Two separate things are happening, and the supplemental tax bill was coming with or without the deck. In California, property taxes are based on the assessed value at the time of purchase. When a home sells for more than the seller's old assessed value, your first tax payments through closing are still calculated on that lower prior value until the county catches up. The county then issues a one-time supplemental tax bill, often split into two installments, to true up the difference for the period you've owned the home. That catch-up is normal. It isn't a penalty, and once it's paid, no further supplemental bills go out for that purchase. A permitted improvement like a deck can trigger its own small supplemental assessment, but that's typically a few hundred dollars. A four-figure bill is almost certainly the purchase-price true-up rather than the deck. The lesson for next time: when you buy new construction or buy above the seller's prior assessed value, set aside cash for a supplemental bill so it arrives as a line item instead of a surprise.