Supplemental Property Tax Calculator
Just bought a home? Estimate the extra property tax bill that shows up a few months later, and when it's due.
After you buy a California home, the county reassesses it at the purchase price and sends a supplemental tax bill for the difference between the new and old assessed values, times your tax rate, prorated for the months left in the fiscal year (July–June). Buy between January and May and you get two bills. The bill goes to you, not your lender.
The old assessed value is the seller's value on the last tax bill (the county's property tax portal shows it). The new value is normally your purchase price, or the value of new construction when it's completed.
Supplemental Property Tax Calculator
Supplemental Tax
| Bill | Proration factor | Amount |
|---|
How this is calculated
Under Revenue and Taxation Code 75.41, the change in ownership is treated as happening on the first day of the next month, and the supplemental tax is the increase in assessed value times your tax rate times a proration factor for the months left in the fiscal year:
| Purchase month | Treated as | Factor | On a $500,000 increase at 1.1% |
|---|---|---|---|
| January | February 1 | 0.42 + 1.00 | $7,810 |
| February | March 1 | 0.33 + 1.00 | $7,315 |
| March | April 1 | 0.25 + 1.00 | $6,875 |
| April | May 1 | 0.17 + 1.00 | $6,435 |
| May | June 1 | 0.08 + 1.00 | $5,940 |
| June | July 1 | 1.00 (next year) | $5,500 |
| July | August 1 | 0.92 | $5,060 |
| August | September 1 | 0.83 | $4,565 |
| September | October 1 | 0.75 | $4,125 |
| October | November 1 | 0.67 | $3,685 |
| November | December 1 | 0.58 | $3,190 |
| December | January 1 | 0.50 | $2,750 |
Purchases treated as February through June also get a full-year supplemental bill for the next fiscal year, because that year's regular bill was set on January 1 using the old value. A June purchase (treated as July 1) gets one full-year bill for the new fiscal year.
Due dates (R&TC 75.52): due when mailed. Mailed July–October: delinquent after 5 p.m. December 10 and April 10. Mailed November–June: the first half goes delinquent at the end of the month after mailing, the second half at the end of the fourth month after that. A Saturday, Sunday or holiday deadline moves to the next business day (this tool moves weekends only). A 10% penalty applies to late payments.
Frequently Asked Questions
What is a supplemental property tax bill?
A one-time extra bill (or two) that the county sends after you buy a home or finish new construction. Your regular bill for the year was based on the old assessed value, so the supplemental bill charges tax on the increase in value for the rest of the fiscal year.
When will I get the supplemental tax bill?
After the assessor sets the new value and the tax collector bills it, so it arrives some time after closing, not with your escrow papers. It goes to you, not your lender: Los Angeles County, for example, sends supplemental bills only to the property owner, even if you have an impound account.
Why did I get two supplemental bills?
If the purchase or completion falls between January and May, the January 1 lien date for the next fiscal year has already passed with the old value. You get a prorated bill for the current fiscal year and a full-year supplemental bill for the next one, until the regular bill catches up.
When is a supplemental tax bill due?
It is due when mailed. For a bill mailed July through October, the first half goes delinquent after December 10 and the second after April 10. For a bill mailed November through June, the first half goes delinquent at the end of the next month and the second half four months after that. Late payments get a 10% penalty.
Can a supplemental bill be a refund?
Yes. If the new assessed value is lower than the old one (for example, a purchase below the prior owner's value), the county issues a supplemental refund. By law the auditor must pay it within 90 days of enrolling the negative assessment.
This tool provides an educational estimate only. The assessor sets the new value and the tax collector issues the bill; exemptions (such as the homeowners' exemption) and your parcel's exact rate change the amount.