Supplemental property tax in California catches many buyers off guard. A few months after closing, you will likely get a supplemental bill. It is separate from the regular annual bill, it goes to you rather than your lender, and on a house the seller owned for decades it can be larger than buyers expect.
None of that is a surprise once you know how Proposition 13 works. This guide explains why the bill exists, how the county calculates it, when it arrives and what to file after you move in, with a focus on buyers of long-held mid-century homes.
Proposition 13 in brief
California voters approved Proposition 13 on June 6, 1978. It rolled local real estate assessments back to 1975 market value levels and limited the property tax rate to 1 percent plus the rate needed to pay voter-approved bonds. After that, a property's base year value can rise each year by the inflation rate or 2 percent, whichever is less.
Property is reassessed to current market value only when it changes ownership or when new construction is completed. When you buy a home, the Board of Equalization says that a purchase usually means 100 percent of the property is reassessed to its current market value. The deed you record is accompanied by a Preliminary Change of Ownership Report, a short questionnaire that tells the assessor about the transfer.
Why the seller's tax bill is a poor guide for a long-held house
If the seller has owned the house for decades, its assessed value has grown each year by no more than the Proposition 13 limit from the base year set when the seller bought it, or from the 1975 level if the house hasn't changed hands since then.
When you buy, your base year value starts over at the market value on your purchase date. So the seller's annual bill tells you little about yours. The supplemental bill is how the county catches up on the difference for the part of the tax year that remains after you take ownership. The longer the seller owned the house, the wider that gap can be.
A practical step at the offer stage: look up the current assessed value on the county assessor's records and compare it with your price. You won't know the new assessed value until the assessor sets it, but the comparison shows whether the catch-up bill is likely to be small or substantial, and it keeps the number from arriving as a surprise.
How the supplemental bill is calculated
The county assessor subtracts the property's prior assessed value from its new assessed value. The difference is the net supplemental assessment. It is usually an increase, but it can be negative, in which case the auditor-controller sends a refund instead of a bill.
Proration. The supplemental amount is prorated by month. It takes effect on the first day of the month after the month in which you took ownership or the new construction was completed.
One bill or two. If the change in ownership happens between June 1 and December 31, the county issues one supplemental bill or refund. If it happens between January 1 and May 31, it issues two: one for the rest of the current fiscal year and one for the next, because the next year's regular roll is based on the January 1 lien date, before your purchase.
Notice first. The county assessor sends a Notice of Supplemental Assessment that shows how the amount was figured. In San Luis Obispo and San Mateo counties, that notice comes about 60 days before the bill, and San Mateo's tax collector says supplemental bills are generally mailed within nine months after a change in ownership or new construction.
Who pays, and why the lender usually doesn't
Supplemental bills are sent directly to the property owner, as state law requires. Lenders who pay your annual taxes from an impound account do not receive the original or a copy of the supplemental bill. The Board of Equalization warns that penalties for a missed payment can't be excused because of a misunderstanding with a lender. If you have an impound account, call your loan servicer, but plan to pay the supplemental bill yourself.
The bill is payable in two installments, and due dates depend on when it is mailed. In San Mateo County, for example, a supplemental bill mailed between July and October has a first installment that becomes delinquent on December 10 and a second on April 10. Bills mailed between November and June have a first installment that becomes delinquent on the last day of the month after mailing, and a second four months later. Your county tax collector's website lists its own schedule.
What to file and check after you close
Homeowners' exemption. The California Constitution provides a $7,000 reduction in taxable value for an owner-occupied home that was your principal residence on January 1. It is a one-time filing with the county assessor on form BOE-266, due by February 15 for the full exemption that year. Some county assessors note the exemption may also apply to your supplemental bill.
Read the supplemental notice. Check the value the assessor used and the date of the change in ownership. Your county assessor's website explains how and when to request a review or file an appeal; in San Luis Obispo County, for example, a review request is due within 60 days of the notice's mailing date.
Set aside the money. Treat the supplemental bill as a closing cost that arrives late. Your agent or escrow officer can help you estimate the new assessed value and the tax rate for your parcel.
Keep your closing papers together. Your settlement statement, the recorded deed and your copy of the change of ownership report make questions with the assessor easier to resolve.
Adding on or building an ADU later
Completed new construction is also a supplemental event. When you add on, the assessor gives the newly constructed portion its own base year value, and the rest of the property keeps its existing base year value. So an addition or a backyard accessory dwelling unit will generate its own supplemental bill once it is complete. Ask the county assessor how it will value your project before you build.
This is general information as of November 2026, not tax advice. Rules and dates vary by county, so check with your county assessor and tax collector, and talk with a tax advisor about your own situation. For more on the buying process, see our buyer resources and mid-mod homes FAQ.
Frequently asked questions
Will my lender pay my supplemental property tax bill?
Usually not. Supplemental bills go directly to the owner, and lenders don't receive copies even when they pay the annual bill from an impound account. Plan to pay it yourself, and talk to your loan servicer if you want it handled another way.
Why did I get two supplemental bills?
If your purchase closed between January 1 and May 31, the county issues two supplemental bills. One covers the rest of the current fiscal year, and the other covers the next fiscal year, whose regular roll is based on the January 1 lien date, before the sale.
Can a supplemental assessment lower my taxes?
Yes. If the new assessed value is lower than the prior assessed value, the net supplemental assessment is negative and the county auditor-controller issues a refund. A refund can't be used as a credit toward an annual bill that is still due.
When should I expect the bill?
It varies by county. In San Mateo County, for example, the tax collector says supplemental bills are generally mailed within nine months of the change in ownership, with the assessor's notice arriving about 60 days before the bill. Your county tax collector's website gives its own timing.
Related reading
Prop 19 Inherited Home Rules: A Guide for Heirs of Mid-Century Houses
Renovation Loan for an Older Home: Options for Mid-Century Houses
Insurance for Older Homes in California: A Mid-Century Owner's Guide
Buying a mid-century home and want help planning for closing costs and taxes? Call or text the Boyenga Team at (408) 373-1660 or (650) 383-8606, or contact the Boyenga Team. We can walk you through our buying assistance process.
