Supplemental property taxes in California are a one-time, prorated tax bill that captures the jump in a property’s assessed value the moment it changes hands or new construction is completed, rather than waiting for the next annual tax roll. If you recently bought a home or finished a major renovation, expect this bill on top of your regular property taxes, covering only the months remaining in the current fiscal year after the triggering event.1California State Board of Equalization. Supplemental Assessment
The system exists because Proposition 13 caps annual assessed value increases at 2%, so when a property sells at a higher market price mid-year, the county would otherwise collect far less tax than the new value warrants until the next January 1 lien date. The supplemental assessment, authorized by Revenue and Taxation Code sections 75 through 75.72, puts the reappraisal into immediate effect and bills you for the difference.
What Triggers a Supplemental Bill
Two events trigger a supplemental assessment: a change in ownership and the completion of new construction.2California State Board of Equalization. Property Tax Annotations – 790.0000 Supplemental Assessment
A change in ownership means the county assessor must reappraise the property at full cash value as of the date the deed is recorded. A standard market sale is the usual trigger, but court-ordered transfers, foreclosures, and certain shifts in corporate ownership can also qualify. The purchase price almost always becomes the starting point for the new assessed value.
New construction covers more than building from scratch. Adding a room, converting a garage into living space, renovating a building to a condition that’s essentially like new, or finishing out raw commercial space for a tenant all count. Routine maintenance and cosmetic repairs typically don’t. The dividing line is whether the work adds square footage, changes the property’s use, or restores it to a substantially new condition. Only the value of the new improvement is reassessed; the underlying land and any preexisting structures keep their existing base year values.
How the Bill Is Calculated
The math has three steps. First, the assessor subtracts the property’s prior assessed value from the newly determined market value. That difference is the net supplemental assessment. Second, the local tax rate is applied. California’s base rate is 1%, and voter-approved bonds and special assessments push the effective rate to roughly 1.05% to 1.25% in most areas. Third, that full-year tax figure is multiplied by a proration factor based on the month the event took effect.
California’s fiscal year runs July 1 through June 30, and the supplemental tax effective date is the first day of the month after the triggering event. A home purchased on August 15 has a tax effective date of September 1, leaving 10 months in the fiscal year. The proration factors are:1California State Board of Equalization. Supplemental Assessment
- July 1: 1.00 (full year)
- 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
A quick example. You buy a home on September 20 for $600,000, and the prior assessed value was $400,000. The net supplemental value is $200,000. At a 1.1% tax rate, the full-year tax on that difference would be $2,200. Because the effective date is October 1, the proration factor is 0.75, so your supplemental bill is $1,650.
Why You Might Get Two Supplemental Bills
This catches many new homeowners off guard. If your change in ownership or construction completion occurs between January 1 and May 31, you’ll receive two supplemental tax bills, not one. The first covers the remaining months of the current fiscal year. The second covers the entire next fiscal year, because the new base year value won’t appear on the upcoming annual roll either.
A purchase in March generates one bill prorated for about four months through June 30 and a second bill for the full twelve months of the following fiscal year starting July 1. Both arrive separately from your regular annual tax bill, and both have their own payment deadlines. The combined amount can be substantial.
When the Bill Arrives and How to Pay
After the triggering event, the county sends a Notice of Supplemental Assessment showing the old and new values. That notice is not a bill; it’s your heads-up and your window to appeal. The actual supplemental tax bill follows, typically three to six months after the purchase or construction completion, though some counties take up to nine months.
Supplemental bills operate on their own timeline, completely independent from the regular annual property tax due dates in December and April. Most supplemental bills are split into two installments with delinquency dates printed on the statement. Miss a deadline and a 10% penalty attaches to the delinquent installment. Second installments that go unpaid also incur a small administrative fee, and continued delinquency adds monthly interest of 1.5% once the bill becomes tax-defaulted.
You can pay online through your county tax collector’s portal, by mail, or in person. These bills go to the property address on file and are easy to overlook amid the paperwork that follows a home purchase.
Your Mortgage Escrow Probably Won’t Pay It
This is where people consistently get burned. A lender’s escrow account almost never covers supplemental tax bills. Escrow accounts are set up to pay the regular annual property taxes on the standard roll. Supplemental bills are generated outside that cycle and mailed directly to the property owner, not the lender.
You’re responsible for paying the supplemental bill out of pocket. Some servicers will make a one-time disbursement from escrow if you send them a copy of the bill, but this is a courtesy, not an obligation, and many decline. When budgeting for a home purchase, set aside funds for the supplemental bill on top of what your escrow will handle. For a property with a large jump in assessed value, the supplemental tax can easily run several thousand dollars, and it arrives months after you’ve already stretched for the down payment and closing costs.
Appealing a Supplemental Assessment
If you believe the assessor’s new value is too high, you have 60 days from the date the Notice of Supplemental Assessment was mailed to file an Application for Changed Assessment with your county’s Assessment Appeals Board.3California State Board of Equalization. Assessment Appeals Frequently Asked Questions If the 60th day falls on a weekend or holiday, the deadline extends to the next business day.
This window is much shorter than the regular annual assessment appeal period, which runs from July 2 through September 15 or November 30 depending on your county. Miss the 60-day window and you lose your right to contest that supplemental value. Gather comparable sales data and, if the numbers justify it, consider hiring an appraiser before the clock runs out. Even if you file an appeal, you still owe the tax by the due date; if you win, the overpayment is refunded.
Transfers That Skip Reassessment
Not every ownership change triggers a supplemental bill. California law excludes certain transfers from reassessment. The most commonly used exclusion involves family transfers under Proposition 19, which took effect on February 16, 2021.4California State Board of Equalization. Proposition 19
A parent can transfer a primary residence to a child without reassessment, but only if the child also uses it as their primary residence and claims the homeowner’s or disabled veteran’s exemption within one year. The child’s new taxable value is the parent’s existing Proposition 13 value plus any market value exceeding the exclusion amount, which is $1,044,586 for events occurring between February 16, 2025, and February 15, 2027. Grandparent-to-grandchild transfers qualify under the same rules, but only if the grandchild’s parents are deceased.
Transfers between siblings, cousins, or other relatives do not qualify. Investment properties and second homes transferred between parents and children no longer receive any exclusion under Proposition 19, a change from the prior law. Interspousal transfers during marriage or as part of a divorce are also excluded from reassessment, as are transfers into most revocable living trusts where the original owner retains control.
When the New Value Is Lower
A supplemental assessment doesn’t always mean you owe money. If the new assessed value is lower than what was on the prior roll, the net supplemental value is negative and you’re entitled to a refund. The county issues the refund using the same proration method. This scenario also arises when a second buyer pays less than the first buyer did within the same fiscal year.
Don’t assume you’ll get a negative assessment just because the market softened. The comparison is between the new market value at the time of your purchase and the value already sitting on the current tax roll, which may already have been reduced.
How Long the County Has to Bill You
Counties don’t have unlimited time to send a supplemental assessment. Under Revenue and Taxation Code section 75.11(d), the standard deadline is the fourth July 1 after the assessment year in which the triggering event occurred. If the property owner failed to file required ownership change documents or the change went unrecorded, the county gets until the eighth July 1.5State Board of Equalization. Statute of Limitations for Supplemental and Escape Assessments
Most supplemental bills arrive within a year of the event. If you bought a property and never received one, don’t assume you’re in the clear; the county may simply be behind. Set money aside for at least a year after closing. A surprise bill three years later with accumulated interest is far worse than one you budgeted for.