Supplemental Tax Bills in California: Why You Get One After Buying
A supplemental tax bill is a one-time property tax bill that California counties send after a change in ownership or completed new construction. It charges tax on the difference between the property’s new assessed value and its old one, for the months left in the fiscal year. It comes in addition to the regular annual bill, it is mailed to the owner rather than the lender, and most buyers owe one in the first year.
The bill exists because of Proposition 13. When you buy, the county reassesses the home to market value as of the first of the month after the purchase, but the annual bill for that year was already set using the seller’s lower value. The supplemental bill covers the gap. The rules are in Revenue and Taxation Code sections 75 through 75.72 and apply the same way in Sonoma, Napa, Marin, San Francisco and every other county, though each county runs its own billing.
This is general information, not tax advice. For questions about your assessment, call the county assessor, and ask a tax adviser how the bill affects your own return.
How the supplemental amount is calculated
The Board of Equalization (BOE) describes three steps:
- Find the new value. The assessor sets the market value as of the event date, usually close to the purchase price.
- Subtract the old value. The prior assessed value comes off. The difference is the net supplemental assessment, which can be positive or negative.
- Apply the tax rate and proration. The county multiplies the net amount by the local tax rate and then by a factor for the months remaining in the July 1 to June 30 fiscal year.
The new value takes effect on the first day of the month after the event. The BOE’s proration factors are:
| Effective date | Factor | Effective date | Factor |
|---|---|---|---|
| July 1 | 1.00 | January 1 | 0.50 |
| August 1 | 0.92 | February 1 | 0.42 |
| September 1 | 0.83 | March 1 | 0.33 |
| October 1 | 0.75 | April 1 | 0.25 |
| November 1 | 0.67 | May 1 | 0.17 |
| December 1 | 0.58 | June 1 | 0.08 |
The rate is not just 1%. Sonoma County’s tax collector notes the general levy is limited to $1 per $100 of assessed value, with voter-approved taxes and district assessments added on top. Your title or escrow officer can tell you the rate for your parcel’s tax rate area.
One bill or two: it depends on the closing month
The county’s annual roll is based on values as of January 1, so timing matters:
- Closing between June 1 and December 31: one supplemental bill, covering the rest of the current fiscal year.
- Closing between January 1 and May 31: two bills. One covers the rest of the current fiscal year, and the second covers the entire next fiscal year, because that year’s roll was already being prepared with the old value.
After that, your regular annual bill reflects the new value. Marin County’s finance department describes the same split and notes that a negative result produces a refund check instead of a bill.
Two worked examples (hypothetical)
Both examples use a $1,000,000 purchase of a home the seller had owned for many years, with a prior assessed value of $400,000 and an assumed total tax rate of 1.1%. Your actual numbers will differ.
| Closes September 18, 2026 | Closes March 10, 2027 | |
|---|---|---|
| Effective date of new value | October 1, 2026 | April 1, 2027 |
| Net supplemental assessment | $600,000 | $600,000 |
| Bill for the rest of FY 2026-27 | $600,000 x 1.1% x 0.75 = $4,950 | $600,000 x 1.1% x 0.25 = $1,650 |
| Second bill for FY 2027-28 | None | Roughly $6,500 (full year, based on new value minus the roll value) |
| Supplemental total | About $4,950 | About $8,150 |
The spring buyer’s larger total isn’t extra tax. It is a full year that would otherwise have been billed at the seller’s old value. In both cases, the regular annual bill you pay in the meantime still reflects the $400,000 value, which is why the supplemental can feel like a surprise.
When the bill arrives and when it’s due
First the assessor mails a Notice of Supplemental Assessment showing the new value, the old value, the event date and the amount of the assessment. The tax bill comes later. Sonoma County’s tax collector says bills are mailed at least 35 days after the date on the notice and go out throughout the year. No statute or county page we reviewed promises a set number of months after closing, so budget for it from day one.
Under Revenue and Taxation Code 75.52, taxes on a supplemental bill are due on the date it is mailed and are payable in two installments:
| Bill mailed | First installment delinquent | Second installment delinquent |
|---|---|---|
| July through October | 5 p.m. December 10 | 5 p.m. April 10 of the next year |
| November through June | 5 p.m. on the last day of the month after the bill is mailed | 5 p.m. on the last day of the fourth calendar month after the first installment’s delinquency date |
For example, a bill mailed in February would have its first installment delinquent after March 31 and its second after July 31. A 10% penalty attaches to any late installment.
Why your impound account doesn’t pay it
This catches many buyers. The BOE says lenders don’t receive supplemental bills, even when they pay your annual taxes through an impound account; the bill goes directly to you. Napa County’s assessor puts it plainly: supplemental bills “are not automatically paid by their mortgage company” even with an impound account. The BOE adds that penalties cannot be excused because of a misunderstanding with your lender.
