enter your address and we'll pull your assessment and run the comparison.
Gather comparable sales
Collect 3 recent sales of similar nearby homes that closed on or before March 31, since later sales can't be used for this year's case.
The Overassessed Filing Guide includes selected comps with professional-grade adjustments, ready to copy onto the form.
File the informal review
Download the Request for Value Review (Prop 8) form, fill in your info and comps, and mail it to the Assessor's office (Office of Assessor, 2530 Arnold Drive, Suite 100, Martinez, CA 94553-4359) by November 30 — it's free.
File the formal appeal (as backup)
Mail the BOE-305-AH with a $40 check to the Contra Costa County Assessment Appeals Board (Assessment Appeals Board, 1025 Escobar Street, 1st Floor, Martinez, CA 94553) by November 30 so your rights are protected if the informal review doesn't go your way — and don't leave the opinion-of-value box blank.
Prepare for your hearing
If your case goes to a hearing, return the notice to confirm you'll appear, and bring your evidence with you to Martinez. Hearings are held in person before the Board on Thursdays at 9:00 AM.
Our Filing Guide gives you copy-ready values for both the informal form and the formal application, plus a deadline tracker.
Check if you qualifyImportant things to know
You have until November 30 — not September 15
Don't trust generic California guides that say September 15 — in Contra Costa County, you have until November 30 to file. The formal window is July 2 – November 30; Contra Costa is an extended-deadline county.
Everything files by mail
There is no online filing for either track. Print the forms, sign the appeal in blue ink, and get them postmarked by the deadline.
The formal appeal costs $40 — the informal review is free
The formal appeal costs $40 whether you win or lose, payable to the Contra Costa County Treasurer — the informal review costs nothing, which is why it's the smart first move.
The free review doesn't stop the clock
The informal review does not preserve your formal appeal deadline. If your case is strong, file the formal appeal by November 30 too — you can withdraw it later.
Comps must close by March 31
A perfect comp that sold in April is worthless for this year's case. The county form itself says sales must be prior to 3-31 — check closing dates before you build your evidence.
Two paperwork traps sink otherwise-good appeals
Leaving the opinion-of-value box (Section 4, Column B) blank gets your application rejected — the form says so itself. And when your hearing notice arrives (~45 days ahead), you must respond to it within 15 days or the hearing is automatically cancelled.
Frequently asked questions about Contra Costa County property tax appeals
What is the deadline to appeal property taxes in Contra Costa County?
Both tracks are due November 30. The formal appeal window opens July 2 and closes November 30, and the free informal value review must reach the Assessor by the same date. A postmark by the deadline counts, and if November 30 falls on a weekend or holiday, the deadline rolls to the next business day. See the county's Assessment Appeals page for details.
How much does it cost to appeal my property tax in Contra Costa County?
The informal value review is completely free. The formal appeal costs $40 per application, paid by cash, check, or money order to the Contra Costa County Treasurer when you file — and it's nonrefundable even if you win. Details are on the county's Assessment Appeals page.
Do I need a lawyer to appeal my property taxes in Contra Costa County?
No. Homeowners can file both the informal review and the formal appeal themselves — the appeal form's agent and attorney section is optional. Most residential cases come down to comparable sales, not legal arguments. See the county's Assessment Appeals page.
What evidence do I need for a property tax appeal in Contra Costa County?
Comparable sales are the core evidence: recent sales of similar nearby homes that closed on or before March 31 of the assessment year. The county's informal review form has room for three comps and lets you attach more. You can pull property details for free from the county's CCMAP tool. The county's Helpful Terms page explains the comp rules.
How long does a property tax appeal take in Contra Costa County?
Formal appeals take time: county rules require your hearing to be scheduled within 22 months of filing, and the Board must decide your case within two years unless you agree to extend. You'll get a hearing notice about 45 days in advance — respond to it within 15 days or the hearing is cancelled. Keep paying your tax bills while you wait; if you win, the county refunds the difference. The county's appeals FAQ covers the timeline.
Does filing the free informal review protect my appeal deadline in Contra Costa County?
No — and this catches people every year. The free informal review doesn't pause or extend the formal appeal deadline. If November 30 passes while the Assessor is still reviewing your request, you've lost your appeal rights for the year. File the $40 formal appeal by November 30 as a backstop; you can withdraw it in writing if the informal review works out. See the county's Assessment Appeals page.
Official county resources: Assessment Appeals, Review Your Value, and every Contra Costa County deadline in one place.
Ready to see if you qualify?
Enter your address and we'll compare your assessed value to comparable sales. It takes about 60 seconds.
Overassessed provides estimates based on publicly available data and AI-generated analysis. This is not a formal appraisal, legal advice, or tax advice. Results are not guaranteed, and appeal outcomes depend on county review. Users file their own appeals. AI-generated estimates may differ from actual market values.