Websearch secured, supplemental and prior year delinquent property taxes : Secured tax bills are payable online from 10/4/2023 to 9/1/2024. Most supplemental tax bills are. Weba supplemental tax bill reflects any increase in property tax generated by a supplemental “event” (change in ownership or new construction). Webalameda , ca — thousands of alameda county homeowners are about to get a pleasant surprise, with a significant reduction in their property taxes.