If you purchased a house in Contra Costa County, expect to receive your own supplemental property tax bill — separate from the regular annual bill your mortgage company handles through an impound account.

This tax bill catches most new residents to Contra Costa County off guard, and many new homeowners assume their mortgage company will handle it the way it handles their regular annual tax bill, which is not the case for this supplemental tax bill.

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Why the Bill Exists in the First Place

Since July 1, 1983, California law has required a reassessment of real property as of the first day of the month following either a change in ownership or the completion of new construction.

Contra Costa County’s own guidance defines new construction broadly — “any substantial addition to real property (e.g., adding a new room, pool, or garage) or any substantial alteration which restores a building, room, or other improvement to the equivalent of new (e.g., completely renovating an outdated kitchen)” — and the county notes that most sales of property result in this kind of reassessment.

The supplemental bill is the mechanism that captures the difference between the property’s old assessed value and its new one for the stub period between the reassessment date and the next regular tax roll, rather than waiting an entire year to adjust the tax.

How the County Actually Calculates It

The Assessor calculates the difference between the new value — set at the time of purchase or when new construction is completed — and the old value, which was set on January 1 of the previous fiscal year.

The county’s own example: a property whose old assessed value was $100,000 and whose new value after a purchase is $120,000 produces a $20,000 supplemental assessment value, which is then prorated based on how many months remain in the fiscal year from the date of the change.

Buy in January instead of July, and the supplemental assessment covers a much shorter remaining stub period, which is part of why the dollar amount on a supplemental bill can look so different from one buyer to the next even on similarly priced houses.

If you filed for the homeowner’s exemption, the full $7,000 exemption amount is deducted from the supplemental assessment before the supplemental tax is calculated — but only if you apply for it within 30 days of the date on the Assessor’s supplemental assessment notice. Missing that 30-day window means losing the exemption on this particular bill, even if you would otherwise have qualified for it on the regular annual roll.

A Worked Example With Real Numbers

Say a Contra Costa County house was assessed at $450,000 on the prior January 1 lien date, and it sells and closes in September for $650,000, becoming reassessed to that new value as of October 1 under the statewide rule.

The supplemental assessment value is the $200,000 difference between the old and new assessed values, prorated for the roughly nine months remaining in the fiscal year from October through the following June 30 — call it about 75 percent of a full year’s worth of that $200,000 difference, or roughly $150,000 of prorated supplemental assessed value.

At a combined property tax rate around 1.1 to 1.2 percent, typical for Contra Costa County once voter-approved local bonds and assessments are included, that works out to somewhere in the neighborhood of $1,650 to $1,800 in supplemental tax, due in two installments, on top of the regular annual bill the new owner will also owe starting the following fiscal year based on the full new assessed value.

Because this sale closed in September, it falls in the June 1 through December 31 window, so the buyer gets exactly one supplemental bill rather than two.

New Construction Triggers the Same Bill, Even Without a Sale

A supplemental assessment is not limited to purchases. A homeowner who adds a bedroom, builds a pool, converts a garage, or substantially renovates a kitchen to the equivalent of new construction triggers the same kind of supplemental reassessment on the completed improvement, even if the underlying property never changes hands.

The practical effect is the same two-tier system: the regular annual bill continues reflecting the pre-improvement value until the next lien date catches up, while a separate supplemental bill captures the added value for the stub period between completion and the next regular roll.

A homeowner planning a major renovation in Contra Costa County before eventually selling should expect this bill to show up well before the house ever goes on the market, completely independent of any future sale.

Disputing the Assessed Value

If the new assessed value behind a supplemental bill looks too high — based on a purchase price that included personal property, a transaction between related parties at a non-market price, or an assessor’s valuation of new construction that seems inflated — the formal remedy is an appeal to the Assessment Appeals Board, not simply calling the Assessor’s office and asking for a change.

That appeal has to be filed within sixty days of the mailing date on the assessment notice itself, through the Clerk of the Assessment Appeals Board, and missing that window generally forfeits the right to challenge that particular assessment through this process.

Because paying the billed installments on time remains required while an appeal is pending, the practical approach most homeowners take is to pay the bill as issued and pursue a refund of the disputed portion if the appeal succeeds, rather than risk the 10 percent late penalty by withholding payment during the appeal.

One Bill or Two, Depending on When You Bought

Contra Costa County’s own rule on this is specific and easy to get wrong: you will receive one supplemental bill if the change in ownership is recorded on the tax roll between June 1 and December 31, but you will receive two supplemental bills if the change is recorded between January 1 and May 31 — and if you bought between January 1 and May 31, you will also receive an additional supplemental bill covering the entire following fiscal year on top of those two.

In other words, the exact same house, bought a few months apart, can generate a different number of supplemental bills purely because of which side of the January 1 line the recording date falls on.

This is a real quirk of how Contra Costa County’s roll-recording calendar interacts with the statewide reassessment rule, not a billing error, however it looks on paper.

Why Your Mortgage Company Never Sees It

Contra Costa County’s Tax Collector sends the supplemental bill directly to the homeowner, not to the mortgage servicer, which is the opposite of how the regular annual secured tax bill usually works for a buyer with an impound or escrow account built into their mortgage payment.

The county’s own guidance is blunt about this: your impound account will generally only cover your annual tax bill, and homeowners need to contact their lender directly to find out whether — and how — the supplemental bill gets paid, because the county is not going to route it there automatically.

A buyer who assumes every property tax bill flows through the same escrow process their lender already set up can easily let a supplemental bill sit unopened, or miss it entirely if it arrives at a prior address during a move.

