California Supplemental Property Tax, Explained
If you recently bought a home or finished new construction in California and a “supplemental” property tax bill showed up — separate from your regular bill and the amount you saw at closing — you're not being double-charged. A supplemental bill is a one-time catch-up for the change in your home's assessed value, and it surprises almost every first-time buyer.
Here's what a supplemental property tax is, why you got one (or two), whether it repeats, and how the amount is figured.
Quick answer
A supplemental property tax bill is a one-time charge that catches up the tax on the increase in your home's assessed value after you buy it or complete new construction. It's separate from your regular annual bill. If the event happened between January and May you may get two supplemental bills — but it does not repeat every year.
Why did I get a supplemental tax bill?
When you buy a home, California resets its taxable value to what you paid — a new Proposition 13 base-year value — as of the closing date. The prior owner may have held a much lower assessed value for years. Your regular tax bill won't reflect the new, higher value until the next fiscal year, so the assessor issues a supplemental assessment for the difference and bills the tax on it from the date you took ownership through the end of the fiscal year (June 30). The same happens when you complete new construction — the added value is assessed and billed supplementally.
Why did I get two supplemental tax bills?
Timing. California's property-tax fiscal year runs July 1–June 30, and the county starts building next year's regular roll early in the calendar year — at the old value. If your purchase or new construction is completed between January 1 and May 31, that next-year roll is already set low, so you receive two supplemental bills: one for the rest of the current fiscal year, and one for the entire next fiscal year. An event between June 1 and December 31 typically produces a single supplemental bill. Two bills is normal — it isn't a mistake or a double charge.
Is the supplemental tax every year?
No — it's a one-time adjustment. Once your supplemental bill (or bills) is paid, the new base-year value is carried on your regular annual bill from then on, rising no more than 2% a year under Proposition 13. You'll only see another supplemental bill if you buy another property or complete more new construction.
How the supplemental tax is calculated
The supplemental amount is the increase in assessed value times your area's tax rate, then prorated for the months left in the fiscal year:
| Step | Example |
|---|---|
| New base-year value (what you paid) | $650,000 |
| Prior assessed value (old owner's base) | $450,000 |
| Supplemental assessment (the increase) | $200,000 |
| Tax at ~1.1% | $2,200 for a full year |
| Proration (e.g. bought in February → 5 months left) | $2,200 × 5/12 ≈ $917 |
The exact proration factor depends on the month of the event. Estimate your own figure with the supplemental property tax calculator.
Estimate your supplemental tax →When it's due and how to pay
A supplemental bill has its own due dates, printed on the bill and based on when it was mailed — it does not follow the regular November 1 / February 1 schedule. Larger supplemental bills are split into two installments with separate delinquency dates. A 10% penalty applies after each delinquency date, so pay by the dates shown on the bill and confirm the amount with your county Treasurer-Tax Collector.
Frequently Asked Questions
Why did I get a supplemental tax bill in California?
Because your property was reassessed. When you buy a home or complete new construction, the county assessor sets a new base-year value as of that date. The supplemental bill charges tax on the difference between that new value and the prior owner's (usually lower) assessed value, prorated from the date of the event to the end of the fiscal year (June 30). It's separate from — and on top of — the regular annual tax bill.
Why did I get two supplemental tax bills?
You get two when the change of ownership or new construction happens between January 1 and May 31. By then the county has already started preparing the next fiscal year's regular roll at the old value, so the assessor issues one supplemental bill for the remainder of the current fiscal year and a second for the entire next fiscal year. An event between June 1 and December 31 usually produces just one supplemental bill.
Will I get a supplemental tax bill every year?
No. The supplemental bill is a one-time adjustment (billed as one or two bills). Once it's settled, your new base-year value is folded into the regular annual property tax bill you'll receive every year going forward — you won't keep getting supplemental bills unless you buy again or complete new construction.
Is the supplemental tax the same as my regular property tax?
No. The regular annual bill (mailed in the fall, due in two installments) taxes your assessed value for the full fiscal year. The supplemental bill is a separate, one-time bill that captures the increase in value from the moment you bought or built until the assessor's records caught up. It has its own due dates printed on the bill, not the November/February schedule.
What triggers a supplemental assessment?
Two things: a change of ownership (a purchase or other reassessable transfer) or completed new construction (a room addition, an ADU, a pool, or a newly built home). Routine maintenance and repairs don't trigger it. If your purchase qualified for an exclusion (for example a Proposition 19 base-year transfer or a parent-child exclusion), the supplemental amount can be small or zero.
How much will my supplemental tax bill be?
It's the increase in assessed value (new value minus the old value) times your area's tax rate, prorated for the number of months left in the fiscal year. On a home bought for $200,000 above the prior assessed value at about a 1.1% rate, a full year would be about $2,200; a purchase in, say, January would be prorated to roughly five months of that. Use the supplemental property tax calculator to estimate yours.
When is the supplemental property tax due?
Supplemental bills carry their own due dates, printed on the bill, based on when it was mailed — they do not follow the regular November 1 / February 1 schedule. Like the regular bill, a supplemental bill of enough size is split into two installments with separate delinquency dates. Pay by the dates on the bill to avoid a 10% penalty.
Learn More
General educational information about California supplemental property taxes, not legal, tax, or financial advice. Proration factors, due dates, and amounts vary by county and by the date of your event; confirm the figures on your bill with your county Assessor and Treasurer-Tax Collector.