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Guide

Supplemental Property Tax: What New Owners Should Expect

By Karla I. Vasquez, REALTOR® · Keller Williams Realty Danville · Reviewed October 2026

Quick answer

A supplemental property tax bill is a one-time extra bill that usually arrives a few months after you buy a home or finish new construction. It covers the difference between the old assessed value and your new value for the rest of the tax year. It does not replace your regular bill, and in Contra Costa County it is mailed to you, not your lender, even if you have an impound account.

What is a supplemental tax bill?

When a home changes ownership or new construction is completed, California law requires the property to be reassessed. The State Board of Equalization explains that a prorated supplemental assessment is issued for that change.

Contra Costa County puts it simply:

  • Supplemental assessment = new base value minus old base value.
  • The bill covers taxes on that increase for the remaining months of the tax year.
  • The reassessment takes effect on the first day of the month after the ownership change or completion of construction.
  • Going forward, the higher value is included in your regular annual bills.

If you bought from a long-time owner, the gap between their assessed value and your purchase price can be large, and so can the supplemental bill.

Why didn't my lender pay it?

This surprises many new owners. Contra Costa County states that supplemental bills are not sent to mortgage companies and are not paid from impound accounts. The bill goes only to the owner.

So even if your monthly payment includes taxes, you'll need to pay the supplemental bill yourself. Some lenders may let you add it to your impound account afterward, so it's worth asking.

Can I get more than one supplemental bill?

Yes. Depending on when you closed in the tax year (July 1 to June 30), you may receive one or more supplemental bills. The County notes that due dates depend on when the bill is mailed, and supplemental bills follow the same penalty rules as annual taxes. Read each bill's due dates carefully.

If the new value is lower than the prior value, the reassessment can produce a refund instead of a bill. The County notes that a refund does not change what's due on your annual bill, which still must be paid on time.

Don't forget the regular bill

New owners can fall through the cracks on the annual bill too:

  • Annual secured bills are mailed once a year, by November 1. If you closed after that, the bill may have gone to the seller.
  • The County says missing a bill doesn't excuse late payment or penalties.
  • In your first year, you may receive an "Information Copy Only" bill tied to the prior owner's lender. Call your lender right away to confirm who's paying.
  • Installments are due November 1 (delinquent after December 10) and February 1 (delinquent after April 10).

Apply for the homeowners' exemption

If the home is your principal residence, you may qualify for a $7,000 homeowners' exemption. The County says buyers may be eligible on the supplemental assessment too if the home didn't already have an exemption and you occupy it within 90 days of purchase. The Assessor typically mails an application within several months; return it promptly.

Karla's tips

  • Set money aside at closing. Ask your escrow officer and lender what the seller's assessed value was versus your price, and estimate the gap with the County's Supplemental Tax Estimator.
  • Watch your mail for several months. Supplemental bills can arrive well after escrow closes. Put a reminder on your calendar.
  • Building or adding on? As Owner & Principal of KCK Construction & Remodeling, I tell clients that completed new construction can also trigger a supplemental bill. Plan for it in your project budget.
  • Keep your closing file. Your settlement statement shows how taxes were prorated with the seller.
  • Questions in Spanish? I can help you read the bill and know who to call. (510) 677-8695.

En español: lo básico

El impuesto suplementario es una factura adicional, de una sola vez, que llega unos meses después de comprar una casa o terminar una construcción nueva. Cubre la diferencia entre el valor anterior y el nuevo valor por el resto del año fiscal. En Contra Costa, esta factura se envía al dueño y no se paga de la cuenta de impuestos (impound) del préstamo, así que usted debe pagarla directamente. Si la casa es su residencia principal, solicite también la exención de propietario de $7,000.

Sources

General information only, not legal, tax or financial advice. Rules and fees change; confirm with the County, the Board of Equalization, your tax professional or an attorney. Karla I. Vasquez is a REALTOR® and Registered LDA, not an attorney.