A supplemental tax bill is a one-time catch-up charge, not a new annual expense. The mechanism (mainly a California and similar-state situation): your property taxes start out based on the prior assessed value, the previous owner's basis, or for new construction the builder's original land value, because the county's reassessment lags the sale. Once the county records the sale, or adds the completed home's value for new construction, it reassesses at the new higher value and sends a supplemental bill covering the difference for the period between closing and reassessment. That catch-up bill is the one-time event. From then on, your regular annual bill simply reflects the new value. Two things to check: - Your escrow account. If your lender set up impounds based on your actual purchase price, there should be enough to absorb the supplemental bill, but confirm that with your escrow or servicing company rather than assume it. - Mello-Roos. Many newer communities carry this separate special assessment for infrastructure. It sits outside the standard property tax and can add a meaningful amount, so make sure it's built into your total tax estimate. Tax rules vary by county and change over time, so confirm the specifics with your county assessor or a tax advisor.