How to Appeal Your Property Tax in San Mateo County
San Mateo County homeowners can reduce their property tax bill by appealing an overassessment. The county offers two paths: an informal Decline-in-Value review through the Assessor's office (free, no hearing) and a formal appeal through the County of San Mateo Assessment Appeals Board ($30 filing fee). In 2023, 97.6% of decided cases were resolved through negotiation — most homeowners never need to appear at a hearing.
What are the two ways to appeal my property tax?
San Mateo County offers two paths: a free informal review through the Assessor's office, or a formal appeal through the County of San Mateo Assessment Appeals Board. Most people start with the informal review and file the formal appeal simultaneously to protect their rights.
What is an informal review?
Through the San Mateo County Assessor-County Clerk-Recorder & Elections
The informal Decline-in-Value review is the simplest way to request a property tax reduction in San Mateo County. You submit a request to the San Mateo County Assessor-County Clerk-Recorder & Elections with your property information, your opinion of its current market value, and supporting evidence like comparable sales. There's no fee, no hearing, and no risk — the Assessor can only reduce or confirm your assessed value, never increase it. You can submit your request online, by email, by mail, or by fax.
- Free to file — no fee
- No hearing required — a certified appraiser reviews your evidence
- Four ways to submit: online form, email to assessor@smcacre.gov, mail to Assessor Division, 555 County Center, Redwood City, CA 94063-1665, or fax
- If the assessor agrees, they adjust your assessed value — and your taxes go down
Submit your informal review request by October 31
The online form at smcacre.gov is the fastest option. Results are mailed by July of the following year.
What is a formal property tax appeal?
Through the County of San Mateo Assessment Appeals Board
The formal appeal is filed with the County of San Mateo Assessment Appeals Board, an independent body separate from the Assessor's office. You can file the BOE-305-AH online through the Board's appeals portal (new in 2026), or print it, sign it, and mail it with a $30 check. The Board can reduce, confirm, or in rare cases increase your assessed value — though increases occurred in fewer than 0.2% of decided cases in 2023.
- Filing window: July 2 – November 30 each year
- $30 non-refundable filing fee (Online: credit card through the AAB portal. By mail or in person: check payable to 'San Mateo County AAB' or cash)
- In-person only (500 County Center, 1st Floor, Redwood City)
- File online through the Board's portal, or on paper by mail or in person
Online filing is new in 2026 — paper still works
San Mateo County now accepts formal appeals through the Board's secure online portal: register as an Applicant, add your mailing address to your profile, fill in the application, and pay the $30 fee by card. The portal also shows the status of your appeal. Prefer paper? Print the BOE-305-AH, sign it (an original signature is required), include a $30 check, and mail or hand-deliver it to: County of San Mateo Assessment Appeals Board, 500 County Center, 5th Floor, BOS 104, Redwood City, CA 94063
Pro tip: File your informal Decline-in-Value request (deadline October 31) and your formal appeal (deadline November 30) at the same time. The informal review does not protect your formal appeal rights — if you wait for informal results and miss November 30, you lose your right to appeal for the entire year.
San Mateo County filing dates and cost, at a glance
- Informal review
- By October 31Free
- Formal appeal
- July 2 – November 30
- Formal filing fee
- $30 per parcel
Want every date for San Mateo County in one place — bill due dates, supplemental assessment clocks, and what to do if you miss a window? See the San Mateo County property tax deadlines page. New to the process? The statewide guide to appealing property taxes in California explains the two tracks before the county specifics.
What evidence do I need for a property tax appeal?
The most effective evidence for a San Mateo County property tax appeal is comparable sales — recent sales of similar homes in your area that sold for less than your assessed value. Use sales from before January 1 or up to 90 days after (no later than about April 1). Focus on homes that are similar in size, age, location, and condition. For owner-occupied single-family homes, the Assessor must prove their value is correct — you don't need a professional appraisal, just good comparable sales data.
“Similar” means comparable in:
- Square footage and lot size
- Bedrooms and bathrooms
- Condition and age
- Proximity to your home
Bring 8 printed copies to your hearing
Do not attach evidence to your formal appeal application. You will present your evidence at the in-person hearing. Bring 8 copies of all evidence documents including comparable sales, photographs, appraisals, and repair estimates.
Free data source: The San Mateo County Assessor-County Clerk-Recorder & Elections provides assessed value data at smcacre.gov/assessor/parcel-information.
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 submit.
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 San Mateo County property tax appeal process has two parallel tracks — an informal review and a formal appeal. We recommend filing both to maximize your chances and protect your rights.
Check if you qualify
Look up your assessed value at the San Mateo County Assessor's parcel information tool and compare it to what your home would actually sell for. If comparable homes are selling for less than your assessed value, you likely have a case.
Overassessed does this for you — enter your address and we'll pull your assessment and run the comparison.
