Fiscal year
California property-tax fiscal years run from July 1 to June 30. The proration on a supplemental bill is tied to that calendar, not to a calendar year.
Home · Free tools · Supplemental property tax
Southern California · November 14, 2026
Calculators
Supplemental property-tax calculator
After a change in ownership or completed new construction, California assessors generally issue a supplemental bill that prorates the new value for the rest of the fiscal year. This calculator is a planning estimate. The official bill can differ, and it is often not paid through a lender's impound account.
Estimate · not a tax bill
Supplemental property tax estimator
Estimate only, not a tax bill. Supplemental rules, exemptions and due dates are set by the county assessor and tax collector. A homeowners’ exemption, exemptions for other transfers, and escape assessments are not modeled here.
Who this is for, and when to use it
Recent buyers, owners who just finished an ADU or another addition, and anyone who has heard they should set money aside for a second tax bill can use this page. Use it after you know a closing or completion date and a likely new assessed value, then compare the result with the official notice when it arrives.
Key terms
Supplemental bill
An additional bill that covers the difference between the old and new taxable values from the event date to the end of the fiscal year, and sometimes into the next fiscal year, depending on the date.
Event date
The date of a change in ownership or the completion of new construction, as the assessor uses it.
New base-year value
The value the assessor sets after the event, which may be the purchase price or another enrolled value.
Impound or escrow account
A lender account that often pays the regular tax installments, but often does not pay a supplemental bill automatically.
How to use this, step by step
- Enter the event date (closing or completion) and the old and new taxable values, using assessor or purchase figures when you have them.
- Enter a tax rate from a recent bill if you have one, rather than leaving a 1% default if your area is higher.
- Read the estimate and note that a second supplemental bill can apply depending on the date.
- Set the money aside; do not assume the lender will pay it.
- When the official bill arrives, pay it as instructed or call the tax collector if the amount or the parcel is wrong.
Costs, timelines, and common pitfalls
A supplemental bill can be a surprise because it arrives weeks or months after a closing or a final inspection. The amount depends on the value change, the date and the tax rate, including local charges. Common pitfalls include spending all of the cash after closing, assuming the homeowners' exemption will cancel the supplemental bill, or ignoring a second bill that covers the following fiscal year. Penalties apply if a billed amount is ignored.
County-by-county notes
Orange County
The Assessor, Clerk-Recorder and Treasurer-Tax Collector are separate offices. Orange County does not publish official blank grant or quitclaim deeds. Confirm exemptions and payment addresses on the official tax collector and assessor sites.
Los Angeles County
The Assessor, Registrar-Recorder/County Clerk and Treasurer and Tax Collector each publish their own lookup tools and forms. Many property-tax claims are filed with the Assessor.
Riverside County
The Assessor-County Clerk-Recorder publishes many of the sample deeds and tax forms used countywide, including in Moreno Valley and Temecula.
San Bernardino County
The Assessor/Recorder and the Tax Collector publish property information and payment pages. Supplemental bills and exemption claims are easy to mix up; read the official letter.
San Diego County
The Assessor/Recorder/County Clerk publishes courtesy deed forms and property-tax claims. Confirm the current packet on the official site before you record or file.
Before you start
- Have the closing or completion date and a reasonable new value (often the purchase price or the permitted work).
- Have a tax rate from a recent bill if possible.
- Know whether you already filed the homeowners' exemption.
- Ask your loan servicer, in writing if needed, whether they will pay a supplemental bill.
- Keep a reserve until the official bill arrives and is paid.
A short real-estate example
Questions
Did escrow already pay this?
Escrow usually prorates the regular tax bill. A supplemental bill is often issued later to the new owner and is a separate payment.
Will my mortgage company pay it?
Often not. Ask your servicer and read the bill.
Does the homeowners' exemption remove the supplemental bill?
It can change later regular bills once it is in place, but you should not assume it cancels a supplemental bill. Confirm with the assessor.
What rate should I enter?
Use the total rate from a recent bill when you can, not only the 1% base levy.
What if I built an ADU?
Completed new construction can trigger a supplemental bill on the added value. Set money aside when the permit is final.
Where are the official exemption forms?
On our property-tax forms page and the assessor's site.
Is this the same as an assessment appeal?
No. An appeal is a challenge to a value and has its own forms and deadlines.
Official resource links
We link only official government or court pages. Confirm the current version on the official site.
Questions about a specific home or transaction? Call or text 833-THIS-BROKER (833-842-7465), text us, or request a call. Dr. Hamid Rowshan, Choice of America. DRE #02004794, company DRE #02240008.
This page is general information for Southern California homeowners, buyers, sellers and renters. It is not legal, tax, insurance or lending advice, not a loan offer, and not a substitute for the official agency, court, lender or insurer that will handle your file. Rules, fees, maps and forms change. Confirm every figure and every filing with the official office before you act.