When you buy a house, the county tax assessor sends you regular property-tax bills, which are based on what you paid for the house – usually around 1% of the purchase price per year.
But in almost all cases, the sellers paid a different amount of taxes, which were based on their purchase price.
Thus, the supplemental property tax makes up the difference.
The escrow company will pay taxes at the old rate, or give you a credit depending on the time of year. When you close a escrow during the period when taxes are due (November 1st – December 10th and February 1st – April 10th), the escrow company just pays the tax for the buyer – but at the seller’s old rate.
The county wants to collect every penny, so they pro-rate the difference between the old and new taxes due for the remainder of the tax period, and they send you a supplemental-tax bill.
We used to have to manually calculate the amount. It is hard enough trying to explain what a supplemental tax bill is, let alone come up with the right amount. Thankfully, the assessor has devised their own calculator, at this link – it is a great tool:
Their FAQs are here: