Supplemental Tax Calculator

How the estimate is worked out

The county subtracts the prior assessed value from the new one and taxes the gap. The State Board of Equalization describes the steps: the assessor values the property at purchase, subtracts its prior assessed value, and the difference is the supplemental assessment. A sale below the prior value produces a negative assessment and a refund instead of a bill.

(purchase price − prior assessed value) × tax rate × proration factor

The factor comes from Revenue and Taxation Code § 75.41 and depends only on the month you close. The bill starts on the first day of the next month and runs to June 30, the end of the fiscal year:

A closing from January through May produces two bills. The upcoming fiscal year's roll was set on the January 1 lien date, before the sale, so it still carries the old value and the county corrects it with a second, unprorated bill for the full year. A June closing takes effect July 1, so the only bill is that full-year one. The supplemental tax bill guide walks through both cases with worked examples.

Choosing the inputs

• Prior assessed value. The seller's current assessed value appears on their property tax bill, which you can ask for during escrow. A seller who has owned the home a long time usually has a value far below the price, and the bill grows with that gap.
• New construction or unknown. Ticking the box sets the prior value to $0, which bills the whole price and gives an upper bound. The county subtracts whatever value was already on the roll, so a new home on a lot that carried an assessed value will owe less.
• Tax rate. The default is the report's county-wide estimate of 1.10%. Each parcel's real rate is the 1% base plus voter-approved debt for its Tax Rate Area, which you can look up in the county's Tax Rate Area search. A full report fetches the parcel's own rate when the county publishes it.
• Close date. Only the month matters for the factor. Moving a closing from May 31 to June 1 changes the result from two bills to one, so try both if your date is near a month boundary.

What the estimate leaves out

Mello-Roos and other Community Facilities District special taxes are fixed annual charges, not a share of assessed value, and the county Assessor describes the special tax as added to the property tax bill at the district's request. The report treats it as a charge that never prorates into a supplemental bill, and the calculator leaves it out too. Both also ignore the homeowners' exemption. The county mails the real bill after the assessor processes the sale, and the Treasurer-Tax Collector is the authority on the amount and due dates. The county's own Assessor billing page links a supplemental estimator for a specific parcel. Every figure here is an estimate, not a tax bill and not tax advice.