California Supplemental Property Tax Calculator

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.

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.

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.

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.

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.

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.

It depends on the month the bill is mailed (Revenue and Taxation Code §75.52). If it is mailed between July 1 and October 31, the two installments are delinquent on the regular dates: after December 10 and after April 10. If it is mailed between November 1 and June 30, the first installment is delinquent on the last day of the month after the mailing month, and the second on the last day of the fourth month after the first becomes delinquent — so a bill mailed in January is delinquent after February 28 (first) and after June 30 (second). The same 10% penalty applies after each date. The California property tax due-dates page has the regular calendar.

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.

Very common — a supplemental bill follows almost every home purchase or completed addition. In 2025, county assessors processed about 586,443 recorded ownership transfers (439,747 single-family) and 308,953 new-construction assessments statewide, led by Los Angeles County. The by-county table above shows the counts; not every one produces a bill (some transfers are excluded from reassessment), but if you just bought or built, expect one.