Supplemental Assessments

Whenever there is a reappraisal due to a change in ownership or completion of new construction, the Assessor is required by law to make a supplemental assessment which reflects the difference between the prior assessed value and the new assessment. This value is prorated based on the number of months remaining in the fiscal year, ending June 30th. This assessment is in addition to the regular tax bill. Notices of supplemental assessments are mailed out to property owners prior to the issuance of the tax bill. Usually there is only one supplemental assessment per event, but events that occur from January through May affect two fiscal years and require two supplemental assessments
The video below explains in more detail.

For more information about supplemental assessments, visit the California Board of Equalization's FAQ on Supplemental Assessments.