When the bill arrives, call your loan servicer. Some will pay it from the impound account if you send a copy, but that can leave the account short and raise your monthly payment. Many buyers simply pay it themselves. Either way, the deadline is yours to meet. Our guide to home financing covers impound accounts more broadly.
Escrow, prorations and who pays
Escrow prorates the regular annual taxes between buyer and seller at closing, giving each credit for their share, as Sonoma County’s tax collector describes. The supplemental bill arrives after closing and is addressed to you as the new owner, so it is usually not part of that proration. Our breakdown of who pays closing costs in California shows which tax items do appear on the settlement statement.
Check the preliminary title report as well. Sonoma County’s tax collector notes that a new owner is responsible for any taxes that were not paid when escrow closed, so ask your escrow officer to confirm every open installment is paid or handled before you sign.
Exemptions, refunds and appeals
- Homeowners’ exemption. If you move in as your principal residence within 90 days of buying, the BOE says the full $7,000 exemption may apply to the supplemental, prorated from the purchase date to June 30. The supplemental notice explains how to claim it; section 75.31 sets a 30-day window from the notice date.
- Negative supplementals. If you paid less than the prior assessed value, the county issues a refund through the auditor-controller. It can’t be used to reduce your annual bill, which must still be paid in full.
- Appeals. You can ask the assessor for an informal review. A formal appeal goes to the county’s assessment appeals board, generally within 60 days of the notice’s mailing date or postmark, whichever is later. In some counties, section 75.31 runs the 60 days from the supplemental tax bill’s mailing date instead, so read the deadline printed on your notice. Sonoma County’s assessor notes that filing an appeal doesn’t postpone the due date.
Other situations change the math. Finishing an addition or an ADU triggers its own supplemental on the new construction; see our guide to buying a home with an ADU. If you plan to carry over an old home’s value under Proposition 19, ask the assessor how your claim will show up; our Prop 19 base year transfer guide explains eligibility. Investors moving rentals into entities can read about property tax for investors.
Estimating the bill before you buy
Use the formula above with your expected price, the seller’s current assessed value from the tax bill or county records, and your tax rate area’s rate. Sonoma County’s assessor links to a third-party estimator from ParcelQuest and notes its results are for reference only. To compare ongoing costs between counties, see our home ownership cost comparison, and first-time buyers can find the broader budget in our first-time homebuyer’s guide.
Questions people ask
When will I get my supplemental tax bill in California?
There is no fixed date. The assessor first mails a Notice of Supplemental Assessment, and the tax collector mails the bill afterward; Sonoma County says at least 35 days after the notice date. Bills go out throughout the year, so set the money aside at closing.
Does escrow pay the supplemental tax bill?
Usually not. Escrow prorates the regular annual taxes at closing, but the supplemental bill is issued after closing to the new owner.
Will my lender pay the supplemental bill from my impound account?
Not automatically. The BOE says lenders don’t receive supplemental bills. Call your servicer to decide who pays, and keep in mind that you are responsible for any penalty if it isn’t paid on time.
Why did I get two supplemental tax bills?
If your purchase closed between January 1 and May 31, the county issues one bill for the rest of the current fiscal year and a second for the full next fiscal year, because that year’s roll was prepared before your purchase was reflected.
Is a supplemental tax bill a one-time charge?
Yes, for each change in ownership or completed new construction. After the supplemental period, the regular annual bill reflects your new assessed value.
Can a supplemental assessment be negative?
Yes. If the new value is lower than the prior assessed value, the county sends a refund instead of a bill. It does not reduce the annual bill.
Can I appeal a supplemental assessment?
Yes. You can request an informal review from the assessor or file a formal appeal with the assessment appeals board, generally within 60 days of the notice’s mailing date. Pay the bill on time while the appeal is pending.
Plan for it when you buy
Ask for a supplemental tax estimate before you remove contingencies, and keep it with your closing cost estimate as you work through the escrow timeline. For more on each step of a purchase, see our home buying process guides, or contact us through our buyer services page to talk through a specific home.
Sources & notes
- California State Board of Equalization: Supplemental Assessment
- California Public Law: Revenue and Taxation Code section 75.52 (supplemental bill due dates)
- California Public Law: Revenue and Taxation Code section 75.31 (notice, exemption claim, appeals)
- County of Sonoma Assessor: Supplemental notices
- County of Sonoma Auditor-Controller-Treasurer-Tax Collector: Property tax FAQs
- Napa County Assessor: Supplemental assessments
- County of Marin Department of Finance: Supplemental tax assessments
Last reviewed October 2026 by Matthew Martinez.
