Weblearn what supplemental tax bills are, when and why they are issued, how they are calculated, and how to pay them. Find out how to dispute a bill if you think it is. Weblearn about supplemental assessment, a type of property tax bill issued when the assessed value changes due to ownership or new construction. Find out how to get a. Weba supplemental tax bill reflects any increase in property tax generated by a supplemental “event” (change in ownership or new construction).