California Supplemental Property Tax: R&T §75 Mechanics

TL;DR

When a property changes ownership in California, the county assessor reassesses to current market value under Revenue & Taxation Code §75 et seq., generating a supplemental property tax bill for the difference between the prior owner's assessed value and the new base year value, prorated for the portion of the fiscal year remaining. This is separate from the annual property tax bill and separate from any escape assessment covering prior years. Supplemental bills are issued within roughly 6 to 24 months after a qualifying transfer, are payable to the county tax collector on the schedule shown on the bill, and are the new owner's responsibility even if arriving after closing. Escrow prorations at closing generally do not cover supplemental taxes because the amount is not yet known.

The three tax bill categories

Per California BOE guidance, property tax practice involves three distinct bills that owners and licensees must distinguish clearly. The annual property tax bill is issued each fall (typically mailed by November 1, with the first installment due November 1 and delinquent after December 10, and the second installment due February 1 and delinquent after April 10) based on the assessed value on the January 1 lien date and reflects the property's regular Proposition 13 assessment plus voter-approved bonds and direct charges. The supplemental property tax bill covers the change-in-ownership or new-construction event that occurred after the January 1 lien date, capturing the value adjustment for the remainder of the fiscal year. The escape assessment bill covers prior tax years where property was under-assessed due to unreported changes in ownership, unpermitted new construction, or clerical errors — it reaches back up to eight years under R&T §532.

The three bills serve distinct purposes and can arrive at overlapping times. A newly purchased property may receive a regular annual bill (from the prior owner's assessment), a supplemental bill (capturing the reassessment from the sale), and potentially an escape bill (if the prior owner had unreported changes). All three are legitimate obligations and must be paid on their respective schedules regardless of what the buyer expected at closing.

How the supplemental bill is calculated

The supplemental assessment under R&T §75 et seq. is calculated by determining the new base year value at the change-in-ownership date (typically the purchase price under Prop 13's fair-market-value trigger), subtracting the prior owner's assessed value (which was often much lower due to Prop 13's 2% annual cap), and applying the county's tax rate to the difference on a prorated basis for the months remaining in the fiscal year at the time of transfer.

Example calculation: Buyer purchases a home on August 15, 2026 for $1,200,000. The prior owner's assessed value was $340,000. The supplemental value increase is $860,000. If the county tax rate is 1.15% (approximate), the annualized supplemental tax is roughly $9,890. The supplemental assessment is effective the first day of the month following the change in ownership — September 1 in this example — so the proration factor is 10/12 (September through June), producing a prorated supplemental bill of roughly $8,242. The buyer will also receive an annual tax bill for the following year at the new $1,200,000 base, plus the Prop 13 2% annual escalation cap thereafter. For related context on how the underlying assessment works, see our California Proposition 13 property tax guide.

Timing and why escrow prorations don't help

Supplemental bills are typically issued 6 to 24 months after the qualifying event, depending on county assessor workload and the transaction volume in the county. Los Angeles County, for example, often takes 12-18 months from close of escrow to issue a supplemental bill. The assessor first processes the Change of Ownership Statement filed with the recorder, then reassesses the property, then transmits the new value to the tax collector, who issues the supplemental bill.

This lag creates a common source of buyer surprise. At closing, escrow prorates the current annual property tax bill between buyer and seller based on the tax already assessed for that fiscal year — but the supplemental tax owed by the buyer for the reassessment is not yet known and cannot be prorated. The buyer pays the closing-day prorated annual tax and then, months later, receives a supplemental bill covering the tax on the value increase from the moment of purchase forward. This is not double taxation; it is the mechanism by which Prop 13's fair-market-value reset takes effect. Buyers should budget for the supplemental bill and should not assume closing prorations settled all tax obligations for the property.

Payment schedule and delinquency

Supplemental bills follow a payment schedule shown on the bill itself, and the number of bills depends on when in the fiscal year the change-in-ownership event occurred. If the supplemental event occurs from June through December, one supplemental bill or refund is generally issued for the remainder of the current fiscal year. If the event occurs from January through May, two supplemental bills or refunds are generally issued: one for the remaining months in the current fiscal year and a second for the entire ensuing fiscal year. Each bill shows its own installment schedule and delinquency dates, which typically mirror the annual property tax cycle of December 10 and April 10 delinquency for the respective installments.

Delinquent supplemental taxes carry the same 10% penalty plus additional charges as delinquent annual property taxes. Prolonged delinquency leads to defaulted status, redemption penalties, and eventually tax sale under the same statutory framework as regular property tax delinquency. Buyers should track the receipt date on any supplemental bill and calendar the delinquency date rather than relying on the November-December-April rhythm of annual bills.

Escape assessments — what they are and when they issue

Escape assessments under R&T §531 through §534 capture value that "escaped" prior years' assessments — usually because a change in ownership was not timely reported to the assessor, new construction was not permitted or was permitted incorrectly, or a clerical error caused under-assessment. An escape bill retroactively raises the assessed value for one or more prior years and issues a bill for the tax difference plus interest.

