What Is a Supplemental Property Tax Bill, and How Much Should I Set Aside After Buying in Kern County?

Under Proposition 13, a California home's assessed value is set when it is purchased and can only rise by up to 2% a year after that. A seller who bought years ago is often paying tax on a value far below today's market. When you buy, the county reassesses the home at your purchase price as of the date ownership changed.

Your first regular tax bill was already calculated on the seller's old value, so the county sends a supplemental bill to collect the difference from your ownership date through the end of the tax year, which runs July 1 to June 30. After that, your regular bills reflect the new assessed value.

Timing matters more than most buyers realize. If you close between June 1 and December 31, you will typically receive one supplemental bill covering the rest of that tax year.

If you close between January 1 and May 31, you will typically receive two. The next year's regular tax roll is locked in on the January 1 lien date, before your purchase, so it still uses the seller's old value. The county sends one supplemental bill for the rest of the current tax year and a second one covering the entire next tax year. Buyers who close in spring are the ones most often caught off guard, because the second bill can be the larger of the two.

Your monthly mortgage payment typically includes an impound for your regular property tax bills, and your servicer pays those for you. Supplemental bills are different: they are mailed to the owner of record, and most servicers do not pay them automatically. If you would prefer your servicer pay it from escrow, call them as soon as the bill arrives, since some will.

Separately, once the county updates your regular bill to the new assessed value, your servicer's annual escrow review will usually raise your monthly payment to match. That is a second, smaller adjustment to plan for in your first year.

The most accurate way is the Kern County supplemental tax estimator, run by the Treasurer-Tax Collector. Enter the property and your purchase details and it estimates the bill.

For a rough idea before you have the details: take your purchase price minus the seller's current assessed value (shown on the property's tax bill or the county assessor's records), then multiply by your area's tax rate. As an illustration only, using a 1.25% rate: a $370,000 purchase on a home assessed at $220,000 is a $150,000 difference, or about $1,875 for a full year, prorated for the months left in the tax year. Your actual rate depends on the home's tax rate area.

If you buy for less than the current assessed value, which is less common, the supplemental assessment can work in reverse and result in a refund.

On a newly built home, the seller's assessed value often reflects only the land, or a partly completed home, so the supplemental bill can be much larger than on a resale. Many newer Kern County subdivisions also carry Mello-Roos or other special assessments, which appear on your regular tax bill rather than the supplemental bill. Ask for the full tax picture before you write an offer on new construction.