In California, how often you get a supplemental tax bill depends on one thing: the month your triggering event happened. A purchase or completed construction between June 1 and December 31 produces one supplemental bill. The same event between January 1 and May 31 produces two. After those bills clear, the new assessed value moves onto your regular annual tax bill and no more supplemental bills arrive for that event.
Supplemental bills are one-time adjustments, not recurring charges. They exist to bridge the gap between your property’s old assessed value and its new one after a reassessment event, so the higher (or lower) tax obligation takes effect right away instead of waiting for the next regular roll.1California State Board of Equalization. Supplemental Assessment
What Counts as a Triggering Event
Only two things generate a supplemental bill: a change in ownership and the completion of new construction. Nothing else. A change in ownership covers sales, inheritances, transfers into or out of trusts, and any other legal event that shifts who controls the property. The assessor reappraises at full cash value as of the date of the change.2California Legislative Information. California Revenue and Taxation Code 75.12
New construction triggers a supplemental only when the project is finished, meaning the earliest of the date it becomes available for the owner to use, the date someone occupies it with the owner’s permission, or the date it can be functionally used for its property type.2California Legislative Information. California Revenue and Taxation Code 75.12 The assessor values the new improvements only and adds that to the existing assessed value.
Events stack. If you buy a home and then finish a major renovation, both events can produce their own supplemental bills. A phased remodel — kitchen this year, addition next year — can generate a supplemental for each completed phase. So the “how often” answer for a single event (one or two bills) can multiply if you have more than one event.
The June–December vs. January–May Rule
California’s fiscal year runs July 1 through June 30, and that calendar drives everything. The assessor prorates the change in value based on how much of the fiscal year is left after your event.
- Event between June 1 and December 31: one supplemental bill. It covers the prorated change from the first of the month after the event through June 30 of the current fiscal year.1California State Board of Equalization. Supplemental Assessment
- Event between January 1 and May 31: two supplemental bills. The first covers the remainder of the current fiscal year. The second covers the entire next fiscal year (the following July 1 through June 30), because the regular tax roll for that upcoming year was already finalized using the prior owner’s assessed value before your event happened.1California State Board of Equalization. Supplemental Assessment
Two bills for an early-year purchase is not double taxation. Each bill covers a different stretch of time. Once the regular annual roll catches up to your new assessed value, the supplemental billing for that event is over.3Orange County Assessor. Supplemental Assessments and Notices
When the Bill Actually Shows Up
Don’t expect a supplemental bill immediately after closing. The assessor has to process the ownership change, verify the transaction, and calculate the new value. Most supplemental bills are mailed within nine months of the triggering event, though counties with heavy transaction volume take longer.
Supplemental bills carry their own due dates, separate from the regular December 10 and April 10 property tax deadlines. Each supplemental bill is split into two installments. Bills mailed between July and October follow the regular delinquency schedule (December 10 and April 10). Bills mailed at other times of the year have different deadlines printed on the notice itself, so check the dates on your specific bill.
Missing a supplemental installment triggers a 10% penalty on the delinquent amount plus administrative fees that vary by county. Penalties are automatic once the delinquency date passes. Unpaid supplemental taxes accrue further penalties and can eventually result in a lien.
A Supplemental Can Be a Refund Instead
If your property’s new assessed value is lower than the old one — which happens when you buy for less than the previously assessed value — the net supplemental assessment is negative and the county auditor-controller issues a refund check.1California State Board of Equalization. Supplemental Assessment The same timing rule governs how many you get: a January-through-May event can produce two refund checks; a June-through-December event produces one.
A supplemental refund does not reduce your existing annual property tax bill. You still owe the full amount on your current annual bill even if the supplemental shows a value drop. The refund arrives separately.
Your Escrow Account Probably Won’t Cover It
Even if your mortgage payment includes an escrow amount for regular property taxes, supplemental bills are almost always your responsibility to pay directly. The county mails the supplemental to the property owner, not to the mortgage servicer, and most lenders treat supplemental taxes as outside the escrow account. They will not adjust your monthly payment to cover them.
Set aside funds for the supplemental bill when you close. If one arrives and you’re unsure whether your servicer will pay it, contact them right away and plan on paying it yourself.
Transfers That Produce No Supplemental Bill
Some ownership changes are excluded from reassessment entirely, meaning no supplemental assessment and no bill at all.
The most common exclusion is the parent-child transfer under Proposition 19. If a parent transfers their primary residence to a child, the child can keep the parent’s lower assessed value, but only if the child uses the home as their own primary residence and files a homeowners’ or disabled veterans’ exemption within one year of the transfer. The child must also file Form BOE-19-P with the county assessor within three years of the transfer date to receive the exclusion retroactively.4California State Board of Equalization. Transfers of Property Between Parents and Children
The exclusion has a value cap. For transfers between February 16, 2025 and February 15, 2027, the property’s current market value cannot exceed the parent’s factored base year value by more than $1,044,586. Any excess above that gets added to produce a higher assessed value for the child.4California State Board of Equalization. Transfers of Property Between Parents and Children
Other excluded transfers include interspousal transfers during marriage or as part of a divorce, and certain transfers into revocable trusts where the original owner keeps control. If your transfer qualifies, the frequency question answers itself: zero supplemental bills.