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Supplemental Property Tax

Supplemental assessments were established pursuant to provisions in Senate Bill 813 (known as the Hughes-Hart Educational Reform Act of 1983), enacted on July 29, 1983, and Article XIII A (Proposition 13) of the California Constitution, passed by the voters on June 6, 1978.

Article XIII A generally requires reappraisal of property at its fair market value as of a change of ownership or completion of new construction.  SB813 requires that a supplemental assessment be enrolled for the amount of the difference in value between that which was on the regular assessment roll(s) and the new fair market value.  SB813 also requires that taxes be collected on the supplemental assessment, in addition to the taxes collected on the regular assessment roll.

The effective date is used to determine the proration factor(s) applied to the tax rate.  The Auditor applies the current year's tax rate adjusted by a proration factor to reflect the portion of the tax year remaining.  In computing the portion of the tax year remaining, the Auditor established the first day of the month following the date on which the change in ownership or completion of new construction occurred.  The following table reflects the proration factor applied for each month.

Proration Factor(s)

Effective Date Current Roll Roll being prepared


July 1
August 1
September 1
October 1
November 1
December 1
January 1
February 1
March 1
April 1
May 1
June 1









Taxes on the supplemental roll become a lien against the real property on the date of the change in ownership or completion of new construction.  If there is another change in ownership before the supplemental billing is made, a portion of the supplemental assessment is entered on the supplemental unsecured roll in the name of the person who would have been the assessee if the additional changes in ownership had not occurred, and are treated and collected like other taxes.  The remaining portion of the supplemental tax remains a lien against the real property.  The taxes on each roll are based upon the proportional days of ownership.

Taxes on the supplemental bill are due on the date mailed and shall become delinquent as provided by law.  For property on the secured roll and certain property entered directly to the unsecured roll, the taxes due may be computed in two equal installments.  The supplemental bill shall become delinquent as follows: