What is a supplemental tax bill?
In addition to annual taxes, you may be responsible for paying supplemental property taxes. State law requires the Assessor to reappraise property upon a change in ownership or new construction. The supplemental assessment reflects the difference between the new assessed value and the old or prior assessed value. If the property is reassessed at a higher value than the old assessed value, a supplemental bill will be issued. If the property is reassessed at a lower value than the old assessed value, a refund will be issued.
The taxes are prorated based on the number of months left in the fiscal year from the date of ownership change or the new construction completion date. If the change in ownership or new construction occurs between January 1st and May 31st, two supplemental tax bills will be issued. The first supplemental bill will be for the remainder of the fiscal year, and the second supplemental bill will be for the fiscal year that follows.
Supplemental tax bills are mailed directly to the property owner and are your responsibility. In general, they are
NOT paid out of your impound account. Please check with your lender.
This source from LA County site
more info LA county site here