Supplemental Taxes & Estimator

Contact the Property Tax Division by phone (530) 621-5470, ext. 4 or email

General Information

Supplemental tax is "catch up" property tax that results from the reassessment of property when a qualifying change in ownership or completion of new construction occurs. Supplemental tax is calculated on the difference between the assessed value of the supplemental event minus the most current assessed value on the roll for the number of full months remaining in the tax year. Supplemental taxes are calculated pursuant to California Revenue and Taxation Code, commencing with Section 75.

Supplemental taxes may not be calculated until 30 days following the Assessors 'Notice of Supplemental Assessment'. Supplemental taxes are calculated throughout the year and are typically secured. Supplemental taxes are in addition to, rather than in replacement of, secured lien date property tax bills. Supplemental taxes will not cause the respective secured lien date property tax bill to be recalculated.

Supplemental tax calculations may result in a negative amount that is commonly referred to as a supplemental refund. A supplemental refund may be eligible to be paid even though other tax bills for the property are not paid.

If a subsequent change of ownership occurs before a supplemental tax is billed, the bill may need to be prorated among multiple assessees/owners (R&T§75.54(c)). If the Auditor is unaware of the subsequent sale at the time the supplemental tax is calculated, this proration doesn't automatically occur and is manually calculated upon notification to the Auditor. Please contact the Auditor-Controller, Property Tax Division for any questions regarding proration of supplemental taxes.

Important information regarding the estimating tools:

  • No warranty is made to the User regarding the estimating tools.
  • Use of these estimating tools is at the sole discretion of the User.
  • These estimating tools are not a tax bill. Do not attempt to pay the Tax Collector based on these estimating tools.

Links & Lookups

Historical Perspective

Prior to supplemental property taxation, reassessments were not effective until the next annual lien date. Thus, properties with a change in ownership or new construction avoided assessed value changes for up to 16 months (18 months once the lien date changed from March 1 to January 1 in 1997). Beginning in September 1983 (Senate Bill 813), changes in ownership or new construction were reassessed as of the date of the supplemental event.

Supplemental taxes were initially a method to address shortfalls in school funding. After several years, the State Legislature changed the distribution of tax revenue derived from supplemental taxes. Today, state statutes direct supplemental taxes to be shared among the majority of taxing jurisdictions who receive part of Proposition 13 lien date property tax roll's 1% general taxes. Any "voter debt" amounts on the supplemental tax is restricted to repay the voter-approved debt.