Property Tax · Calculator · Reviewed July 14, 2026
Prop 13 tax reset estimator
Estimate the reassessment gap that can become one or two supplemental bills after a purchase or completed project.
The short version
The supplemental tax applies to the value change
Start with the new assessed value minus the prior assessed value. Multiply that change by the local value-based tax rate, then use the state factor for the month after the sale or completed construction. Timing decides the count: a January through May event generally makes two supplemental bills or refunds, and a June through December event generally makes one. The math can run both ways, because a lower new value can make a refund instead of a bill. Either way, the regular annual bill remains separate.
What changes the answer: Purchase price can be a useful first estimate, but the assessor sets the value. The 1.1 percent default is only a rough stand-in for the local value-based rate and does not include Mello-Roos, direct charges, penalties, or every exemption.
It is easy to see the seller's old tax bill and think yours stays the same. It often does not. After many sales, the county assessor can set a new value. A supplemental bill may come later.
How it works
A sale or completed project can start the extra bill
A change in ownership or completed new construction can create a supplemental event. The assessor finds the new value for the property or new work and subtracts the value already on the roll. That difference is the net supplemental value.
A positive difference can create tax. A negative difference can create a refund. This is not a second tax on the full home value. It is the timing system that puts the increase or decrease into effect before the next regular annual roll catches up.
Use the assessed-value gap and the local rate
The first part of the estimate is new assessed value minus prior assessed value. For a recent ordinary sale, the purchase price may be a useful starting number. Transfers with exclusions, partial interests, family rules, or unusual terms can produce a different assessed result.
Apply the local value-based property-tax rate to that gap. Proposition 13 starts the basic levy at one percent, while voter-approved debt can raise the value-based rate. Flat charges, parcel taxes, Mello-Roos, and other direct assessments do not belong in this multiplication.
The state factor follows a July-to-June year
California's property-tax fiscal year runs from July 1 through June 30. The supplemental change becomes effective on the first day of the month after the event. A March sale therefore starts on April 1 and has a 0.25 factor for April, May, and June.
The official factors use two decimal places. An October event starts November 1 and uses 0.67. A June event moves to July 1, has no current-roll piece, and uses one full year on the roll being prepared.
A March example shows the two pieces
Suppose the new value is $850,000 and the prior value is $420,000. The gap is $430,000. At 1.1 percent, one full year of tax on that gap is $4,730.
For a March event, the first piece is $4,730 times 0.25, or $1,182.50. The second piece is the full $4,730 for the coming fiscal year. The rough supplemental total is $5,912.50 across two bills. The county can use a different value or rate.
The annual bill and impound account stay separate
Keep paying the regular annual bill as written. A supplemental reduction does not cancel or offset that bill. The new value normally reaches a later annual roll, so the ordinary annual amount can also rise after the supplemental cycle.
A mortgage lender that pays annual taxes from an impound account does not receive the original supplemental bill. Read county mail yourself and ask the lender how it treats the charge. The assessor handles value; the tax collector handles the bill and payment.
First useful action
Start with the calculator
Use the estimate here, then confirm the result with the official source for the address or transaction.
Calculator
Rough tax reset
Enter the new and prior assessed values above to see a rough estimate.
This is a rough estimate, not a tax bill. The county assessor and tax collector control the real value, tax rate, exemptions, add-on charges, proration, and bill.
First moves
- 1
Type in the purchase price or new value.
- 2
Type in the seller's current assessed value if you know it.
- 3
Pick the month the sale or other reset happened.
- 4
Use the county's real tax rate if you have it. If not, start with 1.1 percent.
- 5
Treat the answer like a warning light. Then check the county assessor and tax collector.
Watch for
- 1
Prop 13's 1 percent idea is only the starting point. Local debt, special taxes, Mello-Roos, and direct charges can add more.
- 2
A January to May reset can make two supplemental bills. June to December usually makes one.
- 3
If the new value is lower than the old value, the math can point to a refund instead of a bill.
- 4
The annual bill remains due. A lender that pays annual taxes through an impound account does not receive the original supplemental bill.
- 5
The real bill depends on the date, county timing, exemptions, and assessed value.
- 6
A low old tax bill can reset sharply after a sale.
- 7
This is not a county bill and not a substitute for escrow, assessor, or tax-collector numbers.
Official sources
Where to confirm this
Use the source that matches the step you are on. Current forms, fees, deadlines, and agency decisions can change after this page is reviewed.
Use this as a map. It does not decide your rights, tell you what to file, or say someone broke the law. If a deadline, denial, eviction, firing, injury, tax bill, permit fight, or insurance dispute is on the line, use the official source or a licensed professional.