Why You Got a Supplemental Property Tax Bill in San Francisco
By Danielle Cui · August 8, 2026
You bought a home in San Francisco, paid your property taxes, and then a second bill arrived months later for thousands of dollars you didn't budget for. It's not an error, and it's not a duplicate. It's a supplemental assessment — and it's the single most confusing bill in California property tax.
What it actually is
When you buy, the Assessor reassesses the property to market value as of the transfer date and sets a new base year value. But the tax roll for the current fiscal year was already built on the previous owner's assessed value — often far lower, because Prop 13 had been capping their increases at 2% a year for however long they owned it.
The supplemental bill charges you the difference between the old assessed value and your new one, prorated from the date you took ownership through the end of the fiscal year (June 30).
So the size of your supplemental bill depends on two things:
- How big the value jump was. A long-held home with a low Prop 13 base produces a large supplemental bill for the buyer. This is why buying from a family that owned since 1985 generates a much bigger surprise than buying from someone who bought two years ago.
- When in the fiscal year you closed. Close in August and you're charged for ~11 months; close in May and it's ~2 months.
New construction triggers the same mechanism, measured from the date of completion rather than a transfer.
Why escrow didn't cover it
Two reasons owners get caught:
- Prorations at closing settled the existing bill, based on the seller's assessed value. Nobody prorated a bill that didn't exist yet.
- Impound accounts are sized from the current bill. Your lender collected monthly for the taxes on the roll, not for a supplemental that hadn't been issued. Most lenders don't pay supplemental bills at all — that one is usually yours to pay directly, even if you have an impound account. Check rather than assume.
You may also get two supplemental bills if your purchase straddles the June 30 fiscal year boundary — one for each fiscal year affected. Again, not a duplicate.
When it's due
Supplemental bills don't follow the normal December 10 / April 10 cycle. The due dates depend on when the bill was mailed:
| Bill mailed | 1st installment delinquent after | 2nd installment delinquent after |
|---|---|---|
| July 1 – October | December 10 | April 10 |
| November 1 – June 30 | Last day of the month following mailing | The fourth month |
Read the dates printed on your bill. Late payment carries a 10% penalty on the first installment and 10% plus fees on the second. (The full payment calendar.)
The part almost nobody uses: you can appeal it
A supplemental assessment is appealable, and its deadline has nothing to do with the July–September window:
You have 60 days from the mailing date printed on the supplemental notice. If day 60 lands on a Saturday, Sunday, or legal holiday, you get the next business day.
That window can open in any month, and it's easy to blow through while you're arguing with your lender about escrow. Grounds worth considering:
- The new base year value exceeds market value at the transfer date — for instance, the Assessor enrolled a figure above your purchase price.
- Your purchase wasn't an arms-length transaction — a family transfer, an off-market deal, a purchase from a lender or estate under time pressure.
- The price included non-real-property items — furnishings, business assets, or seller financing at an above-market rate.
- The Assessor's record is wrong on square footage, bed/bath count, or condition, and that error is baked into your new base.
- The proration is wrong — check the effective date against your recorded deed.
That last one is worth a look before you build a valuation argument. Date errors are simpler to fix, and sometimes a call to the Assessor's office resolves it without an appeal at all.
Why the base year value is worth fighting harder than one year's assessment
A Prop 8 decline-in-value appeal gets you a temporary reduction for a single year. A base year value correction lowers the foundation that every future year is factored up from at up to 2% annually. Over a decade of ownership, that difference is substantial.
And you have longer than 60 days for the base year figure itself: it can be appealed during the regular filing period for the first year it's enrolled on the roll, or during any of the next three years — a four-year window. (The full breakdown of appealing after a purchase.)
Two things to do now
File for the homeowners' exemption if this is your principal residence — new owners are usually mailed a claim form, and it's worth about $80 a year plus it's the qualifier for a burden-of-proof advantage at any future hearing. (How to claim it.)
Check whether the enrolled value actually matches the market at your transfer date. CompFinder pulls comparable San Francisco sales around a given date, which is the evidence a supplemental appeal turns on.
Buying into a TIC? The mechanics differ meaningfully — only your fractional interest is reassessed. (How SF TICs are assessed.)
Frequently asked questions
What is a supplemental property tax bill in San Francisco?
A bill for the difference between the previous owner's assessed value and your new base year value, prorated from your ownership date through June 30. It exists because the current year's tax roll was built on the seller's lower Prop 13 value before you bought.
Why didn't my escrow or lender pay my supplemental bill?
Closing prorations settled the bill that existed at the time, based on the seller's assessed value. Impound accounts are also sized from the current bill, and most lenders don't pay supplemental bills — that one is typically the owner's to pay directly. Verify with your servicer rather than assuming.
Can I appeal a supplemental assessment?
Yes, within 60 days of the mailing date printed on the supplemental notice — with the next business day allowed if day 60 falls on a weekend or holiday. This is completely separate from the July 2 to September 15 regular filing window.
Why did I get two supplemental tax bills?
Because your purchase straddled the June 30 fiscal year boundary, so a supplemental assessment was issued for each affected fiscal year. It isn't a duplicate bill.
Is a supplemental bill a one-time charge?
Yes, for that purchase or construction event. Going forward, your new base year value appears on the regular annual bill and rises up to 2% a year under Prop 13. If the market later falls below it, you'd file a Prop 8 decline-in-value appeal.