Supplemental Tax Bill in Walnut Creek: What Buyers Owe

What is a supplemental tax bill, and how much will I owe after buying in Walnut Creek?

A supplemental tax bill is a one-time, separate bill from Contra Costa County that covers the gap between the seller's old assessed value and your purchase price, prorated from the month after closing through June 30. On many Walnut Creek purchases that gap is large, so the bill can run into the thousands of dollars. It arrives months after closing, goes to you — not your lender — and carries a 10% penalty if you pay it late.

By Michael Delehanty — Delehanty Group | DRE #01505346 | September 30, 2026

You closed, you got the keys, and your mortgage payment looks settled. Then a few months later an envelope from the County arrives asking for another four-figure payment you didn't see coming. That's the supplemental tax bill, and it's one of the most common "nobody warned me" moments I hear from East Bay buyers.

Here's the part that surprises people most: your lender's escrow account usually won't pay it. Let's walk through how it works, what it costs, and what to do before you ever write an offer.

How the supplemental bill is calculated

California reassesses a property when it changes hands. Under Prop 13, the seller may have been paying tax on an assessed value set years ago — sometimes decades ago. When you buy at today's price, the County Assessor resets the taxable value to your purchase price, and the difference is billed to you for the rest of the fiscal year, which runs July 1 through June 30.

The formula is simple:

New assessed value (usually your purchase price) minus old assessed value
Multiplied by the tax rate — roughly 1.10%–1.20% in most of Walnut Creek, though your specific tax rate area can differ
Multiplied by the months remaining in the fiscal year, divided by 12

The reassessment takes effect the first of the month after closing. Here's an illustrative example — not a quote, because every parcel is different:

You buy a Walnut Creek home for $1,050,000
The seller's assessed value was $400,000
The difference is $650,000
At a 1.15% effective rate, that's $7,475 a year
You close October 15, so the new value starts November 1 — 8 months left in the fiscal year
Your supplemental bill: about $4,983

Now the part I want you to hear twice: that $7,475 a year is also your new ongoing tax increase. The supplemental bill is just the catch-up for the months between closing and the next July. Your regular annual bill will reflect the full new value starting with the next fiscal year.

If the seller had owned the home a short time — say, a 2022 purchase — the gap is much smaller. If they've owned it since the 1990s, the gap is huge. That's why the old assessed value matters more than almost any other number when you're budgeting.

Closing between January and May? The bill can span two fiscal years, which can mean two supplemental bills.

When it arrives, and why escrow won't cover it

The County notes that supplemental bills are sent only to the property owner, even if you have an impound account with your lender. Lenders don't receive a copy. Escrow accounts typically cover the regular annual bills, not supplemental ones. Servicers like PHH state plainly that supplemental bills aren't part of the standard real estate taxes and are the homeowner's responsibility.

Timing varies. Expect the bill roughly three to nine months after closing, though some arrive sooner. Contra Costa's due dates depend on when the bill is mailed:

Mailed July 1 – October 30: first installment delinquent after December 10, second after April 10
Mailed November 1 – June 30: the dates are printed on the bill, with the first installment due the last day of the following month

Miss a deadline and the County adds a 10% penalty per late installment, plus an additional charge on a late second installment. Don't let a bill sit on the kitchen counter because you assumed the lender handled it.

There's one more trap. If you refinance in the first year, your lender bases the new payment on the updated tax liability — which can raise your monthly payment versus what you assumed from the seller's lower bill. The same thing is behind many of the payment surprises I covered in why your Walnut Creek mortgage payment just went up.

What to do before and after closing

After 15 years running a contracting firm here in the East Bay, I learned that the expensive surprises are the ones nobody put in the budget. Here's how I walk buyers through this one:

Ask for the seller's current assessed value before you write the offer. It's on the tax bill in the disclosure package. The bigger the gap from your price, the bigger your supplemental bill and your new annual tax.
Use the County's supplemental tax estimator to get a rough number, then set that cash aside in a separate account the day you close.
Tell your lender you expect a supplemental bill and ask how they want it handled, so a payment doesn't fall through the cracks.
File for the homeowner's exemption. If you occupy the home as your principal residence within 90 days of purchase, you may qualify for a $7,000 exemption on the supplemental assessment — apply within 30 days of the notice. It's small, worth roughly $80 a year at typical rates, but it's free.
Know your appeal window. You have 60 days from the assessment notice mailing date to file an appeal if you believe the new value is too high. The Clerk of the Assessment Appeals Board is at (925) 655-2008.

