California Supplemental Property Tax Bill, Explained
The surprise bill new California homeowners get a few months after closing, and how to plan for it.
A supplemental property tax bill is an extra, one-time bill California counties send after you buy a home or finish new construction. It taxes the increase in assessed value (your price minus the seller's old assessed value) for the months left in the July–June fiscal year. Buy between January and May and you get two. It's mailed to you, not your lender, and it has its own due dates.
Why it exists
Under Prop 13, a home is reassessed at market value when it changes hands. The county's regular bill for the year was already set from the January 1 assessment, which used the seller's lower value. Revenue and Taxation Code section 75 and the sections after it close that gap: the assessor enters a supplemental assessment for the difference, and the tax collector bills it separately.
How it's calculated
Supplemental tax = (new value − old value) × your tax rate × proration factor. The change is treated as happening on the first day of the next month, and the factor is the share of the fiscal year left after that date (R&TC 75.41).
Say you buy for $850,000 and the seller's assessed value was $320,000. The increase is $530,000; at a 1.1% rate that's $5,830 a year of extra tax.
| You close in | Supplemental bills | Total |
|---|---|---|
| September | One bill: $5,830 × 0.75 = $4,373 | $4,373 |
| April | $5,830 × 0.17 = $991 for this fiscal year, plus a full $5,830 for next year | $6,821 |
| June | One full-year bill for the fiscal year starting July 1: $5,830 | $5,830 |
Why two bills for a spring purchase? The January 1 lien date for next fiscal year has already passed with the seller's value, so next year's regular bill is also too low. The second supplemental bill covers that whole year.
When it's due
A supplemental bill is due when it's mailed and is paid in two installments (R&TC 75.52):
- Mailed July through October: the first half goes delinquent after 5 p.m. on December 10 and the second after April 10, the same dates as the regular bill.
- Mailed November through June: the first half goes delinquent at the end of the month after the bill is mailed, and the second half at the end of the fourth month after that.
A deadline on a weekend or holiday moves to the next business day. Late installments get a 10% penalty.
Your lender probably won't pay it
Even with an impound (escrow) account, supplemental bills go to the owner. Los Angeles County's tax collector says it sends supplemental bills only to the property owner, even if you have an impound account. If you want your lender to pay, contact it as soon as the bill arrives, and don't let the deadline pass while you sort it out.
How to plan for it
- Look up the seller's assessed value on the county's property tax portal before you close. The bigger the gap from your price, the bigger the bill.
- Set the money aside. As the example shows, it can easily run to thousands of dollars.
- File for the homeowners' exemption with the assessor if the home is your main residence; it lowers future bills.
Run your numbers
Our Supplemental Property Tax Calculator works out each bill from your closing date, both values and your county's rate, and gives the installment deadlines once you know the mailing date. For the ongoing annual bill, use the property tax calculator.
Frequently Asked Questions
Why did I get a supplemental property tax bill?
Because your home was reassessed at the purchase price (or after new construction), but the regular tax bill for the year was based on the old assessed value. The supplemental bill charges tax on the difference for the rest of the fiscal year.
Does my mortgage impound account pay the supplemental bill?
Usually not. The bill is mailed to you, the owner. Los Angeles County, for example, says it sends supplemental bills only to the property owner, even if you have an impound account. Pay it yourself or ask your lender whether it will.
Is the supplemental bill a one-time charge?
Yes. Once the regular annual bill reflects your new assessed value, there are no more supplemental bills for that purchase. Depending on the month you bought, that takes one or two supplemental bills.
What if I miss the supplemental bill deadline?
A 10% penalty is added to the late installment, and a cost charge applies after the second installment goes delinquent.
Can I get a supplemental refund?
Yes. If the new assessed value is lower than the old one, the county issues a negative supplemental assessment and refunds the difference. State law gives the auditor 90 days from enrolling it to pay.
This article is for general information only. Your county assessor sets the new value and your tax collector issues the bill. Sources: Revenue and Taxation Code 75.41, 75.43 and 75.52; Los Angeles County Treasurer and Tax Collector.