On July 1, 1983, Senate Bill 813 changed the state Revenue and Taxation Code to create what are known as Supplemental Assessments. This new law changed the manner in which changes in assessed value were billed by requiring that any increase or decrease in taxes due to a change in ownership or completed new construction became effective as of the first day of the month following the date of change in ownership or the date new construction was completed rather than on the next annual tax bill.
Supplemental assessments result in a one-time tax bill that is "in addition to" (that is, supplemental to) the annual property tax bill sent to property owners in October. Changes in ownership or completed new construction that trigger supplemental assessments are referred to as "supplemental events."
A supplemental event results in a reassessment. A reassessment may be an assessed value increase resulting in a supplemental bill, an assessed value decrease resulting in a supplemental refund or no change in value.
Depending on the date of the supplemental event, either one or two supplemental tax bills will be produced. Supplemental events that occur between January 1 and May 31 will generate two supplemental bills. Supplemental events that occur between June 1 and December 31 will generate one supplemental bill.
You may also receive more than one supplemental tax bill if more than one supplemental event has occurred in a fiscal year. If this occurs the bills are pro-rated between each owner for the period of time the property was owned.