Due Dates Depend on When the Bill Was Mailed, Not a Fixed Calendar Date

Unlike the regular annual bill’s fixed December 10 and April 10 delinquency dates, a Contra Costa County supplemental bill’s due dates shift depending on when the Tax Collector actually mails it.

For a supplemental bill mailed between July 1 and October 30, the schedule matches the familiar annual pattern: the first installment is delinquent at 5 p.m. on December 10, and the second at 5 p.m. on April 10.

For a bill mailed between November 1 and June 30, the first installment becomes delinquent at 5 p.m. on the last day of the month following the month it was mailed, and the second installment is due on the last day of the fourth month after that first delinquency date.

A bill mailed in, say, February would have its first installment due at the end of March and its second roughly four months after that — a schedule that does not map cleanly onto the December 10 and April 10 dates most Contra Costa County homeowners already have memorized from their regular bill.

Missing either installment comes with a 10 percent penalty added to whatever installment was not paid on time, plus an additional $20 charge specifically on a late second installment. The bill itself comes in two installments with payment stubs for each one, and it is paid separately from — not combined with — the regular annual secured tax bill.

A County-Specific Delay Worth Knowing About

Contra Costa County’s own tax office notes that because of the large number of parcels and the frequency with which property changes hands in the county, there are often delays in placing new assessments on the roll — meaning the supplemental bill for a given sale does not necessarily arrive right away, and the proration dates used to calculate it should be checked directly with the county rather than assumed from the closing date alone.

A homeowner who has not received a supplemental bill within a few months of closing should not assume none is coming; it may simply be delayed in processing, and interest and penalties still accrue from the original delinquency dates once the bill does arrive, regardless of how late it was sent.

If more than a few months have passed since closing with no bill in hand, calling the Assessor’s office directly to check on the status of the reassessment is a better approach than simply waiting and hoping it never arrives.

Who to Call for What

Contra Costa County splits responsibility for the supplemental tax process across a few different offices. The Assessor’s office, reachable at (925) 313-7400, handles the actual valuation and the homeowner’s exemption deadline. The Auditor-Controller, at (925) 608-9300, handles proration calculations and corrections if the math on a specific bill looks wrong.

If you want to formally appeal the assessed value itself, that goes to the Clerk of the Assessment Appeals Board at 1025 Escobar Street, 1st Floor, Suite 150, Martinez, CA 94553, or by phone at (925) 655-2008, and the appeal has to be filed within sixty days of the mailing date shown on the assessment notice — filing an appeal does not pause the obligation to pay the installments on time while it is pending, so the safest approach is to pay as billed and seek a refund later if the appeal succeeds.

What This Means If You’re Buying, Not Just Selling

Most of the Contra Costa County property tax topics sellers search for are about what they owe at closing.

This one works in the other direction: it is a cost that follows the buyer after the sale closes, and a seller who wants to be straightforward with a buyer, particularly in a market where buyers increasingly research these things themselves, benefits from at least mentioning that a supplemental bill is coming separately from the prorated taxes handled in escrow.

Escrow prorates the existing tax bill between buyer and seller at closing based on the old assessed value; the supplemental bill, calculated off the new post-sale value, is a separate obligation that shows up later and is the buyer’s alone to pay.

How This Fits With the Rest of a Contra Costa County Sale

A supplemental tax bill is only one piece of what changes hands financially in a Contra Costa County sale.

If the property is also going through probate, the county’s own probate court has its own local procedures separate from this tax question — see Contra Costa County Probate: The Courthouse, the Fee, and the Forms You’ll Actually Need for how that side of a sale works.

And if Prop 19’s inherited-property rules are part of why the house is being sold in the first place, Prop 19 and Inherited Property: Why Your Tax Bill Just Jumped explains the reassessment trigger that can apply on top of everything covered here.

Every County Runs This the Same Way, With Its Own Local Details

The basic, state-wide rule is that there is a reassessment on the first of the month after a change of ownership, and it is prorated for the rest of the fiscal year. So this is the same rule for all counties in California.

Therefore, if you recently moved to Contra Costa County from another county in California, you have probably seen a bill similar to this before.

What is different from county to county is what Contra Costa County’s own pages spell out here: is the tax bill sent through the mortgage servicer, or is it sent to the homeowner? What is the delinquency date formula for the tax bill?

And which office processes tax bill disputes? We have detailed how this is handled for Los Angeles and Orange Counties. The due dates for Contra Costa County bills are based on when the Tax Collector mails out the bill.

You cannot assume a due date for a different county’s bill also applies to Contra Costa County.

Before You Close

Ask your lender directly whether your impound account will cover a supplemental bill — most will not
File for the homeowner’s exemption within 30 days of the Assessor’s supplemental notice if you qualify and haven’t already
Note whether your purchase recorded before or after January 1 — it changes how many supplemental bills you should expect
Keep an eye out for a bill addressed to the property even after you’ve moved, especially if you bought in the first half of the calendar year
If you plan to dispute the new assessed value, calendar the sixty-day appeal deadline the day the notice arrives, not the day you decide to act on it

If an unexpected supplemental tax bill is part of why you’re looking at selling a Contra Costa County house instead of holding onto it, Cash Home Buyers CA can make a no-obligation cash offer and we pay the standard closing costs, including the county and city transfer taxes, escrow and title.

We buy directly from homeowners throughout Concord and the rest of Contra Costa County, and you can find Cash Home Buyers CA on Google to see how we work before you call.

Contra Costa County’s Supplemental Tax Bill: Why It Skips Your Mortgage Company