In California, when a property changes hands or completes new construction mid-year, the county issues supplemental tax bills to capture the increase in assessed value. Depending on when the change occurs you may even receive two separate supplemental bills: one prorated from the date of change through the end of the current fiscal year (June 30), and a second covering the period from July 1 until the next regular roll is posted. Under Proposition 13, base values only reset on sales or significant improvements, so these supplemental bills make sure you pay the correct prorated taxes on your property’s new assessed value.

This calculator, powered by ParcelQuest, makes it easy: start by entering an address or parcel number to see a light overview of parcel information—APN, owner and mailing address, legal description, assessed values (land, structures, total), exemptions, tax rate area, sale history and more. From there you can estimate your supplemental tax bill by simply entering the date of sale (or, for new construction, the completion date) and the purchase price or post-construction value; the tool then computes the prorated supplemental tax due for the rest of the year.