Estimate Your Supplemental Tax Bill

Supplemental Assessments may be issued when a property changes ownership or new construction is completed. If you received a Notice of Supplemental Assessment, the Assessor was required to reassess your property. 

This Supplemental Assessment will reflect the difference between the prior assessed value and the new assessed value (the value used to calculate your tax bill). It covers the time from the ownership change or new construction completion date to the following June 30th. It is important to understand that these Supplemental Assessments are ‘in addition’ (supplemental) to the annual property tax bill. Your regular tax bill is still due and payable at its stated time.

Have you recently purchased a property?
Please visit our Supplemental Tax Estimator page for assistance in estimating your supplemental assessment.

The tax bills or refunds for these assessments are mailed directly to the new owner of the property. They are not available during the escrow process, and are the responsibility of the new owner. This is true even if the regular annual property taxes are paid by a lender through an impound account. Please discuss this matter carefully with your lender. In any case, do not presume that your lender will automatically pay these bills.

Learn the History of Supplemental Assessments

Supplemental assessments were created to address the perceived inequalities in the Proposition 13 tax system. Under Proposition 13, property can only be reappraised when there has been an ownership change or new construction. Prior to supplemental assessment, assessed value (the value used to calculate your tax bill) changes from reappraisable events would not be taxed until the following tax year. This resulted in tax treatment that was significantly different for reappraisable events that may have been only a day apart. Supplemental assessments were created to address this concern.

The Notice of Supplemental Assessment explains how the reappraisal of your property affected your assessment. If the new value1 is greater than the existing value2, you will receive a bill for the increase in value. If the new value1 is lower than the existing value2, you will receive a credit or refund.

Detailed Explanation of Supplemental Assessments

Your Notice of Supplemental Assessment will display a grid similar to the following:

Date of Change of Ownership or Completion of New Construction Table

Suppose there is a change of ownership on September 10, 2013. On that date, the seller’s assessed value was $107,203. The purchase price was $215,000. On the first day of the month following the change of ownership, there are nine months remaining in fiscal year 2013 to 2014. The proration factor for this Supplemental Assessment is 0.75, based upon the nine remaining months of the twelve-month fiscal year. The resulting bill would be derived by multiplying 0.75 (9/12) times $107,797 (the value increase), times the applicable tax rate as set by the Board of Supervisors.


In some cases you may receive two or more supplemental tax bills. The number of supplemental bills is dependent on the date the ownership changed or new construction was completed. If the date is:

  • June 1 to December 31: You will receive 1 supplemental assessment prorated from your date of event to the following June 30th.
  • January 1 to May 31: You will receive 2 supplemental assessments. The first will be prorated from your date of event to the following June 30th. The second will be for the entire subsequent assessment year from July 1 to June 30th.

Related Information

State Board of Equalization’s Supplemental Assessment - Frequently Asked Questions