By Amrit Singh, REALTOR® · The Oberoi Group at Intero Real Estate · Published October 21, 2026
THE SHORT VERSION
- When you buy, your home is reassessed at what you paid. Your regular bill doesn’t catch up right away, so the county sends a separate bill for the difference.
- Your lender doesn’t pay it. It’s mailed to you.
- Close June through December and you get 1 supplemental bill. Close January through May and you get 2.
- Estimate yours →
A few months after closing, a lot of new buyers open a property tax bill they didn’t see coming. It’s not a mistake and it’s not a scam. It’s the supplemental tax bill, and if you just bought in Fremont, Union City, Newark, Hayward or anywhere else in Alameda County, one is probably on its way to you.
Why it happens
Under Prop 13, your property tax is based on your home’s assessed value. When a home changes hands, the Assessor resets that value to the purchase price.
The problem is timing. Your regular annual bill is based on the value on the books as of January 1. If you bought after that, the annual bill still reflects the seller’s old value, often a fraction of what you paid.
The supplemental bill closes that gap. It taxes only the difference between your purchase price and the old assessed value, and only for the months left in the tax year, which runs July 1 to June 30.
What it looks like in real numbers
Say you buy a home in Fremont for $1,600,000. The seller bought decades ago, so it’s assessed at $450,000. The difference is $1,150,000. At Fremont’s 2026 to 2027 rate of about 1.1746%, that’s about $13,508 a year in extra tax, roughly $1,126 a month that isn’t in your regular bill yet.
If you closed in October, your new value kicks in November 1. That’s 8 months left in the tax year, so your supplemental bill is about $9,005.
If you closed in March, your new value kicks in April 1. You get two supplemental bills: about $3,377 for April through June, plus about $13,508 for the next full tax year, because that year’s regular bill was set before you bought.
Either way it adds up to the same thing: about $18,800 a year, the tax on what you actually paid. The supplemental bill isn’t extra tax. It’s your real tax, catching up.
Estimate your supplemental bill
Enter what you paid, the assessed value before you bought, the month you closed and your city.
The catch up is about $13,508 a year, roughly $1,126 a month. Your lender won’t pay these bills unless you send them, so set the money aside now.
Which months each bill covers
The tax year runs July 1 to June 30. Your new value starts the first of the month after you close.
Tax year you closed in
Same home, different closing month
How much arrives as separate supplemental bills, by the month you close. It isn’t extra tax. Close January through May and the catch up for the next full year comes as a second bill instead of on your regular one.
Tallest bar: about $19,136. Your month is outlined.
The part that catches people: your lender doesn’t pay it
If you have an impound account, your lender pays your regular property tax for you. But supplemental bills are mailed straight to you, not to your lender. You’re responsible for paying it, and the county won’t waive a late penalty because you thought your lender had it covered.
Some lenders will pay it out of your impound account if you send them the bill. Call yours and ask. Just don’t assume.
When it’s due
Supplemental bills are paid in two installments, like your regular bill. The due dates depend on when the bill is mailed:
- Mailed July through October: first half due by December 10, second half by April 10.
- Mailed November through June: first half due by the last day of the month after it’s mailed, second half 4 months later.
Miss a deadline and there’s a 10% penalty, plus a fee on the second installment. Your postmark has to be on or before the due date.
When it shows up
It often lands months after closing, and it can even come before your first regular bill. That gap is exactly why people forget about it. By the time it arrives, the closing costs are paid and the furniture is bought.
Why your escrow proration didn’t cover it
At closing, property taxes get split between you and the seller. That split is based on the seller’s old tax bill, not your purchase price. So escrow settled up on the old number, and the supplemental bill is the county billing you for the difference.
What can lower it
The homeowners’ exemption. If the home is your primary residence, $7,000 of assessed value is exempt. It’s small, about $80 a year in Fremont, and on a supplemental bill it only applies if the seller didn’t already use it that year. File within 90 days of moving in.
An appeal, if the value is wrong. You get a Notice of Supplemental Assessment before the bill. If you think the value is off, you have 60 days from that notice to appeal with the Alameda County Assessment Appeals Board.
If you paid less than the old value, the supplemental is negative and the county sends you a refund instead. Rare in the Bay Area, but it happens.
My advice for every buyer
Plan for it on day one. Take the difference between your price and the seller’s assessed value, multiply by your city’s tax rate, and set aside your share for the months left in the tax year. Or use the estimator above.
JUST CLOSED AND NOT SURE WHAT’S COMING?
Text me the address. I’ll look up the old assessed value and tell you roughly what your bill will be.
Questions people ask
Why did I get a supplemental tax bill?
Because you bought a home or finished new construction. The county reassessed it at the new value and billed you for the difference for the rest of the tax year, which runs July 1 to June 30.
Is the supplemental tax bill included in my mortgage payment?
No. It is mailed to you, not your lender, even if your lender pays your regular property tax from an impound account. Some lenders will pay it from the impound account if you send it to them, so call and ask.
How much will my supplemental tax bill be?
Roughly: your purchase price minus the old assessed value, times your tax rate, times the number of months left until June 30, divided by 12. The estimator on this page does the math for any city in Alameda County.
Why did I get two supplemental bills?
You closed between January and May. The first bill covers the rest of that tax year. The second covers the next full tax year, because that year’s regular bill was set on January 1, before you bought.
When is the supplemental tax bill due?
In two installments. If the bill is mailed July through October, the halves are due by December 10 and April 10. If it is mailed November through June, the first half is due by the last day of the month after mailing and the second half 4 months later.
What happens if I pay the supplemental bill late?
A 10% penalty, plus a fee on the second installment. The county will not waive it because you thought your lender was paying.
Can I appeal a supplemental assessment?
Yes. You get a Notice of Supplemental Assessment before the bill, and you have 60 days from that notice to appeal with the Alameda County Assessment Appeals Board.
Do I get a supplemental bill if I paid less than the old value?
No. You get a refund instead.
SOURCES
Alameda County Assessor, supplemental assessments ↗
California State Board of Equalization, supplemental assessments ↗
Supplemental bill due dates by mailing month (state rule, as published by San Mateo County) ↗
I’m a REALTOR®, not a tax advisor or attorney. This article and the estimator are general information, not tax or legal advice. Estimates use each city’s main 2026 to 2027 tax rate and leave out the homeowners’ exemption and fixed charges. Checked October 21, 2026. Your bill, the Assessor and the Tax Collector have the final word.