Buyers 55 and older should also check whether they qualify to carry over an existing tax base — I broke that down in Prop 19 explained for Walnut Creek homeowners. And if your purchase price has slipped below the assessed value in a soft condo market, read is your Walnut Creek property tax bill too high.

For the rest of the cash you'll need at the table, my guide to buyer closing costs in Walnut Creek covers what shows up on the settlement statement. The supplemental bill is the one that doesn't — which is exactly why it's worth planning for. And if you're on the other side of the table, what you'll net selling your Walnut Creek home explains what the seller's tax proration looks like at closing.

Your exact number depends on the seller's assessed value, your price, your closing date, and your tax rate area. That's the kind of math I run with buyers before they make an offer, not after.

Frequently Asked Questions

Is the supplemental tax bill included in my mortgage escrow payment?

Usually not. Contra Costa County sends the supplemental bill only to the owner, and lenders don't get a copy. Escrow accounts typically cover regular annual bills, so contact your loan servicer when the bill arrives and plan to pay it directly by the due date.

How much is a supplemental tax bill in Walnut Creek?

It depends on the gap between the seller's old assessed value and your purchase price. Multiply that gap by roughly 1.10%–1.20%, then by the months remaining until June 30 divided by 12. On a $1,050,000 purchase with a $400,000 prior assessed value, closing in mid-October, the estimate is about $5,000.

When is the supplemental tax bill due in Contra Costa County?

If it's mailed between July 1 and October 30, the first installment is delinquent after December 10 and the second after April 10. If it's mailed between November 1 and June 30, the due dates are printed on the bill. Late installments carry a 10% penalty.

Can I appeal a supplemental assessment?

Yes. You have the same appeal rights as with an annual assessment, and the application must be filed within 60 days of the assessment notice mailing date. Contact the Clerk of the Assessment Appeals Board at (925) 655-2008 for the process.

Do sellers pay the supplemental tax bill?

No. The supplemental bill results from the change in ownership and is billed to the new owner. Sellers are responsible for their own regular property taxes through the closing date, which escrow prorates on the settlement statement.

Next step

If you're trying to figure out what this means for your specific situation, I'm happy to walk you through it. Text or email me directly — (510) 697-3900 or michael@delehantyre.com — and we'll talk through the numbers.

About Michael Delehanty — Delehanty Group | DRE #01505346

Michael Delehanty is a Walnut Creek-based real estate agent with Compass, specializing in buying and selling homes across the East Bay — including Walnut Creek, Concord, Pleasant Hill, Danville, Orinda, and the surrounding communities.

Before becoming a real estate agent, Michael spent 15 years running his own contracting firm in the East Bay, working on thousands of homes and major projects across the Bay Area. That hands-on construction background gives his clients a distinct advantage: when Michael walks through a property, he sees what most agents simply can't. From structural details to renovation potential, his experience translates directly into sharper pricing, smarter negotiation, and fewer surprises at the inspection table.

Michael has been a licensed Realtor since 2005, bringing more than 20 years of experience to every transaction. He has successfully guided clients through complex situations including short sales, bank-owned properties, investment transactions, and competitive multiple-offer scenarios. Whether you are a first-time buyer, a move-up seller, or an investor, Michael brings the market knowledge and problem-solving skills to get deals done.

What sets Michael apart is his deep roots in this community. He has lived in Walnut Creek for nearly 30 years and is genuinely invested in the people here — not just the properties. He served four years as Auction Chair and Athletic Boosters President at Las Lomas High School, and has been a member of a local book club for eight years. His two daughters grew up here, attending Las Lomas before going on to the University of Washington and Cal Poly San Luis Obispo. When Michael helps you buy or sell a home in Walnut Creek or the surrounding East Bay communities, he is not just doing a transaction — he is working in the neighborhood where he has built his own life.

michael@delehantyre.com | (510) 697-3900 | michaeldelehanty.com