Journal ·
Why Did I Get a Supplemental Property Tax Bill After Buying a House?
Short answer: because California law requires the county to reassess your property as of the first day of the month after your purchase closed, and the supplemental bill charges you the difference between the new assessed value and the seller's old one for the rest of the fiscal year. It is a separate bill from the annual one. Your impound account almost certainly does not cover it, and it usually lands within about six months of closing.
What is a supplemental property tax bill in California?
Proposition 13 caps annual increases in assessed value, which means a long-held home can carry an assessed value far below what you paid. When ownership changes or new construction is completed, the county assessor reappraises, and the gap between the old assessed value and the new one becomes a supplemental assessment.
Los Angeles County states the rule plainly on its property tax portal: the supplemental bill is in addition to the Annual Secured Property Tax Bill, and both must be paid by the dates shown. It does not replace anything. Buyers who assume it replaced the annual bill end up with a 10 percent penalty.
The mechanics sit in Revenue and Taxation Code Section 75.41, which tells the auditor exactly how to compute the amount.
How is the supplemental tax amount calculated?
Four steps, in this order.
- The assessor sets a new assessed value as of the reassessment date and subtracts the prior assessed value. Exemptions come off. What is left is the net supplemental value.
- The auditor applies the current year's tax rate to that net value and computes what a full year would cost.
- The auditor multiplies by a proration factor set by statute, based on the presumed date, which is the first day of the month after closing or completion.
- The result is split into two equal installments.
The proration factors are fixed in the statute, not estimated:
| Presumed date | Factor on the current roll |
|---|---|
| August 1 | 0.92 |
| September 1 | 0.83 |
| October 1 | 0.75 |
| November 1 | 0.67 |
| December 1 | 0.58 |
| January 1 | 0.50 |
| February 1 | 0.42 |
| March 1 | 0.33 |
| April 1 | 0.25 |
| May 1 | 0.17 |
| June 1 | 0.08 |
| July 1 | no current-roll supplemental |
A worked example on Los Angeles County numbers. Say a Glendale house carried a prior assessed value of $420,000 and you buy at $1,150,000. The net supplemental value is $730,000. At a general levy of 1 percent, a full year on that difference is $7,300 before voted indebtedness and direct assessments. Close escrow on September 12 and the presumed date becomes October 1, factor 0.75, so the current-roll supplemental runs about $5,475, billed in two installments. Your actual rate will sit above 1 percent once voter-approved debt is added, so treat this as a floor.
Why did I get two supplemental bills?
Because of when you closed. A change in ownership or completion of new construction between January 1 and May 31 produces two supplemental assessments and two bills. The first covers the remainder of the fiscal year in which the event happened. The second covers the following fiscal year, at a factor of 1.00, because the annual roll for that year was already prepared using the seller's old value.
That second bill is the one that ambushes people. It arrives roughly a year after a spring purchase, long after the transaction feels finished.
There is also an Adjusted Supplemental Property Tax Bill, which replaces a supplemental bill for the same fiscal year when something changes, typically an exemption filed after the first bill went out. The county explains the fields on its adjusted supplemental page.
Does escrow or my impound account pay the supplemental bill?
Usually not, and this is the single most common misunderstanding.
Escrow prorates the annual property taxes between buyer and seller at closing, based on the bill that exists at that moment, which reflects the seller's assessed value. The supplemental bill does not exist yet. It is generated months later and mailed to the assessee of record.
Impound accounts are funded from the lender's estimate of the annual bill. Most servicers do not automatically advance supplemental bills, because the amount is unknown at funding. Some will pay it if you send it in, some will not. Check the servicing agreement rather than assuming, and if the servicer does pay it, expect an escrow analysis and a payment increase afterward.
The practical version: set money aside at closing. Roughly 1.2 percent of the difference between your purchase price and the seller's prior assessed value, multiplied by the statutory factor, gets you close enough to plan around.
When are supplemental bills due and what happens if I miss one?
Supplemental bills carry their own delinquency dates printed on the bill, and they do not follow the familiar November and February rhythm of the annual bill. Miss the first installment date and a 10 percent penalty applies. Miss the second and it is 10 percent plus a $10 cost. A returned check adds $50.
There is a cancellation provision, but it has no dollar figure in it. Revenue and Taxation Code Section 4986.8 says only that on recommendation of the tax collector, the auditor may cancel any tax bill if the amount is so small as not to justify the cost of collection. The threshold is a judgment call by the county, not a number written into the statute, so do not plan around it.
Mail forwarding is the quiet risk. The bill goes to the mailing address on file with the assessor. Buyers who move, or who bought a rental and never updated the mailing address, find out about the bill through the penalty. The county runs a mailing address change form and an e-notification service for exactly this reason.
How do I estimate the bill before I close?
The Los Angeles County Assessor publishes a supplemental tax estimator that takes the purchase price and closing month. Run it during your contingency period, not after.
Purchase prices in this part of the county set the size of the gap, and current local figures are in the August 2026 Burbank market update.
Two inputs make the estimate meaningful. First, pull the prior assessed value from the assessor's parcel record rather than guessing from the last sale price, because the Prop 13 base has been adjusting by up to 2 percent a year since that sale. Second, look at the direct assessments line on the current bill. Those are flat charges for things like sewer maintenance and lighting districts, they are not based on assessed value, and they carry over to you unchanged.
Does a supplemental bill mean my annual taxes went up too?
Yes, and the sequence confuses people. The supplemental bill covers the gap for the current fiscal year only. The annual bill then catches up at the next lien date, January 1, and the following year's Annual Secured Property Tax Bill reflects the new base value in full. So a buyer typically sees three separate documents in the first eighteen months: a prorated annual bill at the seller's old value, one or two supplemental bills, and then a permanently higher annual bill.
New construction triggers the same machinery. Finish an ADU and the assessor adds the value of the new construction to the base, without reassessing the rest of the house. The supplemental bill that follows covers only that added value.
Q: How long after closing does the supplemental tax bill arrive?
Los Angeles County says often within six months of the close of escrow or the completion of new construction, and notes the timeline varies considerably by property. Purchases in busy filing periods run longer.
Q: Is the supplemental bill a one-time charge?
For a change of ownership, yes, though a January-to-May closing generates two of them. After that the higher value simply appears on the regular annual bill.
Q: Can I pay the supplemental bill in installments?
The bill is computed in two equal installments with separate delinquency dates printed on it. Paying in full by the first delinquency date is also allowed.
Q: Does Proposition 19 change any of this?
Prop 19 affects whether a reassessment happens at all in certain transfers, including some parent-to-child transfers and base-value transfers for owners over 55. If no reassessment occurs, there is no supplemental assessment. Eligibility is fact-specific and has filing deadlines, so confirm with the assessor before assuming.
Q: What if I think the new assessed value is wrong?
The county runs a formal process for contesting value, starting with the assessor's regional office listed on the bill and escalating to the Assessment Appeals Board. Appeal windows are short and the bill remains due while an appeal is pending.
This article is general information about California property tax mechanics, not tax or legal advice. Figures are illustrative and rates vary by tax rate area. Confirm your own numbers with the county assessor or a tax professional. Call Alla at (818) 699-5367 or use the enquiry form.
Neighborhood-level context for buyers is in the Glendale area guide, and Alla's published commentary is collected on the press page.