Supplemental Assessment
A supplemental assessment is a reassessment of your property's value that happens between the regular annual cycles, triggered by a change in ownership or completed new construction. In California, when you buy a home or finish a major build, the county reassesses it to current market value and sends a separate, prorated "supplemental" tax bill covering the rest of the fiscal year — in addition to your normal bill.
Why it exists
Under California's Proposition 13, a property is reassessed to market value when it changes ownership or is newly constructed. Rather than wait until the next January 1 lien date to capture that change, the supplemental roll puts the new value into effect immediately. The bill reflects the difference between the old assessed value and the new one, prorated from the first day of the month after the triggering event through the end of the fiscal year (BOE — Supplemental Assessment).
A simple example
Say you buy a home in October that the previous owner had assessed at a much lower Prop 13 value. The county reassesses it to your purchase price, then issues a supplemental bill for the gap in value — covering roughly November through the end of June. Because timing affects the proration, two buyers paying the same price can owe different supplemental amounts depending on the month they closed.
How it affects your tax — and your appeal
A supplemental bill can be a surprise, especially for first-time buyers who budgeted only for the regular bill. The good news: a supplemental assessment is appealable, just like a regular one. If you believe the reassessed market value is too high, you can challenge it — but supplemental appeals often have their own filing deadline tied to the date the supplemental notice was mailed, separate from the regular July–September window. Read your Notice of Supplemental Assessment carefully for the exact deadline.
Supplemental assessments are a California mechanism; other states handle mid-year ownership changes differently or not at all. For the official rules, see the BOE Supplemental Assessment overview.
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