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.
How it Works
When a supplemental event occurs, the county assessor determines the current market value of the property that changed ownership or that was newly constructed. The assessor then subtracts the property's prior assessed value from its newly assessed value, and the difference between the two is the net supplemental value that will be assessed and enrolled as a supplemental assessment. The supplemental assessment may be either a positive amount or, in the case of a reassessment that is less than the prior assessed value, a negative amount.
If the net supplemental assessment is positive, the increase in taxes will be calculated by the county auditor-controller based on the change in value. One, or possibly two, supplemental tax bill(s) will be generated and mailed by the county tax collector. If the net supplemental assessment is negative (a reduction in value), the auditor-controller will issue one, or possibly two, supplemental refund(s).
Once the new assessed value of your property has been determined, the county assessor will send a "Notice of Supplemental Assessment." This notice will show what the net supplemental assessment amount is and how it was calculated.
A supplemental reduction in value will not reduce (nor can it be used as a credit toward) the amount still due on an existing annual tax bill. The amount of tax shown on the existing annual tax bill must be paid even if the assessed value of the property was reduced by a supplemental assessment.
Supplemental bills (or refunds) are calculated based on the number of months remaining in the current fiscal year after the month in which the supplemental event occurs. A fiscal year runs from July 1 through June 30.
If a supplemental event occurs between June 1 and December 31, only one supplemental tax bill or refund check is issued. This bill, or refund, accounts for the property's change in value for the period between the first day of the month following the event date and the end of the current fiscal year (i.e., the following June 30). If, however, a supplemental event occurs between January 1 and May 31, two supplemental tax bills or refunds are issued. The second bill or refund accounts for the property's change in value for the entire 12 months of the coming fiscal year, beginning on the following July 1.
The tax or refund amount resulting from a supplemental assessment becomes effective on the first day of the month following the month in which the supplemental event took place; monthly proration factors are used to calculate the taxes owed. Taxes supplemental to the current roll are computed by first multiplying the net supplemental assessment by the tax rate, and then multiplying that amount by a monthly proration factor.