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Why Did I Get a Supplemental Property Tax Bill After Buying a House?
You got a supplemental bill because Los Angeles County reassessed the house as of the first day of the month after your purchase closed, and the bill charges you the difference between the new assessed value and the seller's old one for the rest of the fiscal year. It usually lands within about six months of closing, it is separate from the annual bill, and your impound account almost certainly does not cover it. To size it before you close, take the gap between your price and the seller's prior assessed value, multiply by the tax rate for the parcel's tax rate area, which for 2025/2026 runs from about 1.09 percent in parts of Glendale to 1.19 percent in parts of the City of Los Angeles, then multiply by the statutory proration factor for your closing month in the table below.
What is a supplemental property tax bill in California?
Proposition 13 caps annual increases in assessed value, so a long-held home can carry an assessed value far below what you paid. When ownership changes or new construction is completed, the assessor reappraises, and the gap between the old value and the new one becomes a supplemental assessment. The mechanics sit in Revenue and Taxation Code Section 75.41.
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. Buyers who assume it replaced the annual bill end up with a 10 percent penalty.
Supplemental tax bill vs annual property tax bill
They come from different offices, on different schedules, for different periods, and one does not cancel the other.
| Annual Secured bill | Supplemental bill | |
|---|---|---|
| Covers | July 1 to June 30 | the year after your closing only |
| Based on | the roll value at January 1 | new value minus old value |
| Arrives | every October | once, within about six months |
| Due dates | November 1 and February 1 | printed on the bill |
| Paid by impound | usually yes | usually no |
It is a one-time catch-up charge, not a recurring tax.
What is a Notice of Supplemental Assessment?
Before the bill arrives, the assessor mails a separate document called a Notice of Supplemental Assessment. People mistake it for the bill, set it aside, and lose two deadlines.
Revenue and Taxation Code Section 75.31 lists what it must show: the new base year value, the taxable value already on the current roll, the date of the change in ownership or completion, the amount of the supplemental assessments, any exempt amount, and the date the notice was mailed.
Two clocks start here, and they are not the same clock.
- Thirty days to claim an exemption, counted from the date on the notice, under subdivision (b). The homeowners' exemption is the usual one.
- Sixty days to appeal, counted from a different document in Los Angeles. Subdivision (c)(1) runs 60 days from the mailing date on the notice. Subdivision (c)(2) names the County of Los Angeles specifically and runs those 60 days from the mailing date printed on the tax bill instead.
That second point is written into the statute and it is easy to miss. An owner who applies the statewide rule here will get the wrong date.
How is the supplemental tax amount calculated?
Four steps, in this order.
- The assessor subtracts the prior assessed value from the new one and takes off exemptions. What is left is the net supplemental value.
- The auditor applies the current year's tax rate to that net value.
- The auditor multiplies by a statutory proration factor keyed to the presumed date, the first day of the month after closing.
- 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. A Glendale house carries a prior assessed value of $420,000 and you buy at $1,150,000, so the net supplemental value is $730,000. The rate in step two is not a flat 1 percent. It is the rate the Auditor-Controller publishes for the parcel's tax rate area, and for a Glendale parcel in area 04045 the 2025/2026 rate is 1.085398 percent, which makes the full-year figure $7,923. Close on September 12, the presumed date becomes October 1, factor 0.75, and the current-roll supplemental runs about $5,943 in two installments.
The rate moves with the city line. For the same year the Auditor-Controller's lookup shows 1.107902 percent for a Burbank parcel in area 02530 and 1.187380 percent for a City of Los Angeles parcel in area 00013. How that line splits one neighborhood is worked through in Toluca Lake, Burbank or Los Angeles.
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 ambushes people. It arrives roughly a year after a spring purchase, long after the transaction feels finished.
An Adjusted Supplemental Property Tax Bill replaces a supplemental bill for the same year when something changes, usually 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 taxes at closing using the bill that exists at that moment, which reflects the seller's value. The supplemental bill does not exist yet. The escrow timeline ends long before the assessor reaches your file.
Impound accounts are funded from the lender's estimate of the annual bill, and most servicers do not advance a supplemental bill because the amount is unknown at funding. Check the servicing agreement rather than assuming, and if the servicer does pay it, expect an escrow analysis and a higher payment afterward.
Set money aside at closing instead. The parcel's tax rate, between about 1.09 and 1.19 percent across Glendale, Burbank and Los Angeles in 2025/2026, times the gap between your price and the seller's prior assessed value, times the statutory factor, is close enough to plan around. Running that number before you write an offer is part of representing a buyer.
When are supplemental bills due and what happens if I miss one?
Supplemental bills carry their own delinquency dates and do not follow the November and February rhythm of the annual bill. Miss the first installment 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 address on file with the assessor, so buyers who moved learn about it through the penalty. The county runs a mailing address change form 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.
Two inputs make it meaningful. Pull the prior assessed value from the assessor's parcel record, not the last sale price, because the Prop 13 base has been adjusting by up to 2 percent a year since then. Then read the direct assessments line: flat charges for sewer maintenance and lighting districts that carry over to you unchanged.
The price side of the gap starts with a home valuation, and current figures are in the August 2026 Burbank market update.
What if I think the new assessed value is wrong?
You appeal to the Assessment Appeals Board, and in Los Angeles County those 60 days run from the mailing date on the tax bill. Since October 1, 2021 the county charges a $46 non-refundable filing fee per application, with a hardship waiver on request.
This is a different window from the regular annual appeal and its November 30 deadline. Confusing the two is the usual way owners miss a supplemental appeal. The bill stays due while an appeal is pending, so pay it and seek a refund rather than withholding payment.
Does a supplemental bill mean my annual taxes went up too?
Yes, and the sequence confuses people. The supplemental bill covers the current fiscal year only. The annual bill catches up at the next lien date, January 1, and the following year's Annual Secured bill reflects the new base value in full. A buyer typically sees three documents in eighteen months: a prorated annual bill at the seller's old value, one or two supplemental bills, then a permanently higher annual bill.
New construction runs the same machinery. Finish an ADU and the assessor adds only that new value to the base.
Proposition 19 can remove the reassessment entirely in some transfers, including certain parent-to-child transfers and base-value transfers for owners over 55. No reassessment means no supplemental assessment, but eligibility is fact-specific and carries its own deadlines.
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 sellers planning around a new assessment can start with preparing a sale.