What Is a Supplemental Property Tax Bill, and Why Did I Get One After Buying a Home?

When you buy a home in California, the county assessor resets its assessed value to your purchase price, and the gap between the old assessed value and the new one generates a one-time supplemental property tax bill separate from your regular annual bill. It isn't a mistake, a scam, or a duplicate charge — it's how Proposition 13's reassessment-on-sale rule catches your property taxes up to the new purchase price for the remaining part of the fiscal year you've owned the home. Here's how the bill is calculated, when it's due, and what happens if you ignore it.

 

Why Do I Get a Supplemental Tax Bill After Buying a Home?

 

A supplemental assessment is triggered any time there's a change in ownership or newly completed construction on a property. Under Proposition 13, a property's assessed value normally only rises a small, capped amount each year while the same owner holds it — which means a seller who has owned a home for a decade or more may still be paying taxes based on a much lower assessed value than the home's current market price. When you close on the purchase, the county assessor resets the assessed value to your purchase price, and the increase between the seller's old assessed value and your new one is billed separately, on top of the regular annual tax bill you'll also receive.

 

How Is the Supplemental Tax Amount Calculated?

 

The supplemental tax bill applies the standard 1% tax rate to the "net supplemental value" — the difference between your new assessed value and the prior assessed value, minus any exemptions you qualify for, such as a homeowners' exemption. That amount is then prorated based on how much of the current fiscal year (which runs July 1 through June 30) remains after your purchase closes. Buy in August and you'll owe roughly eleven-twelfths of a full year's difference; buy in May and you'll owe closer to one-twelfth, though a second bill for the following fiscal year often follows.

 

When Is the Supplemental Tax Bill Due?

 

Los Angeles County supplemental tax bills come in two installments. The first is due November 1 and becomes delinquent (with a 10% penalty) after December 10. The second is due February 1 and becomes delinquent (with a 10% penalty plus a $10 fee) after April 10. If your purchase or new construction closes between January 1 and May 31, state law requires two separate supplemental assessments — one for the remainder of the current fiscal year and one for the following fiscal year — so you may receive two supplemental bills instead of one.

 

Does My Mortgage Lender Pay This for Me?

 

Usually not automatically. Even if you have an impound or escrow account with your lender that collects money monthly for your regular property taxes, the county tax collector sends the supplemental bill directly to you, the property owner — not to your lender. Many impound accounts aren't set up to anticipate this one-time bill, so it's worth budgeting for it separately and confirming with your lender whether they'll cover it or whether it's on you to pay directly.

 

What Happens If I Ignore the Supplemental Tax Bill?

 

The bill doesn't go away if you don't act on it. Failing to receive a bill in the mail doesn't excuse you from paying on time or from the late penalties, so if you've recently purchased a home and haven't seen a supplemental bill yet, it's worth checking with the county assessor or tax collector directly rather than assuming none is coming. Unpaid supplemental taxes can eventually lead to the same collection consequences as unpaid regular property taxes.

 

Frequently Asked Questions

 

What if I never received my supplemental tax bill in the mail?

Not receiving the bill doesn't excuse late payment or waive the penalty. Contact your county assessor's office or tax collector directly to check whether a supplemental bill has been issued for your property.

 

Will I get a refund if my new assessed value is actually lower?

Yes. If your purchase price results in a lower assessed value than the property previously carried, the county issues a refund rather than a supplemental bill.

 

How many supplemental bills should I expect?

Most buyers receive one. If your purchase or completed construction falls between January 1 and May 31, expect two, covering two different fiscal years.

 

Is the supplemental bill something I'll pay every year?

No — it's a one-time bill tied specifically to the change in ownership or completed construction. Your ongoing annual property taxes are billed separately going forward, based on your new assessed value.

 

Updated for September 2026.