What is the supplemental property tax in California, and how much will I owe?
When you close on a home in California, the county reassesses the property at your purchase price under Proposition 13. The supplemental tax bill is a one-time charge covering the difference between the prior assessed value and your new value, prorated for the months remaining in the fiscal year. The Napa County effective property tax rate runs about 1.28% of assessed value, and Solano County rates are similar. The bill arrives three to nine months after closing — and your escrow account does not pay it.
By Kasama Lee, REALTOR® | RE/MAX Gold | DRE #01408667 | May 10, 2026
If you've recently closed on a home in American Canyon, Vallejo, Napa, Fairfield, Benicia, or anywhere else in southern Napa or Solano County — or you're about to — there's a property tax bill heading your way that nobody talks about until it shows up in your mailbox.
It's called the supplemental property tax. And it's the single most common closing surprise I see new homeowners face.
Here's what you need to know so it doesn't catch you off guard.
Why most California buyers don't see this bill coming
When escrow closes, your settlement statement shows prorated property taxes — the seller pays through the closing date, and you cover the rest of the fiscal year. Many buyers walk away thinking property taxes are squared away.
They're not.
Proposition 13 limits annual increases on assessed value to 2% per year — until a property changes hands or new construction is completed. When either happens, the county assessor reassesses the property at the new value (your purchase price) and sends a separate bill covering the difference between the old assessed value and the new one, prorated for the rest of the fiscal year.
That bill is the supplemental tax. And almost every California buyer is surprised by it for one specific reason:
Your escrow impound account does not pay it.
If you have a mortgage with an impound account, your lender collects monthly for the regular annual property tax bill — but the supplemental bill is mailed directly to you, not your lender. A copy isn't even sent to your servicer. You're responsible for paying it on time, and if you miss the deadline, the county can hit you with a 10% penalty plus a 1.5% monthly accrual once the property goes into tax default.
How much is the supplemental tax bill in Napa or Solano County?
The math is straightforward, but the result depends on three things:
- The difference between the prior assessed value and your purchase price
- Your county's tax rate
- The proration factor based on your closing month
In Napa County, the median effective property tax rate runs about 1.28% — meaningfully higher than the 1.21% California state median. Solano County rates are similar. Your specific tax rate area can be slightly higher or lower depending on local bond debt, school district overlays, and any 1915 Act assessments.
Here's how the proration factor works. If the effective date of your reassessment is August 1, the proration factor is .92. September 1 is .83. October 1 is .75. November 1 is .67. December 1 is .58. January 1 is .50. February 1 is .42. March 1 is .33. April 1 is .25. May 1 is .17. June 1 is .08. July 1 closings get a full annual factor of 1.0, but most California closings land mid-fiscal-year, so you'll usually see one of the partial factors above.
A real example. You buy a home in American Canyon for $850,000 in August. The prior assessed value was $620,000. The increase is $230,000. At a 1.28% effective tax rate, the full annual supplemental tax would be roughly $2,944. With an August 1 effective date, the proration factor is .92, so your supplemental bill comes in at about $2,708.
In Solano County, the math works the same way. The Solano formula spelled out by the county assessor: take the new assessed value minus the prior assessed value, multiply by your parcel's tax rate, then multiply by the BOE proration factor for your effective month.
This is one place where it pays to understand your specific tax rate area before you close. Vallejo, Fairfield, Vacaville, and Benicia each carry their own combination of school bonds and overlay assessments. Two homes at the same purchase price in different cities can produce different supplemental bills.
When does the bill arrive — and what do you do with it?
Counties don't issue the bill the moment escrow closes. The new assessed value takes effect the first day of the month after your closing month, and then the assessor has to process the reassessment, send a notice, and finally mail the bill. In Napa and Solano counties, that timeline typically runs three to nine months — sometimes longer if the assessor's office is backed up.
When the bill arrives, you'll see two installments listed on it, similar to the regular annual property tax cycle. You can pay it in full or split it. Just don't ignore it. The penalty schedule is real: 10% if the first installment is late, another 10% plus a $20 cost if the second installment is late, and 1.5% per month once the property goes into tax default.
If your closing happened between January 1 and June 30, you'll often receive two supplemental bills — one for the current fiscal year and one for the upcoming year. That second bill catches a lot of buyers off guard because they've already paid the first and assume they're done.
Why this matters more for Napa and Solano buyers in 2026
Two specific factors make this conversation more important right now in our market.
First, prices have moved. The median Napa County sale price is around $900,000 in spring 2026, and Solano County is closer to $570,000. For long-held homes especially, the gap between prior assessed value and the new sale price can be enormous — which means the supplemental bill is bigger than buyers expect. With more inventory popping up across the region this spring, more buyers are landing on homes that have been owned by the same family for 15, 20, or 30 years — homes where Prop 13 has held the assessed value far below current market value.
