California Supplemental Property Tax Calculator
When you buy a home or complete new construction in California, the assessor reassesses the property and sends a one-time supplemental tax bill for the difference between the new assessed value and the old one — prorated for the rest of the fiscal year. It arrives on top of your regular property tax, and it surprises almost every new owner.
This calculator estimates that supplemental bill (or refund), and shows whether you'll get one bill or two.
How your supplemental tax bill is calculated
The assessor subtracts your property's prior assessed value from its new assessed value. The difference — the net supplemental value — is taxed at the county rate and prorated for the part of the fiscal year that remains:
The formula
(new value − prior value) × tax rate × (months remaining ÷ 12)
The proration factor depends on when the change happened, because the supplemental period runs from the first of the following month through June 30:
| Change occurs in | Months remaining | Proration factor |
|---|---|---|
| July | 11 | 0.92 |
| August | 10 | 0.83 |
| September | 9 | 0.75 |
| October | 8 | 0.67 |
| November | 7 | 0.58 |
| December | 6 | 0.50 |
| January | 5 | 0.42 (+ second bill) |
| February | 4 | 0.33 (+ second bill) |
| March | 3 | 0.25 (+ second bill) |
| April | 2 | 0.17 (+ second bill) |
| May | 1 | 0.08 (+ second bill) |
| June | 12 | 1.00 (full next year) |
Why some buyers get two supplemental bills
If your purchase or completed construction falls between January 1 and May 31, you'll receive two supplemental bills instead of one. The first covers the remaining months of the current fiscal year. The second covers the entire next fiscal year — because the regular tax roll for that year was locked in at the old, lower value before your change was recorded, so the county catches up the difference with a second supplemental bill.
Example (State Board of Equalization)
A home is purchased in March for $300,000; the prior assessed value was $261,000. The net supplemental value is $39,000, which at about 1.025% is roughly $400 a year.
- First bill (rest of this year): $400 × 0.25 = $100
- Second bill (full next year): $400
Frequently Asked Questions
What is a supplemental property tax bill?
A one-time bill for the change in your property's assessed value after a purchase or completed new construction. It covers the difference between the new value and the prior value, prorated from the date of the change through the end of the fiscal year (June 30). It is separate from, and in addition to, your regular annual tax bill.
How much is a typical supplemental tax bill?
There is no fixed amount — it depends on how much your assessed value rose and when the change happened. As a rough guide, at about a 1.1% rate every $100,000 of increase in assessed value is roughly $1,100 for a full year, then prorated for the months left in the fiscal year. Example: buying a $700,000 home that was previously assessed at $350,000 is a $350,000 increase (~$3,850/year); a purchase in December (factor 0.50) would produce a first supplemental bill of about $1,925. Use the calculator above for your own numbers.
Why did I get two supplemental tax bills?
If your change of ownership or new construction occurred between January 1 and May 31, you receive two supplemental bills: one prorated for the remaining months of the current fiscal year, and one for the entire following fiscal year. That second bill exists because the regular tax roll for the next year was already prepared at the old, lower value before your change was recorded.
How is the supplemental tax calculated?
Net supplemental value = new assessed value − prior assessed value. That is multiplied by the county tax rate to get a full-year amount, then multiplied by a proration factor equal to the number of months left in the fiscal year divided by 12.
How is the proration factor determined?
The supplemental assessment takes effect on the first day of the month after the change, and the factor is the months remaining until June 30 divided by 12. For example, a March purchase is effective April 1, leaving 3 months, for a factor of 0.25.
Is the supplemental bill a yearly bill?
No. It is a one-time catch-up for the period between your change and when the new value appears on the regular roll. Going forward, your normal annual bill reflects the new assessed value.
Does the supplemental tax include Mello-Roos or parcel taxes?
This estimate applies a county-level ad-valorem rate to the change in assessed value. Fixed charges like Mello-Roos, parcel taxes, and direct assessments are generally billed separately and are not included here.
Learn More
This is an educational estimate of California supplemental property tax under Revenue & Taxation Code § 75 et seq., not an official bill or tax advice. County assessors and tax collectors determine the official supplemental assessment and amount, which may differ. Rates shown are county-level estimates and exclude fixed direct charges.