A supplemental property tax bill is a one-time charge you get in California when a property changes hands or new construction is completed, covering the tax difference between the old assessed value and the new market value for the months left in the current fiscal year. It exists because Proposition 13 caps annual increases to the existing assessment at 2%, so when a sale or major improvement resets the value, a separate bill closes the gap until the regular annual bill catches up.
What Triggers the Bill
Two things trigger a supplemental assessment: a change in ownership or the completion of new construction. Ownership changes include sales, gifts, some inheritances, and certain changes in how title is held. New construction means work that permanently adds value—an added room, a pool, an ADU, expanded square footage. When one of these events happens, the county assessor sets a new market value and compares it to the value already on the tax rolls.1California State Board of Equalization. Supplemental Assessment
Routine upkeep does not count. Repainting, replacing carpet, or patching a roof generally does not qualify as new construction because it does not substantially add to the property’s value or useful life.
Transfers That Do Not Trigger a Bill
Several common transfers are excluded from reassessment, which means no supplemental bill is generated at all.
- Transfers between spouses, including transfers arising from divorce or the death of a spouse, are not reassessed.2California State Board of Equalization. Property Tax Annotations – 220.0268
- Under Proposition 19, a parent can transfer their primary home to a child (or to a grandchild if the middle generation is deceased) without full reassessment, provided the child uses the home as their own primary residence and the market value does not exceed the existing taxable value plus an inflation-adjusted amount. For transfers through February 15, 2027, that amount is $1,044,586. If market value exceeds the threshold, only the excess is added.3California State Board of Equalization. Proposition 19
- Rebuilding after a disaster generally does not trigger reassessment of the reconstructed portion, provided reconstruction begins within the required timeframe.
One boundary worth knowing: parent-child transfers of anything other than a primary residence no longer qualify for the exclusion. Investment properties, vacation homes, and commercial property passed within a family are reassessed at current market value, and a supplemental bill follows.3California State Board of Equalization. Proposition 19
How the Amount Is Calculated
The supplemental assessment is the difference between the new market value and the prior assessed value on the rolls. If the property was assessed at $200,000 and the assessor sets the new value at $250,000, the supplemental assessment is $50,000. The local tax rate applied to that $50,000 gives you what a full year of additional tax would be.1California State Board of Equalization. Supplemental Assessment
The bill is then prorated, because the event almost never falls on the first day of the fiscal year. California’s fiscal year runs July 1 through June 30, and proration is based on the months remaining after the month the event occurs. Buy a home in October and the bill covers November through June, roughly two-thirds of the annual figure. The assessor multiplies the annual supplemental tax by a proration factor (about 0.66 in that example) to arrive at the amount due.1California State Board of Equalization. Supplemental Assessment
When You Get a Refund Instead
A supplemental assessment can go the other way. If the new market value is lower than the prior assessed value—sometimes the case when you buy for less than the previous owner’s assessment—the supplemental assessment is negative and the county issues a refund rather than a bill.1California State Board of Equalization. Supplemental Assessment
One catch: a negative supplemental assessment does not reduce your regular annual tax bill or create a credit against it. You still owe the full amount on the annual bill. The refund arrives separately as a check from the county auditor-controller.1California State Board of Equalization. Supplemental Assessment
Whether You Get One Bill or Two
The timing of the triggering event determines how many bills you see.
- If the event occurs between June 1 and December 31, you get one supplemental bill (or refund) for the remaining months of the current fiscal year.
- If it occurs between January 1 and May 31, you get two. The first covers the rest of the current fiscal year; the second covers the entire upcoming fiscal year that begins the following July 1.
Two bills show up in the January-to-May window because the new value has to be reflected on both the current roll and the next roll before the regular annual bill catches up. Each bill carries its own due dates and delinquency deadlines.1California State Board of Equalization. Supplemental Assessment
Deadlines and Penalties
Supplemental bills arrive separately from your regular annual statement, often several months after the triggering event while the county processes the deed or the construction permit. Delinquency dates depend on when the county tax collector mails the bill. Bills mailed early in the fiscal year (July through October) generally follow the same schedule as annual tax bills, with a first installment delinquent after December 10 and a second after April 10. Bills mailed later follow a different schedule tied to the mailing month.
Miss a deadline and a 10% penalty attaches to that installment. A smaller administrative fee is added if the second installment goes unpaid. If both installments are still unpaid on June 30, the property goes into tax default, and additional penalties of 1.5% per month—18% per year—accrue along with a redemption fee. Penalties are not waived because you did not know about the bill or because you assumed your lender was paying it.1California State Board of Equalization. Supplemental Assessment
Why Your Mortgage Escrow Probably Won’t Cover It
Supplemental bills go directly to the property owner, not to the mortgage lender. Lenders do not receive the original or a copy of the supplemental bill, even when they handle your annual property taxes through an escrow or impound account.1California State Board of Equalization. Supplemental Assessment
Most escrow accounts are sized only for the standard annual taxes estimated at closing and do not build in a cushion for supplemental assessments. When a supplemental bill arrives, you can pay it directly or ask your lender whether they will pay it from your escrow balance. Either way, the legal obligation to pay on time rests with you as the owner. If the bill goes unpaid because of a miscommunication with the lender, state law does not treat that as grounds to waive the late charges.1California State Board of Equalization. Supplemental Assessment
How to Appeal the Assessment
If you think the new market value is too high, you can file an appeal with your county’s Assessment Appeals Board. The deadline is 60 days from the date the supplemental assessment notice (or the supplemental bill, depending on the county) is mailed. Miss it and you generally lose the right to challenge the valuation for that assessment period.
Useful evidence includes a recent independent appraisal, comparable sales from around the time of your triggering event, and documentation of conditions that reduce value, such as needed repairs. Filing fees vary by county, ranging from about $50 to several hundred dollars, and some counties charge nothing. If the board reduces the assessed value, you get a refund for any overpaid supplemental taxes. If you disagree with the board’s decision, you may be able to appeal further to superior court.
Federal Deductibility
Supplemental property taxes are deductible on your federal return as real estate taxes if you itemize. They follow the same rules as your regular property taxes: deduct them in the year you actually pay them. The combined total of state and local taxes you deduct—property, state income, and sales—is subject to the federal SALT cap, which is $40,400 for 2026, or $20,200 if you are married filing separately. If regular property and income taxes already put you at the cap, a supplemental bill may not add much on the deduction side.