How do I appeal a supplemental assessment in California?

A supplemental assessment is issued when a property changes ownership or completes new construction mid-year, capturing the value change from the date of the event rather than waiting for the next January 1 lien date. Because it arrives off the normal cycle, it has its own appeal clock.

Under Cal. Rev. & Tax. Code §75.31, the supplemental notice must advise you of the right to an informal review and the right to appeal, and the appeal "shall be filed within 60 days of the date of mailing printed on the notice or the postmark date therefor, whichever is later." In counties that have adopted the relevant option (and in Los Angeles County), the 60 days runs from the date printed on the supplemental tax bill instead.

Key points:

• File a BOE-305-AH with the clerk of the board within that 60-day window — do not wait for the regular July–November period, which does not govern supplemental assessments.
• Late notice protection: if you did not receive the notice at least 15 days before the regular §1603 deadline, §75.31 lets you file within 60 days of the tax bill with an affidavit that the notice was not timely received.
• Stipulated error: if you and the assessor agree there is an error, an application may be filed within 12 months of notification.
• A supplemental assessment usually reflects a change in ownership or new construction, so the underlying dispute is often whether a reappraisable event actually occurred, not just the value.

The state BOE assessment appeals page covers the general filing mechanics.