California Property Tax Appeal
How to Appeal Your Property Taxes in Contra Costa County (2026)
Contra Costa County stretches from the East Bay suburbs of Walnut Creek and Concord out toward the Delta, and its housing values move with the broader Bay Area — up fast, and sometimes down just as fast in specific neighborhoods. An assessment set at the top of a cycle can quietly overstate what your home is worth today.
One Contra Costa reminder matters more than most: keep paying your property tax installments while your appeal is pending. An appeal doesn't pause the bill, and a missed installment adds penalties that can wipe out your savings. The Assessor offers an informal Decline-in-Value review first, which is the fastest path to a correction.
This page lays out the Contra Costa County process — the December 1 deadline, the form, the fee, and the informal-first route — plus an honest take on whether your home is genuinely overassessed.
Contra Costa County appeal — at a glance
- Appeal deadline
- July 2 – December 1
- Filing fee
- $40
- Appeal form
- BOE-305-AH
- Assessor's office
- (925) 313-7400
Check your Contra Costa County home in minutes
Enter your address and get an honest, comparable-sales analysis of whether you're overassessed — $9.99, with 3 years of monitoring included.
Analyze my assessmentFlat fee. You keep 100% of your tax savings.
Is your Contra Costa County home overassessed?
California property tax bills run roughly 1.1%–1.25% of a home's assessed value. At that rate, an assessment that's $50,000 too high costs a homeowner on the order of $550–$625 every single year — and it compounds until someone corrects it.
The only way to know is to compare your assessed value against what similar nearby homes actually sold for. That's exactly what a Numa analysis does: real comparable sales, an honest overassessed / fairly-assessed verdict, and a conservative savings estimate — for $9.99, with 3 years of monitoring included.
How to appeal in Contra Costa County, step by step
- 1
Confirm your home is overassessed
Your case rests on recent sales of comparable homes. Gather a few similar properties nearby that sold recently, adjust for differences in size, age, and condition, and check whether they came in below your assessed value. This is the most time-consuming part of an appeal, and the quality of those comparables is what decides it.
- 2
Request an informal review with the Assessor
In Contra Costa County, start with an informal Decline-in-Value (Proposition 8) review — often the fastest path, resolved without a hearing. Get the informal review form.
- 3
File a formal appeal if needed
If the informal review doesn't resolve it, file form BOE-305-AH with the Assessment Appeals Board — before the December 1 deadline. You can file by mail or in person.
- 4
Present your evidence
Your comparable-sales evidence is what wins the appeal. Numa's $24.99 packet assembles it into a ready-to-file case — and if the informal review fails, the formal Appeals Board packet is included at no extra charge.
Do it yourself, or let Numa do the hard part
You can absolutely appeal on your own — the deadline, form, and steps above are all you need to file. The catch is Step 1: building the comparable-sales evidence that actually wins. Here's the honest trade-off.
Do it yourself
- Free
- Typically 5–10 hours of work
- You find, adjust, and score the comps yourself
- Amateur evidence is the top reason appeals fail
- No easy way to know if you should file at all
With Numa
- $9.99 analysis, done in minutes
- Real comparable sales, scored the way the county does
- An honest verdict — including “don't file, you're fairly assessed”
- 3 years of monitoring included
- Optional $24.99 ready-to-file evidence packet if you're overassessed
Contra Costa County key dates
- Appeal window: July 2 – December 1
- Continue paying tax installments during appeal to avoid penalties.
- Prop 8 (decline in value): Same as regular appeal deadline.
Contra Costa County property tax appeal — FAQ
What is the property tax appeal deadline in Contra Costa County?
The regular appeal window runs July 2 through December 1. Continue paying tax installments during appeal to avoid penalties.
How much does it cost to appeal property taxes in Contra Costa County?
The filing fee is $40.
Do I have to go to a hearing to lower my assessment?
Often not. In Contra Costa County you can request an informal review (a Decline-in-Value / Proposition 8 request) with the Assessor first — many reductions are resolved this way without a formal hearing. If the informal review doesn't succeed, you can file a formal appeal with the Assessment Appeals Board.
How do I know if my Contra Costa County home is overassessed?
Your home is overassessed if comparable homes recently sold for less than your assessed value. Numa runs that comparison for you — the same comparable-sales method the county uses — and gives you an honest verdict for $9.99, with 3 years of monitoring included.
What form do I use to appeal in Contra Costa County?
Formal appeals use form BOE-305-AH, filed with the Assessment Appeals Board. Numa's packet fills in the evidence and shows you exactly how to file it.
Check your Contra Costa County home in minutes
Enter your address and get an honest, comparable-sales analysis of whether you're overassessed — $9.99, with 3 years of monitoring included.
Analyze my assessmentFlat fee. You keep 100% of your tax savings.