Supplemental Assessment

When the Assessor's Office reappraises your property due to a change in ownership or completion of new construction, you will be notified by mail of the new assessed value. This notification is known as a “Notice of Supplemental Assessment.” If the new value (Base Year Value) is greater than the taxable value on the roll, the supplemental assessment will result in a bill. The supplemental tax bill is issued only on the added value, and unlike the regular tax bill which covers the entire fiscal year (July through June), the supplemental bill is prorated to cover the period from the date of ownership change or completion of new construction to the end of the fiscal year.

It is important to understand that supplemental tax bills are in addition to the regular annual tax bill (that is, supplemental to) and as required by law, are mailed directly to the owner of the property. Unlike the annual property tax bills that are mailed in October, lenders do not receive the supplemental tax bill. These bills are the responsibility of the owner even if the regular annual property taxes are normally paid by a lender through an impound account.

Supplemental Tax Estimator

To estimate what your supplemental taxes might be, click here.

Frequently Asked Questions

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• What is the law that allows supplemental assessments?
  • On July 1, 1983, the California State Legislature passed Senate Bill 813 to amend the state Revenue and Taxation Code to create what are known as "supplemental assessments". The intent of the State Legislature was to fully implement Proposition 13 and to promote increased equity among taxpayers by enrolling and making adjustments of taxes resulting from changes in assessed value due to changes in ownership and completion of new construction at the time they occur. Revenue and Taxation Code sections 75-75.72 detail the laws governing the supplemental assessment process.
• What does “change in ownership” or “new construction” mean?
  • Please refer to the topics Change in Ownership Reappraisal and New Construction Reappraisal for definitions and more information.
• What is a supplemental tax bill?
  • A supplemental tax bill reflects any increase in property tax generated by a supplemental “event” (change in ownership or new construction). A supplemental assessment becomes effective on the first day of the month following the month in which the supplemental event takes place. For example, if an assessable transfer of title occurred on October 10th, any increase or decrease in taxes resulting from that event becomes effective November 1st. In this example the supplemental bill is prorated to cover the increased assessment from November 1 through June 30, the end of the fiscal year.
• What happens if a change in ownership causes the base year value of the property to decrease?
  • If the new Base Year Value is lower than the Taxable Value on the roll, the result is an assessment decrease. The Notice of Supplemental Assessment will indicate the decrease with a negative sign in the “Amount of this Supplemental Assessment.” Assuming that the regular tax bill is paid based on the value on the assessment roll, the decrease will generate a one-time supplemental refund.
• When will I get my supplemental bill?
  • The Tax Collector will issue a bill after the event has been reappraised by the Assessor. There is no statutory timeline for the issuance of supplemental bills as there are for annual bills. Supplemental bills are issued year-round and most bills are mailed within 12 months after a change in ownership or completion of new construction.
• Am I entitled to a Homeowners' Exemption on my supplemental tax bill?
  • If the property is not already receiving a homeowner’s exemption for the same fiscal year the supplemental assessment applies to, you might be eligible. Please call our Homeowner’s Exemption staff at (510) 272-3770 for more information.
• Can I file an appeal of a supplemental assessment?
  • Appeals (also called an “Application for Reduced Assessment”) of supplemental assessments must be filed with the Assessment Appeals Board (not the Assessor) within sixty (60) days of the mailing date shown on the Assessor’s “Notice of Supplemental Assessment.” However prior to filing an appeal, if you believe the value is incorrect, we recommend that you contact our office and request an “informal review” as soon as possible after receiving your notice. If you can provide the Assessor with convincing evidence that the assessment is incorrect, it could be corrected without filing an appeal and attending a hearing.