San Mateo County property owners can file tax assessment appeals online through a new portal the county launched Monday, Sept. 21.

The portal at smcgov.org/aab lets residents file an appeal, pay fees, upload supporting documents and track their case status at any hour, according to the county's official post on X. Filing also eliminates the paper forms the county previously required, the county said in a post on Instagram. Property owners with an appeal already on file from before June 1, 2026, must email the AAB clerk to activate their account in the new system.

The filing window for regular 2026 assessment appeals closes Monday, Nov. 30.

Property owners who believe their 2026 assessed value is too high have two paths. The office of Assessor-County Clerk-Recorder Mark Church is accepting informal Decline in Value applications through Saturday, Oct. 31. Owners should first contact the Assessor's Division within 15 days of receiving their assessment notice to request an informal review. If the assessor does not reduce the value, owners can then file a formal appeal with the Assessment Appeals Board, according to the county assessor's website.

The Assessment Appeals Board (AAB) is an independent three-member panel appointed by the Board of Supervisors. It hears evidence from both the property owner and the Assessor's Division before setting a value.

The formal appeal window opened July 2 and runs through Nov. 30. Appeals on supplemental assessments must be filed within 60 days of the mailing date on the supplemental notice.

A third-party property tax guide, AppealDesk.com, lists the filing fee at $30 per parcel, non-refundable. That figure was last verified in July, before the portal launched, so residents should confirm the current fee through the portal or by contacting the AAB clerk.

San Mateo County homeowners pay well above the national average in property taxes. AppealDesk estimates the county's median annual bill at roughly $8,637 to $9,758, compared with a national median of $2,400.

The county's old Assessment Appeals Board webpage now redirects to the new portal. Residents with questions can email the AAB clerk at [email protected] or call (650) 363-4573.