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Supplemental Property Taxes

California State law requires the reassessment of property as of the first day of the month following an ownership change or the completion of new construction. New construction is categorized as any substantial addition to real property (e.g., adding a new room, pool, or garage) or any substantial alteration which restores a building, room, or other improvement to the equivalent of new (e.g., completely renovating an outdated kitchen). In most cases, this reassessment results in one or more supplemental tax bills being sent to the property owner in addition to the annual property tax bill.

Property owners who have impound accounts should contact their lender regarding payment of supplemental tax bills.

Most changes in ownership caused by the sale of property result in reassessment. However, there are a few specific circumstances that are excluded from reassessment. For example inter-spousal transfers, the transfer, sale, or inheritance of property between parents and their children, and the addition of joint tenants do not result in the reappraisal of property values.

Additional circumstances are outlined by Proposition 19, which was passed by voters in November 2020. This proposition allows persons over age 55, or severely disabled of any age, to transfer the “taxable value” of their primary residence (original) to a replacement residence anywhere in the state. Proposition 19 also allows victims of a wildfire or natural disaster to transfer the taxable value of their primary residence to a replacement residence anywhere in the state. Finally, proposition 19 excludes transfers of property between parents and children and between grandparents and grandchildren, under limited conditions, from reassessment. For further information, please review the California State Board of Equalization’s Proposition 19 web page. If you have questions about eligibility or applying for a Proposition 19 base transfer, please contact the Office of the Assessor at (707) 565-1888.

Computation of Supplemental Tax Bills

The Assessor first determines the new value of the property based on current market values. The Assessor then calculates the difference between the new value (set at the time of purchase or completion of new construction) and the old value (set on January 1 of the previous fiscal year). The result is the supplemental assessment value. Once the new assessed value of your property has been determined, the Assessor will send you a notification of the amount to be assessed.


  • New value at date of purchase or completion of new construction $120,000
  • Assessed value for current fiscal year $100,000
  • Supplemental assessment value will be $20,000

This reassessment usually results in an increase in property value, in which case your supplemental taxes will be calculated by the Auditor-Controller based on the change in value. After the taxes are calculated, one or more supplemental tax bills will be created and mailed to you by the Tax Collector.

In some instances the reassessment results in a reduction in value, in which case a refund will be prepared by the Auditor-Controller and mailed to you. Please note: A reduction in value will not reduce the amount due on the annual tax bill. The annual tax bill must be paid in the amount originally billed.

Notice of Supplemental Assessment

The Assessor will send a Notice of Supplemental Assessment after a value has been determined.  The Supplemental tax bills will be mailed at least 35 days after the date on the notice of supplemental assessment. The supplemental bills will have due dates that do not correspond with annual secured due dates.

Due Dates

The date on which supplemental bills become delinquent varies depending upon when they are mailed by the Tax Collector.

If the bill is mailed between July 1st and October 30th the taxes become delinquent following the schedule as for annual tax bills mailed in November

  • The first installment is delinquent after 5:00 p.m. on December 10th.
  • The second installment is delinquent after 5:00 p.m. on April 10th.

If the bill is mailed between November 1st and June 30th, the delinquency dates - which are printed on the bill - are determined as follows:

  • The first installment is delinquent after 5:00 p.m. on the last day of the month following the month the bill was mailed.
  • The second installment is delinquent after 5:00 p.m. on the last day of the fourth month after the first installment delinquency date.

Reading Supplemental Tax Bills

When reviewing your these additional bills, it is important to carefully read the important messages as well as review the County Values section. The important messages will provide a brief description of the event which caused the reassessment, the event date and the time period the bill covers. In the county values you will see values in each of the 3 columns, (Prior, Current, This Bill), showing the change in value and the value difference that is being taxed. Please visit our section on Reading Your Tax Bill for more detail.

Should you have questions about a valuation, please contact the Assessor at 707-565-1888. If you have questions about the tax bill or how to pay it, please contact the Tax Collector at 707-565-2281.


Supplemental bills follow the same penalty structure as annual secured bills. Penalties of 10% are added to any installment that is not paid on time, and an additional $20 charge is added to a late second installment. For more detailed information visit our Due Dates and Penalties section.

The supplemental tax bill is sent in addition to the annual tax bill and both must be paid as specified on the bill. For information on any of these topics, see our FAQ.