How to Calculate Your Supplemental Property Tax Bill

To calculate your supplemental property tax in California, subtract the prior assessed value from the new base year value, multiply that difference by your local tax rate, and multiply the result by a proration factor tied to the month the change took effect. The proration factor exists because California’s fiscal year runs July 1 through June 30, and a supplemental bill only covers the months left in that year after your purchase or new construction.

The Four Numbers You Need

Before running the math, pull together four pieces of information.

The new base year value is usually the purchase price on the recorded deed, or the assessor’s appraised value of new construction on its completion date.¹

The prior assessed value is the taxable value already on the county’s roll before the change. It appears on the most recent annual property tax statement and on the county assessor’s website.

Your local tax rate starts at the 1% base cap set by California law, with voter-approved bonds pushing the total higher. Most homeowners see something between 1.1% and 1.25%, though it varies by tax rate area, and the exact breakdown is printed on your annual tax bill.²

The date of the change is the recording date of the deed or the date construction was completed. This sets the proration factor.

Step 1: Find the Supplemental Assessment

Subtract the prior assessed value from the new base year value. The result is the supplemental assessment, which is the slice of value the county will tax for the rest of the fiscal year.

Say you bought a home for $600,000 and the prior assessed value was $400,000. Your supplemental assessment is $200,000. That figure is the base for the calculation, not the bill itself.

If the new value comes in below the old one, the subtraction produces a negative number. A negative supplemental assessment means the county owes you a refund for the months you would have been overtaxed at the higher value.

Step 2: Subtract the Homeowners’ Exemption If It Applies

If the property will be your primary residence and it wasn’t already receiving the homeowners’ exemption under the prior owner, you can knock $7,000 off the supplemental assessment before applying the tax rate. You have to occupy the home within 90 days of purchase to qualify on the supplemental bill.³ File form BOE-266 with your county assessor; it’s a one-time filing, and submitting by February 15 secures the full exemption for that fiscal year.⁴

If the prior owner was already claiming the exemption, you don’t get a second one on the supplemental bill. Your own exemption picks up starting the next fiscal year. The exemption also cannot be applied to a negative supplemental assessment.³

Step 3: Apply the Proration Factor

The assessor uses the first day of the month after the event as the effective date, then applies a factor based on how many months are left in the fiscal year:³

July: 1.00 (12 months remaining)
August: 0.92 (11 months)
September: 0.83 (10 months)
October: 0.75 (9 months)
November: 0.67 (8 months)
December: 0.58 (7 months)
January: 0.50 (6 months)
February: 0.42 (5 months)
March: 0.33 (4 months)
April: 0.25 (3 months)
May: 0.17 (2 months)
June: 0.08 (1 month)

The full formula is: supplemental assessment × tax rate × proration factor = supplemental tax. If the homeowners’ exemption applies, subtract the $7,000 from the supplemental assessment first.

Example Without the Homeowners’ Exemption

You buy a home in October. The prior assessed value was $350,000 and the purchase price was $500,000, so the supplemental assessment is $150,000. Your local tax rate is 1.2%. The effective date is November 1, and the proration factor for November is 0.67.

$150,000 × 1.2% = $1,800. Then $1,800 × 0.67 = $1,206. That’s your supplemental tax for the current fiscal year.

Example With the Homeowners’ Exemption

Same facts, but the prior owner wasn’t claiming the exemption and you moved in within 90 days. Subtract the $7,000 first: $150,000 − $7,000 = $143,000. Then $143,000 × 1.2% = $1,716. Multiply by 0.67 and you get $1,149.72.³

When the Math Produces Two Bills

If your change in ownership or construction completion falls between January 1 and May 31, the county issues two supplemental bills instead of one.⁵ The first bill covers the remaining months of the current fiscal year (through June 30) using the proration factor from the table. The second bill covers the entire upcoming fiscal year starting July 1, using a factor of 1.00 because it applies to a full twelve months.³

For events between June 1 and December 31, only one supplemental bill goes out, covering the remainder of the current fiscal year. The split exists because the county’s roll for the next fiscal year is already being prepared by the time a January-through-May event happens, so the supplemental process has to pick up both years.

Suppose you close on a home March 15. The effective date is April 1. The first supplemental bill covers April through June at a factor of 0.25. The second covers the full next fiscal year at a factor of 1.00. Both use the same supplemental assessment, but the second bill will be four times the first. Budget for that.

When You Won’t Owe a Supplemental Bill at All

Not every transfer triggers reassessment, and if yours is excluded, there’s nothing to calculate. Transfers between spouses, including adding a spouse to a deed or transferring property as part of a divorce, are automatically excluded.⁶ Under Proposition 19, a parent can also transfer a family home to a child without reassessment, provided the child uses it as a primary residence and files for the homeowners’ or disabled veterans’ exemption within one year. That exclusion is capped at the home’s existing taxable value plus $1 million (adjusted annually since 2023), and any market value above the cap is added to the child’s tax base.⁷

1
California Legislative Information. California Revenue and Taxation Code RTC 75.10
2
California State Board of Equalization. California Property Tax An Overview
3
California State Board of Equalization. Supplemental Assessment
4
California State Board of Equalization. Homeowners’ Exemption
5
California Legislative Information. California Revenue and Taxation Code RTC 75.11
6
California Legislative Information. California Revenue and Taxation Code RTC 63
7
California State Board of Equalization. Change in Ownership – Frequently Asked Questions