Second, home insurance costs and other carrying costs in Napa and Solano counties have climbed in the past two years. Adding a $2,000–$4,000 supplemental tax bill on top of an already-stretched first-year budget is the difference between feeling settled and feeling squeezed.
If you're using your tax refund as part of your closing costs — something I covered in an earlier post on tax refunds and home buying — set aside a portion specifically for the supplemental bill. It's the single most-missed line item in a first-year homeowner budget.
How to plan for the bill before you close
Here's the approach I walk every buyer through:
- Estimate the bill before you write your offer. Take the difference between the prior assessed value (your title preliminary will show it) and your offer price. Multiply by 1.28% (Napa) or your specific Solano tax rate. That's your full-year ceiling. Apply the proration factor for your expected closing month to get a realistic estimate.
- Add the supplemental tax estimate to your reserves. Don't count on impound. Don't count on escrow. Set the money aside in a separate account so it's there when the bill shows up.
- Update your address with the county tax collector. If the seller's mailing address is still on file, the bill could go to them. They have no obligation to forward it. Confirm your address is on file the moment you close.
- Watch the mail for 12 months. That's the window. If you haven't received a bill within 9–12 months, contact the Napa County or Solano County assessor's office directly to confirm where things stand.
Your specific number depends on your purchase price, the prior assessed value, your tax rate area, and your effective month — there's no shortcut around the math. That's the kind of thing I run with every client before we write an offer, so we know exactly what the first 12 months of ownership will cost.
Frequently Asked Questions
Will my escrow account pay the supplemental tax bill?
No. Supplemental tax bills are mailed directly to the homeowner, and a copy is generally not sent to your mortgage lender. Even if you have an impound account funded for the regular annual property tax, the supplemental bill is your responsibility to pay separately.
How much is the supplemental tax bill in Napa or Solano County?
The bill equals the difference between the prior assessed value and your purchase price, multiplied by your tax rate (around 1.28% in Napa County, similar in Solano), multiplied by the proration factor for your effective month. For a $230,000 increase in assessed value at a 1.28% rate with an August 1 effective date, the bill comes in at roughly $2,700.
When does the supplemental tax bill arrive?
In Napa and Solano counties, supplemental bills typically arrive three to nine months after closing. If your closing falls between January 1 and June 30, expect two supplemental bills — one for the current fiscal year and one for the year that begins July 1.
Can the supplemental tax bill be paid in installments?
Yes. The bill is payable in two equal installments, similar to the regular annual property tax. The county lists both due dates on the bill itself. Pay attention to the deadlines — a 10% penalty applies if the first installment is late, and additional penalties accrue if the second is missed.
What happens if I sell the house before the supplemental bill arrives?
The supplemental tax follows the period of ownership it covers, so even if you've already sold, you can be billed for the months you owned the home. Title and escrow typically address this in the closing of the new transaction, but it's something to confirm with your escrow officer before signing.
The bottom line
The California supplemental property tax isn't hidden, and it isn't optional — it's part of how Proposition 13 reassessment works every time a home changes hands. The reason buyers feel ambushed by it is simple: nobody connects the dots between "escrow handled property taxes at closing" and "a separate bill is heading to your mailbox in eight months."
Now you know. Build it into your budget. Watch the mail. Pay it on time.
If you're navigating a purchase in American Canyon, Napa, Vallejo, Fairfield, Benicia, or Vacaville, I'd love to talk it through with you in a private, confidential buyer consultation. We can map out your offer strategy, run a realistic first-year cost estimate (including the supplemental tax), and figure out the best path forward in today's market — no pressure, just a real conversation. Schedule a consultation at https://kasamasells.com/contact.
If you're also thinking about selling your current home as part of the move, a private listing consultation is the right place to start. We can talk pricing, timing, and what your specific home looks like in today's market. You can begin with a free home valuation at https://kasamasells.com/home-valuation.
About Kasama Lee, REALTOR®
Kasama Lee is a RE/MAX Gold Realtor® serving American Canyon, Napa, Vallejo, Fairfield, Benicia, Suisun City, and the broader Vallejo-Fairfield-Napa metro since 2004. A Best of Napa County 2024 award-winning team leader and certified real estate coach for Tom Ferry International, Kasama specializes in helping sellers and buyers navigate single-family homes, new construction, and 55+ active adult communities across southern Napa and Solano counties. With more than two decades of local market experience and a partnership with her husband Barton, a CPA, she brings both negotiation expertise and financial clarity to every transaction. Connect with Kasama at kasamasells.com.
Kasama Lee, REALTOR® | RE/MAX Gold | DRE #01408667
