A supplemental tax statement is the extra, one-time property tax bill California sends you after you buy a home or finish new construction. It covers the difference between the prior assessed value and your property’s new value, prorated for the months left in the fiscal year, and it arrives separately from your regular annual property tax bill with its own payment deadlines. Most buyers see one within a few months of closing. Some see two.
Why You Got the Bill
Two events force the county assessor to reappraise your property and issue a supplemental assessment: a change in ownership and the completion of new construction.1California Legislative Information. California Revenue and Taxation Code 75.10 For a sale, the assessor uses your purchase price to set a new base year value. For construction, the trigger is the date the improvement becomes usable, not when the permit issued or the contractor got paid. The supplemental assessment is the gap between the old assessed value and that new one, and the tax on that gap is what shows up in your mailbox.
Why You Might Get Two Bills
The California fiscal year runs July 1 through June 30, and the number of supplemental bills depends on where your closing date falls in that year.
- If the event happened between June 1 and December 31, you get one supplemental bill covering the rest of the current fiscal year.
- If it happened between January 1 and May 31, you get two. The first covers the months left in the current fiscal year. The second covers the entire next fiscal year.
The reason for the second bill is timing: the tax roll for the upcoming fiscal year has already been printed using the old assessed value, so a second supplemental bridges the gap until your new value lands on a future annual roll.2California State Board of Equalization. Supplemental Assessment Each bill is its own obligation with its own due dates.
How the Amount Is Calculated
A supplemental bill is not a full extra year of tax. The county auditor takes the supplemental assessment, applies the current tax rate, and multiplies by a proration factor tied to when the triggering event occurred.3California Legislative Information. California Revenue and Taxation Code 75.41 The proration treats the event as if it happened on the first day of the following month. Here are the factors published by the Board of Equalization:2California State Board of Equalization. Supplemental Assessment
- June event: 1.00
- July event: 0.92
- August event: 0.83
- September event: 0.75
- October event: 0.67
- November event: 0.58
- December event: 0.50
- January event: 0.42
- February event: 0.33
- March event: 0.25
- April event: 0.17
- May event: 0.08
A worked example. You buy a home in October for $800,000. The prior owner’s assessed value was $500,000, so the supplemental assessment is $300,000. The base California property tax rate is one percent of assessed value, plus any local voter-approved bonds.4California State Board of Equalization. California Property Tax An Overview One percent of $300,000 is $3,000 for a full year. Multiply by the October factor of 0.67 and the supplemental for this fiscal year comes to roughly $2,010. Your actual bill will be a bit higher because local bonds ride on top of the base rate.
When You Have to Pay
Each supplemental bill splits into two equal installments. The deadlines are tied to when the bill was mailed, not the fixed November-and-February schedule you may already know from your annual property tax.3California Legislative Information. California Revenue and Taxation Code 75.41 For bills mailed between November and June, the first installment goes delinquent at the end of the month after the mailing month, and the second goes delinquent four months later.
So a supplemental mailed in March is delinquent for the first installment after April 30 and for the second after August 31. There is no universal calendar date. Read the delinquency dates off the bill itself.
Miss a deadline and a ten-percent penalty attaches to the unpaid installment immediately.5Justia Law. California Revenue and Taxation Code 75.52 There is no grace period. Continued non-payment can result in a lien on the property, the same consequence that follows any delinquent California property tax.
Don’t Assume Your Escrow Will Pay It
This is where new homeowners lose money. Supplemental tax bills are mailed directly to you, not to your mortgage company. Your escrow account was funded to pay the regular annual property tax bill, and it does not anticipate the supplemental. Your lender will not pay it automatically. If the envelope sits in a stack because you assumed the bank had it handled, the ten-percent penalty is coming.
Some lenders will pay a supplemental from escrow as a one-time courtesy if you send a written request with a copy of the bill. That almost always creates an escrow shortage, because the account was not funded for the extra expense. The lender then recalculates your monthly mortgage payment upward to refill the shortfall, sometimes by a lot. Paying the supplemental directly, out of your own account, keeps your monthly payment stable.
County tax collector websites accept e-check payments (usually free) and credit cards (with a convenience fee in the range of two to two-and-a-half percent). You can also pay by mail or in person. The bill lists the options for your county.
When You Get a Refund Instead
Not every supplemental assessment produces a bill. If the new assessed value is lower than the old one, which can happen when someone inherits a property assessed above current market value, the supplemental assessment is negative and the auditor-controller issues a refund check using the same proration.2California State Board of Equalization. Supplemental Assessment
One catch. A negative supplemental does not offset anything you still owe on your regular annual property tax bill. You pay the annual bill in full. The refund comes as a separate check, and the two are handled independently.
If the Valuation Looks Wrong
Your supplemental notice shows the new base year value next to the prior assessed value, along with the parcel number and the date of the ownership change or completion.6California Legislative Information. California Revenue and Taxation Code RTC 75.31 It also tells you about your right to an informal review with the assessor and your right to file a formal appeal.7California Legislative Information. California Revenue and Taxation Code 75.51
The appeal window is typically 60 days from the mailing date on the notice or the bill, depending on the county. That deadline is short and absolute. If the assessed value overshoots what you actually paid or what the property is worth, file the appeal first and work things out with the assessor after. Pay the bill on time regardless; a pending appeal does not stop the delinquency clock.
Deducting It on Your Federal Return
Supplemental property taxes are deductible the same way regular property taxes are, as state and local taxes subject to the SALT cap. For tax year 2026, the SALT deduction is capped at $40,400 ($20,200 if married filing separately), with a phasedown that begins once modified adjusted gross income exceeds $505,000. If your regular property tax plus state income tax is already near the cap, a large supplemental in the same year may push the total past what you can deduct.
You deduct the supplemental in the tax year you actually pay it, not the year of the triggering event. Buy in November 2025, pay the bill in 2026, deduct on your 2026 return. Keep the bill and your payment confirmation. The supplemental amount will not appear on the Form 1098 your lender sends, because your lender did not make the payment.
Transfers That Don’t Trigger a Bill
Some ownership changes are excluded from reassessment, so no supplemental follows. Parent-to-child transfers of a primary residence can qualify, but Proposition 19 narrowed the exclusion significantly starting in February 2021: the child must use the home as their own primary residence and file a homeowners’ exemption within one year, and the excluded value is capped (the property’s current taxable value plus an inflation-adjusted amount, currently $1,044,586 for transfers through February 15, 2027). Anything above the cap gets added to the taxable value and produces a partial supplemental assessment.8California State Board of Equalization. Proposition 19 Fact Sheet Rental properties, vacation homes, and commercial real estate transferred between parents and children no longer qualify at all. Interspousal transfers and certain transfers into revocable trusts also avoid reassessment, though the specifics depend on how the transfer is structured.