Escape assessments can reach back multiple prior years, with longer look-back periods in cases involving failure to report, misrepresentation, fraud, or willful concealment. Because the exact period depends on the statutory basis under R&T §532 and related sections, buyers should treat escape assessments as a separate, fact-specific risk from supplemental bills. A new owner may face an escape bill dating back to the prior owner's period of ownership — the tax follows the property, not the prior owner. Buyers should investigate prior ownership history and unpermitted construction during due diligence. Title insurance sometimes covers escape assessments arising from pre-closing events; buyers should confirm coverage with their title company.

The Change of Ownership Statement (PCOR) and its role

At close of escrow, the buyer files a Preliminary Change of Ownership Report (PCOR) with the county recorder along with the deed. The PCOR discloses the transaction details the assessor needs to determine whether a reassessable change in ownership has occurred, what the transfer amount was, and whether any exclusion (parent-child, spousal, senior transfer under Prop 19, etc.) applies. Failure to file the PCOR does not prevent recording, but it triggers the county assessor to send a Change of Ownership Statement (COS) form separately, and failure to return that form within 90 days can trigger a penalty under R&T §482 — up to $5,000 for property eligible for the homeowners' exemption and up to $20,000 for other property — plus an estimated assessment that must be corrected later through appeal or amendment.

The PCOR is one of the most important documents new owners handle. A completed and accurate PCOR speeds the supplemental assessment process, ensures exclusions are properly claimed, and prevents estimated assessments that may need to be corrected later. For Prop 19 base-year transfers and parent-child exclusions, timely claim filing on the PCOR (or in a separate BOE-form filing within the deadline) is critical to preserving the exclusion.

Frequently Asked Questions

Is the supplemental tax bill something the seller pays?

No. The supplemental bill covers the tax on the value increase from the moment of purchase forward — the buyer is the owner during that period and is responsible for the tax. The seller had no ownership interest during the period covered by the supplemental bill. Escrow does not typically prorate supplemental taxes because the amount is not yet known at closing.

How long does it take for a supplemental bill to arrive?

Typically 6 to 24 months after close of escrow, depending on county assessor workload. Los Angeles County, San Francisco, and other high-volume counties often take 12 to 18 months. The bill is triggered by the recorder's transmission of the change-in-ownership document to the assessor and the assessor's completion of the reassessment. Buyers should budget for the bill regardless of when it arrives.

Can I dispute a supplemental assessment?

Yes. The supplemental assessment is subject to the same appeal process as the annual assessment — the property owner can file an Assessment Appeal Application with the county Assessment Appeals Board, generally within 60 days of the notice of supplemental assessment. Grounds for appeal include disputes about the base year value (typically the purchase price), whether an exclusion should apply, or whether the change-in-ownership event actually occurred.

What's the difference between a supplemental bill and an escape assessment?

A supplemental bill captures the value increase from a current-year change in ownership or new construction, prorated for the months remaining in the fiscal year. An escape assessment covers prior years where property was under-assessed due to unreported changes, unpermitted construction, or clerical error, and can reach back up to eight years. Both are separate from the annual bill and can arrive together or separately.

Does refinancing trigger a supplemental bill?

No. Refinancing is not a change in ownership under R&T §60 et seq. and does not trigger reassessment or a supplemental bill. Only actual transfers of ownership (with limited exceptions such as spousal transfers, parent-child transfers under Prop 19, and same-owner LLC conversions) count as change-in-ownership events for Prop 13 reassessment purposes.

What happens if I don't file the PCOR at closing?

Recording proceeds regardless, but the assessor will send you a Change of Ownership Statement separately, and failing to return that within 90 days can result in a penalty under R&T §482 (up to $5,000 for homeowners' exemption property and up to $20,000 for other property), so the risk scales with the property's tax treatment. The assessor may also proceed with an estimated assessment that must be corrected later through appeal or amendment. Filing the PCOR at closing is faster, cheaper, and preserves any exclusion claims.

Bottom Line

Supplemental property tax bills capture the Prop 13 reassessment triggered by a change in ownership, prorated for the months remaining in the fiscal year at the transfer date. They are issued 6 to 24 months after close of escrow, are separate from the annual property tax bill, and are the buyer's responsibility — escrow prorations at closing do not cover them because the amount is not yet known. Escape assessments are a distinct mechanism that can reach back multiple prior years, with the look-back period depending on the statutory basis and facts, including failure to report, misrepresentation, fraud, or willful concealment, and can affect new owners for events that occurred before closing. Filing an accurate PCOR at close of escrow speeds the process and preserves any base-year exclusion claims. For the underlying assessment framework, see our Proposition 13 property tax guide. For the seller-disclosure framework that runs alongside tax assessment issues at closing, see our California Transfer Disclosure Statement guide, and our Natural Hazard Disclosure Statement guide covers a related disclosure package buyers receive at the same closing.

Source: California Revenue & Taxation Code §75 et seq. — Supplemental Assessments · California R&T Code §531-534 — Escape Assessments · California BOE — Change of Ownership Reporting Forms