A supplemental property tax bill is a one-time catch-up that arrives after you buy, and it surprises a lot of California buyers. Under Proposition 13, a home's assessed value can rise a maximum of 2 percent a year while the same owner holds it. When the home sells, the county does not reassess on your closing date. For a stretch after you close, you are still being billed at the seller's old, lower assessed value. Somewhere between a few months and about a year later, the county reassesses the property to your actual purchase price. The supplemental bill covers the gap between what you were charged at the old value and what you actually owed at the new one for that interim period. It is tied to the change of ownership, so it is a one-time event rather than a new recurring annual charge. Your county assessor can confirm the exact timing and amount. If you have an impound account for taxes and insurance, the bill usually stings less than it sounds, because the lender sets up your escrow using the expected reassessed tax figure rather than the seller's old one. That builds a cushion that typically absorbs the supplemental bill, with any difference trued up at the annual escrow analysis. If you pay taxes on your own, set money aside for it so the bill does not catch you flat-footed.