02 What you need to know
How California supplemental property taxes work
In California, a home is reassessed to its market value when it changes ownership or when new construction is completed. The rules behind this are in the Revenue and Taxation Code, starting at section 75, and are often called “supplemental assessment” rules.
The problem is timing. Your regular annual tax bill is based on the value as of the January 1 lien date, so it can still reflect the seller’s lower value. The supplemental bill collects the tax on the difference between your new assessed value and the old one.
How the bill is calculated
- The county takes the increase in assessed value and multiplies it by your area’s tax rate.
- That annual amount is prorated for the months left in the fiscal year, which runs July 1 through June 30.
- The proration starts on the first day of the month after the purchase or completion date.
When you get two bills
If you close between January 1 and May 31, you generally get two supplemental bills. One covers the rest of the current fiscal year. The second covers the full next fiscal year, because that year’s regular roll was already set using the January 1 value, before you bought.
Who pays it
An impound account (also called escrow) is the account your lender uses to pay your annual taxes and insurance. LA County’s Treasurer and Tax Collector says lenders do not usually pay supplemental bills. The bill is mailed to you, and paying it is on you.
Supplemental bills have their own due dates, printed on the bill, which may not line up with the regular November and February installments. If the new value is lower than the old one, the supplemental can be a refund instead of a bill.
03 Real-world example
Estimating a supplemental tax bill in LA County
Say the home you buy was assessed at $500,000 under the prior owner, and you pay $1,000,000. Your assessed value goes up by $500,000.
Case 1: you close in mid-October
The supplemental period starts November 1 and runs through June 30, which is 8 of 12 months. On the proration table printed on LA County supplemental bills, the factor for a November 1 start is 0.67, which is 8/12 rounded.
| Step | Amount |
|---|---|
| New assessed value | $1,000,000 |
| Prior assessed value | $500,000 |
| Increase | $500,000 |
| × illustrative 1.2% tax rate | $6,000 per year |
| × 0.67 proration factor (Nov 1 to Jun 30) | $4,020 |
Using a straight 8/12 you’d get $4,000. The county’s rounded factor makes it $4,020.
Case 2: you close in mid-February
Now the period starts March 1, leaving 4 months in the fiscal year (factor 0.33). Because you closed between January 1 and May 31, a second bill covers the full next fiscal year.
| Bill | Period | Amount |
|---|---|---|
| Supplemental bill 1 | Mar 1 to Jun 30 (0.33) | $1,980 |
| Supplemental bill 2 | Full next fiscal year (1.00) | $6,000 |
| Total supplemental | $7,980 |
Assumptions: illustrative 1.2% total rate (1% base plus voter-approved debt; your actual rate depends on your tax rate area). Ignores the homeowners’ exemption, direct assessments and small value adjustments. The county’s actual figures will differ.
04 What most people overlook
What buyers miss about supplemental tax bills
- It arrives late. LA County says processing generally takes around six months, so the bill can show up well after you’ve stopped thinking about closing costs.
- It’s not a mistake or a duplicate. It’s in addition to your regular annual bill, not a replacement for it.
- Your impound account usually won’t cover it. If you forward it to your servicer and assume it’s handled, it can go delinquent and pick up penalties.
- Your escrow payment may still rise later. Once the annual roll catches up to your purchase price, your servicer’s annual escrow review will reflect it.
- The seller’s tax bill isn’t your budget. See why your purchase price sets your property taxes.
Set aside the estimated supplemental amount in savings right after closing. When the bill comes, it’s already handled.
05 How I’d approach it
How I’d plan for a supplemental tax bill
The supplemental bill isn’t a surprise if we put it on the calendar before you close.
I pull the seller’s current assessed value from the county and compare it to your purchase price. That gap is what drives the supplemental.
Then I estimate it using your property’s actual tax rate area, not a generic number, and note your closing month so we know whether to expect one bill or two.
- Estimate the supplemental amount and keep it in reserves after closing.
- File your homeowners’ exemption promptly, since it can reduce the supplemental too.
- Run your full payment at the new value in the mortgage payment calculator so the ongoing escrow amount isn’t a surprise either.