A supplemental assessment is a mid-year or after-sale extra bill when value changes. The regular roll often still shows the old figure until next cycle.
A supplemental property tax assessment is California's mid-year or after-sale extra bill that captures a value change the regular roll has not caught yet. Not every state issues one. A purchase is the usual trigger. New construction can be another. The extra invoice covers the gap until the next regular roll.
What is a supplemental assessment after buying a house?
California's Board of Equalization describes a supplemental assessment as the difference between the property's new value after a change in ownership or completed new construction and the value already on the roll. Other states use different stamps for the same mid-year catch-up. The regular January cycle is not the only time a new number can produce a bill.
Example: the roll still says $200,000, the sale resets value to $250,000, and 10 mills on the $50,000 gap is $500 for a full leftover year. That pairing is an example, not one county's 2026 invoice.
Look up if you are overpaying.
Is a supplemental assessment the same as the yearly property tax bill?
No. The yearly bill follows the regular roll. A supplemental bill is the extra slice for a value change that arrived in between. A property tax appeal, when one is filed, is still aimed at a printed value, including a new one on a supplemental notice. The property tax glossary keeps the regular roll next to the mid-year catch-up. You pay only if FairAppeal saves you money, and there are no upfront costs with Fair Appeal.