How to Appeal Property Taxes in San Mateo County, California (2026 Guide)

For San Mateo County’s regular 2026 assessment roll, file your formal property tax appeal by Monday, November 30, 2026. File through the County’s Assessment Appeals Board online portal, or submit the signed paper Assessment Appeal Application / Application for Changed Assessment with the $30 fee to the San Mateo County Assessment Appeals Board at 500 County Center, 5th Floor, BOS 104, Redwood City, CA 94063. (smcgov.org)

How assessments work in San Mateo County

San Mateo County values property as of the January 1 lien date for the upcoming fiscal year. For most homes, the taxable value is the lower of two numbers: your factored base year value—usually purchase price, plus annual inflation capped at 2% under Proposition 13—or the home’s fair market value on January 1 under Proposition 8. If the market value is lower, the Assessor can enroll a temporary Prop 8 reduction; if the market later recovers, that temporary value can rise by more than 2% until it reaches the Prop 13 factored base value. (smcacre.gov)

For the 2026-27 roll, the Assessor announced a countywide assessed roll of $357.6 billion, up 4.85%, and said 2025 decline-in-value reviews found 6,572 properties qualifying for temporary reductions, totaling about $3.5 billion below their 2025-26 Prop 13 values. That is not a published “appeal win rate,” but it is useful local evidence that San Mateo County is actively processing Prop 8 reductions where market value is below assessed value. (smcacre.gov)

Two local dates matter in 2026. The regular formal appeal window opened July 2, 2026 and closes November 30, 2026. The Assessor’s informal Decline in Assessed Value / Decline in Value Reassessment program is separate and has an October 31, 2026 request deadline; if you have not received informal results by November 10, 2026, the County recommends filing a formal appeal to preserve your rights. (smcgov.org)

Whether you should appeal

Appeal if your evidence shows the property’s market value on January 1, 2026 was below the value on your 2026-27 assessment or tax bill. The cleanest residential evidence is comparable sales: similar size, quality, condition, location and amenities. San Mateo County’s hearing guidance says comparable sales should be arm’s-length transactions and, for a decline-in-value appeal, sales before the valuation date are relevant; sales within 90 days after the valuation date may also be useful under California rules. (smcgov.org)

Start with the Assessor before paying for a formal appeal. The County specifically encourages owners to contact the Assessor’s Office within 15 days after receiving an assessment notice; if a reduction is proper, the Assessor may adjust the value without the Board hearing track. But do not let the informal review make you miss November 30—the County says the informal process does not extend the AAB deadline. (smcacre.gov)

Also check exemptions because they reduce the bill without disputing value. San Mateo County’s Homeowners’ Exemption removes $7,000 from assessed value for an owner-occupied principal residence, at no filing charge. The Disabled Veterans’ Exemption for 2026 can exempt up to $180,671, or up to $271,009 for qualifying low-income 100% disabled veterans or unmarried surviving spouses. (smcacre.gov)

Step-by-step how to file

1. Confirm the assessment you are appealing. Use your tax bill or notice to find the Assessor’s Parcel Number and the “value on roll.” San Mateo County says one application covers only one parcel and one tax year. (smcgov.org)

2. Use the right form. For a formal appeal, use the County’s Assessment Appeal Application, also referred to in County materials and BOE materials as the Application for Changed Assessment. For an informal Prop 8 review, use the Assessor’s “Decline in Value” Reassessment Application Form; that is not the same as a formal AAB appeal. (smcgov.org)

3. Choose the filing route. The County’s AAB site now directs taxpayers to a secure Assessment Appeals Board online portal where you can submit applications, check appeal status, view hearing schedules and download forms. For paper filing, County instructions say to submit the original wet-signed application with the fee by mail or hand delivery to: San Mateo County Assessment Appeals Board, 500 County Center, 5th Floor, BOS 104, Redwood City, CA 94063. The FAQ says you may email AAB@smcgov.org to request an application; because the current instructions distinguish online filing from original paper filing, do not rely on email alone as your filing method unless the AAB Clerk confirms it in writing. (smcgov.org)

4. Pay the fee. Each formal appeal requires a non-refundable $30 processing fee. If filing by paper, checks should be payable to San Mateo County AAB. Separate applications and fees are required for separate parcels and roll years. (smcgov.org)

5. Watch special deadlines. Regular 2026 assessments are due November 30, 2026. Supplemental and escape assessments are generally due within 60 days of the notice mailing date or postmark date, whichever is later; calamity reassessment appeals have a six-month window after the proposed reassessment notice. (smcgov.org)

What happens after

The appeal authority is the San Mateo County Assessment Appeals Board (AAB), an independent three-private-citizen board appointed by the Board of Supervisors. It hears evidence from the property owner and the Assessor’s Division and determines the value; the Board can reduce, leave unchanged, or increase the enrolled value based on the evidence. (smcacre.gov)

After a complete application is accepted, the AAB Clerk forwards it to the Assessor for review. Many cases settle by stipulation—a written agreement on value signed before the hearing and submitted to the AAB for approval. If approved, you do not attend a hearing. San Mateo County’s published pages do not advertise a homeowner “paper-only” non-oral hearing option; if there is no stipulation, you or an authorized representative must appear, or the appeal may be denied for lack of appearance. (smcgov.org)

Expect a pre-hearing conference first unless the parties are ready for an evidentiary hearing. The Clerk gives at least 45 days’ notice of the hearing. Pre-hearing conferences are generally roll-call style and not scheduled to an exact personal time; County FAQs say pre-hearing conferences are at 9:30 a.m. and the hearing location is the Board of Supervisors’ Chambers, 500 County Center, 1st Floor, Redwood City. Bring eight copies of exhibits if you go to hearing. (smcgov.org)

The whole process can take time. The County says a hearing may be set in approximately 2-3 months after processing, but due to volume it can be longer, and the entire process can take up to two years. If you win and you checked the refund claim box, the County can process the refund; you still must pay property taxes on time while the appeal is pending. (smcgov.org)

Local tips

Use your actual Tax Rate Area from the bill. San Mateo County’s 2025-26 Tax Rate Book shows the 1.0000 general tax rate plus voter-approved debt rates, with different composite rates by area. For example, a San Mateo Tax Rate Area such as 012-032 shows a 1.1201% composite rate. If a homeowner in that area reduces the enrolled value from $1,650,000 to $1,500,000, the ad valorem savings estimate is $150,000 × 1.1201% = $1,680 for that year, before considering fixed charges that do not change with assessed value. (smcgov.org)

Do not pad your appeal with weak comps. A 1,200-square-foot condo sale in San Bruno is not persuasive for a renovated single-family home in San Carlos. Your best packet is usually a one-page summary, 3-5 comparable sales close to January 1, photos or MLS sheets, and a short adjustment explanation for size, condition, lot, view and school district. If your strongest evidence is a professional appraisal dated near January 1, include it, but be ready to explain why it reflects San Mateo County market value on the lien date.

Finally, treat November 30 as a hard deadline. The County says there are no deadline extensions or exceptions, and amendments to regular appeals are due by 5:00 p.m. on the final filing date. If you are waiting on the informal Decline in Value program in November, file the formal AAB appeal anyway and withdraw later if the Assessor gives you the value you requested. (smcgov.org)