The State of California passed the supplemental assessment law in 1983 to provide additional funding primarily for schools. It merely accelerates the effective date of reappraisals made according to Proposition 13 laws. 

Since then, whenever there is a change in ownership or completion of a new construction, the Assessor shall reappraise the property at its full cash value (unless there is a qualifying exclusion or exemption from reassessment). The appraised value then becomes the new base year value of the property.

The Assessor then sends a supplemental assessment notice which reflects the difference between the prior assessed value and the new assessed value. The value difference is prorated based on the number of months remaining in the fiscal year ending June 30. A supplemental assessment is in addition to and separate from the regular/annual tax bill.

You pay the supplemental tax bill sent by the Treasurer-Tax Collector.

A supplemental tax bill is an additional property tax bill based on the difference between the prior assessed value and the new assessed value of real property. A supplemental tax bill is in addition to the regular tax bill, which is based on the assessed value as of January 1. The supplemental tax bill is sent directly to you by the Treasurer-Tax Collector rather than to your mortgage company as may be the case with the regular property tax bill.

The supplemental tax bill covers the period of time from the change of ownership date or completion of new construction to the end of the fiscal year (June 30). If the change of ownership or new construction occurs between January 1 and May 31, a second supplemental assessment will be required for the next fiscal year (July 1 - June 30). If two assessments are required, two separate bills will be issued.

In some cases it can take up to a year for the supplemental tax bill to be issued after the close of escrow, or completion of new construction. If the new value of the property is less than the previous assessed value, there will be a refund issued to the owner. If another sale or transfer of the property occurs before the mailing of the supplemental tax bill, the supplemental taxes will be prorated between the owners.

If you disagree with the value, you must file an assessment appeals application with the Clerk of the Board within 60 days of the mailing date of the supplemental value notice.