What Is a Supplemental Property Tax Bill and When Will It Hit My New Purchase?

Published August 10, 2026

A supplemental bill is a one time, prorated charge for the gap between a property's old assessed value and its new purchase price. Buy between January 1 and May 31, and California law requires two separate supplemental bills covering two different fiscal years.

The short answer

A supplemental property tax bill is a one time, prorated charge covering the gap between a property's old assessed value and its reassessed value after you buy it. California law requires reassessment as of the first day of the month after your purchase closes. If you close between January 1 and May 31, the law requires two separate supplemental bills, covering two different fiscal years.

Last verified: August 10, 2026

Why the bill exists at all

California reassesses real property to market value only when it changes ownership or is newly constructed, per Proposition 13 and Revenue and Taxation Code chapter 3.5 of part 0.5 of division 1. Outside of that, assessed value only rises by up to 2 percent a year.

The regular property tax roll is set once a year, as of the January 1 lien date. If you buy a house in June, the seller's old, lower assessed value would otherwise sit on the books until the following January 1, more than six months of tax bills calculated on the wrong number.

The supplemental roll exists to close that gap immediately instead of waiting for the next lien date. Per BOE Publication 29, the supplemental assessment is a prorated assessment that reflects the increase or decrease in assessed value from the reappraisal, covering the portion of the fiscal year that remains after the date of the change in ownership or completed new construction.

How the amount is calculated

The supplemental assessment is the difference between the property's new base year value, determined as of the date of the change in ownership, and the value already on the existing roll. State law requires reassessment as of the first day of the month following the change in ownership.

Concretely: if the prior owner's assessed value was lower than what you paid, the difference generates a supplemental tax bill for the increase. If the reassessment produces a lower value than what was on the roll, the county issues a refund instead. The Los Angeles County property tax portal confirms the same basic mechanics locally, describing the general tax levy as 1 percent multiplied by the net taxable value, prorated for the supplemental period.

The two bill scenario, explained

This is the detail most new buyers never see coming, and it can mean two unexpected bills instead of one.

Per BOE Publication 29: a change in ownership occurring between January 1 and May 31 results in two supplemental assessments, and therefore two supplemental tax bills. The first bill covers the remaining portion of the fiscal year in which the purchase happened. The second bill covers the entire following fiscal year, because the new value would not otherwise be reflected on the regular roll until the January 1 lien date after that, and the state prorates it separately instead of waiting.

A purchase that closes between June 1 and December 31, by contrast, generates one supplemental bill, because the coming January 1 lien date is close enough that the regular roll will pick up the new value directly.

| Close of escrow window | Supplemental bills generated | Why |

|---|---|---|

| January 1 through May 31 | Two | The regular roll would not reflect the new value until the second following January 1 lien date, so the law prorates two separate periods |

| June 1 through December 31 | One | The next regular January 1 lien date falls soon enough to pick up the new value directly |

When the bill actually arrives, and how it gets paid

BOE Publication 29 does not give a fixed number of weeks or months from close of escrow to mailing; the timing depends on when the assessor completes the reassessment, which varies by county workload and property complexity. What the publication does confirm is the delinquency schedule once a bill is mailed:

• If the bill is mailed between July 1 and October 31, it becomes delinquent on the same dates as the regular secured roll, December 10 for the first installment and April 10 for the second.
• If the bill is mailed between November 1 and June 30, the first installment becomes delinquent on the last day of the month following the month it was mailed, and the second installment becomes delinquent on the last day of the fourth month after that.

A delinquent supplemental bill defaults the entire property, even if your regular annual taxes are fully paid, and accrues redemption penalties of 1.5 percent a month.

Why this catches buyers, not just landlords

Most mortgage lenders escrow for the regular, known property tax bill based on the assessor's current published value at the time of loan approval. That figure is almost always the seller's old, lower assessed value, because the supplemental assessment has not been calculated yet. The supplemental bill arrives separately, is not built into most escrow accounts, and is due on its own schedule. Budgeting for it as a one time, out of pocket cost at some point after closing, rather than assuming your monthly mortgage payment already covers it, avoids the most common surprise.

FAQ

Does every home purchase in California generate a supplemental bill?

Any purchase where the reassessed value differs from the prior owner's assessed value generates a supplemental assessment, which results in either a supplemental tax bill for an increase or a refund for a decrease. Since Proposition 13 caps annual increases at 2 percent, most purchases after a property has been held for several years produce an increase and therefore a bill.

Can I estimate the bill before I close?

This article does not confirm a specific estimator tool from a primary source retrieved directly. Los Angeles County has historically published a supplemental tax estimator; confirm the current tool and its assumptions directly with the Los Angeles County Assessor before relying on any estimate.

Does the supplemental bill replace my regular annual property tax bill?

No. It is a separate, one time bill layered on top of the regular secured roll bill, covering only the prorated gap for the period described above.

What happens if I do not pay the supplemental bill on time?

Per BOE Publication 29, a delinquent supplemental tax leads to default of the entire property, even if regular taxes are current, with monthly redemption penalties of 1.5 percent and potential sale of the property after five years of continued nonpayment.

Do new construction projects get the same two bill treatment?

Yes. Publication 29 applies the same January 1 through May 31 two bill rule, and the same one bill rule for June 1 through December 31, to completed new construction, not only to changes in ownership.

Will refinancing my mortgage trigger a supplemental bill?

A refinance alone is not a change in ownership and this article does not address it further. Supplemental assessments are triggered by ownership transfers or completed new construction, not by financing activity.

This article is general information about California property tax law, not tax advice, and it is not a substitute for guidance specific to your purchase. Confirm the expected supplemental assessment, billing timeline, and payment schedule for your property with the Los Angeles County Assessor's office or a CPA before closing.

Topics: taxes, supplemental property tax, Los Angeles County, property purchase, South Bay, closing costs

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