Supplemental assessments are the way taxes are divided between new and former property owners.
The supplemental roll provides a mechanism for placing property subject to Proposition 13 reappraisals due to change in ownership or completed new construction into immediate effect. Changes in ownership or completed new construction are referred to as 'supplemental events' and result in supplemental tax bills that are in addition to the annual property tax bill.
The increase (or decrease) in assessed value resulting from the reappraisal is reflected in a prorated assessment (a supplemental bill) that covers the period from the first day of the month following the supplemental event to the end of the fiscal year. A fiscal year runs from July 1 through June 30.
A supplemental assessment is an assessment that splits the taxes between two parties or between two appraisal events