Gather comparable sales
Find 3 or more comparable sales in your area that closed near the January 1 lien date — sales more than 90 days after that date are not admissible. These are the foundation of your case.
The Overassessed Filing Guide includes selected comps with professional-grade adjustments, ready to submit.
File the informal review
Submit a Decline in Value Reassessment Application to the Assessor via the online form at smcacre.gov, email to assessor@smcacre.gov, mail to Assessor Division, 555 County Center, Redwood City, CA 94063-1665, or fax. The deadline is October 31.
File the formal appeal
File the BOE-305-AH online through the Board's portal, or print it, sign it, include a $30 check payable to "San Mateo County AAB," and mail to County of San Mateo Assessment Appeals Board, 500 County Center, 5th Floor, BOS 104, Redwood City, CA 94063. Postmark by November 30. Check "Yes" on Box 8 (Claim for Refund).
Prepare for your hearing
If your formal appeal goes to a hearing, prepare your evidence package and bring 8 printed copies to the hearing at 500 County Center, 1st Floor, Redwood City. Most cases settle at the pre-hearing conference stage.
Our Filing Guide gives you pre-filled forms for both the informal and formal filings, plus a deadline tracker.
Check if you qualifyImportant things to know
Two ways to file the formal appeal
San Mateo County added an online filing portal for the formalBOE-305-AH appeal in 2026. You can still print, sign, and mail the application with a $30 check. Overassessed generates a pre-filled form you can print and sign, plus every value to copy into the portal.
Check “Yes” on Box 8 (Claim for Refund)
This ensures you automatically receive a refund if your appeal succeeds. Failing to check this box requires a separate filing to get your money back and changes the timeline for any Superior Court appeal.
Dual deadlines — October 31 and November 30
San Mateo County has two separate deadlines: October 31 for the informal Decline-in-Value request and November 30 for the formal appeal. The informal review does not protect your formal appeal rights — always file both.
You may not receive an assessment notice
San Mateo County does not mail annual assessment notices to all property owners — only when there's been a change in ownership, new construction, or a prior Prop 8 adjustment. Check your assessed value proactively at the Assessor's website.
Hearings are held in person
The Board's 2026 schedule holds every hearing in person at 500 County Center, 1st Floor, Redwood City (2nd and 4th Thursdays, 9:30 am). Most cases settle before a hearing — 97.6% of decided cases in 2023 resolved through stipulation.
Auto-renewal for existing Prop 8 enrollees
If your property already received a Decline-in-Value reassessment for the current year, you do not need to file a new informal request — the Assessor automatically reviews your property. However, you should still file a formal appeal by November 30 if you disagree with the new value.
Frequently asked questions about San Mateo County property tax appeals
What is the deadline to appeal property taxes in San Mateo County?
San Mateo County has two deadlines. The informal Decline-in-Value request must be submitted to the Assessor by October 31. The formal BOE-305-AH must be postmarked or hand-delivered between July 2 – November 30. If November 30 falls on a weekend or holiday, the deadline extends to the next business day. Filing both is recommended — the informal review does NOT protect your formal appeal rights.
How much does it cost to appeal my property tax in San Mateo County?
The informal review is free. The formal appeal costs $30 per application (Online: credit card through the AAB portal. By mail or in person: check payable to 'San Mateo County AAB' or cash). This fee is non-refundable. If you want Written Findings of Fact (needed for Superior Court appeals), those cost $500 per hour of hearing time.
Do I need a lawyer to appeal my property taxes in San Mateo County?
No. Most residential property owners in San Mateo County represent themselves. You do not need a lawyer, tax consultant, or agent. If you prefer to have someone represent you, you can authorize an agent using the county's Agent Authorization form. In 2023, 97.6% of decided cases resolved through stipulation — a direct negotiation between the homeowner and the Assessor's representative.
What evidence do I need for a property tax appeal in San Mateo County?
The best evidence is three or more comparable sales of similar properties in your area. Sales should be as close to the January 1 lien date as possible — the Board cannot consider sales more than 90 days after that date. Do NOT attach evidence to your formal application. Bring 8 printed copies of all evidence to the hearing.
How long does a property tax appeal take in San Mateo County?
The informal review results mailed by July of the following year. For formal appeals, most cases are resolved through negotiation within 2–3 months. The Board has up to 2 years to hear and decide, but cases rarely take that long. Once decided, the written decision is mailed within 120 days, and refunds are processed within about 15 days if you checked "Yes" on the Claim for Refund.
Can the County of San Mateo Assessment Appeals Board increase my property taxes?
Technically, yes — the County of San Mateo Assessment Appeals Board has the authority to increase your assessed value based on evidence presented at the hearing. In practice, this is extremely rare. In 2023, only 1 out of 618 decided cases resulted in an increase. The vast majority of cases are resolved through negotiated settlements, and nearly all cases that reach a value decision result in a reduction. The informal review is safer: the Assessor can only reduce or confirm your value, never increase it.
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.