Three to nine months after closing on a San Diego home, an envelope arrives from the County Treasurer-Tax Collector. Inside is a bill for thousands of dollars — sometimes more than $10,000 — that was never quoted at closing, isn't part of the impound account, and isn't optional. This is the supplemental property tax bill, and almost no first-time buyer sees it coming.

It's not a scam. It's not a billing error. It's the legally required reconciliation of the prior owner's tax assessment with your purchase price. The county's own homeowner guide says it plainly: do not expect your lender to pay this bill.1 Here's what it is, why it shows up, and how to budget for it before it surprises you.

What it actually is

California Revenue and Taxation Code §§ 75–75.72 requires county assessors to reassess any property whenever ownership changes or new construction is completed.2 When you buy a home, the county compares the prior owner's assessed value (the basis they were paying tax on) to your purchase price (your new basis), and bills you for the difference — prorated for whatever portion of the fiscal year you owned the home.

The county's calculation, in plain English:3

  1. Take your purchase price.
  2. Subtract the prior owner's most recent assessed value.
  3. Multiply the difference by your effective tax rate (roughly 1.02%–1.19% in San Diego County, depending on the tax rate area).4
  4. Prorate by the number of months remaining in the fiscal year (which runs July 1 through June 30).

If the previous owner had been there 15 years and the home appreciated significantly, the gap between their assessed value and your purchase price can be enormous — and your supplemental bill is calculated against that gap.

A worked example

Consider a typical scenario: you buy a $1,000,000 home in San Diego that closes on October 1. The prior owner had owned it since 2010 and was paying tax on an assessed value of $520,000 (that's their original basis plus the maximum 2% Proposition 13 inflation adjustments).

Calculation stepAmount
Your purchase price (new assessed value)$1,000,000
Prior owner's assessed value$520,000
Supplemental assessment (the increase)$480,000
Effective tax rate (approx)1.18%
Annual tax on the increase$5,664
Months remaining in fiscal year (Oct–Jun)9 months
Supplemental bill$4,248

That's just the first one. If you closed between January 1 and May 31, you'll get two supplemental bills — one covering the remainder of the fiscal year you bought in, and a second covering the next fiscal year (which starts before the assessor finishes the reassessment).5 A buyer in this scenario who closed in early March could see total supplementals of $8,000–$12,000.

Why this catches buyers

The closing disclosure shows estimated property tax based on the prior owner's assessment. That's the figure your lender used to size your impound account. The supplemental bill closes the gap retroactively — and it almost never goes through escrow. It comes directly to the homeowner, with a payment deadline.

Why it isn't escrowed

Your monthly mortgage payment includes property tax via your impound account, but the impound is sized to cover the annual secured tax bill — not supplementals. Some lenders will accept supplemental bills into escrow on request, but the default is that the bill goes to you, payable in two installments with hard delinquency dates.1 Miss the date and a 10% penalty applies. Miss both installments and the penalty escalates further.

How to budget for it before closing

Three steps that take less than 30 minutes total:

  1. Pull the seller's current assessed value. The San Diego County Assessor's online parcel lookup shows the assessed value the seller has been paying tax on. This is the figure you'll be reassessed against.
  2. Calculate the gap × your effective rate. Subtract the seller's assessed value from your purchase price. Multiply by 1.18% (or your specific tax rate area). That's your annualized supplemental.
  3. Estimate the proration. If you're closing in October, expect ~9/12 of the annualized figure. If closing in March, expect a full bill plus a partial second one.

The San Diego County Assessor publishes a free supplemental tax estimator that does this calculation for you.6 Run it on every property you're seriously considering — not just the one you're under contract on.

Two things people get wrong

"I'll just pay it from my impound account." You probably can't. Most impound accounts are sized for the annual bill only. The supplemental arrives as a separate invoice that you pay personally.

"My lender will tell me about it." Many won't, or will mention it once at closing in a way that doesn't register. The county's guidance is explicit: do not expect your lender to pay the supplemental tax bill.1

Run a full San Diego payment estimate, supplementals included.

Open the calculator →

The bottom line

The supplemental tax bill isn't a fee or a penalty — it's the catch-up payment that brings your tax basis in line with what you actually paid. On a typical San Diego purchase, budget for $4,000–$10,000 in supplemental bills arriving 3–9 months after closing, on top of anything escrowed. Set the cash aside before you close. The bill will come.

Supplemental tax calculations are illustrative; your actual bill will depend on the specific parcel, prior assessed value, tax rate area, and closing date. Always verify with the San Diego County Treasurer-Tax Collector. Educational content only — not legal, tax, or financial advice.

References

  1. San Diego County Treasurer-Tax Collector. (n.d.). Supplemental property taxes. Retrieved April 28, 2026, from https://www.sdttc.com/content/ttc/en/tax-collection/supplemental-taxes.html
  2. California Revenue and Taxation Code §§ 75–75.72. Retrieved April 28, 2026, from https://leginfo.legislature.ca.gov/faces/codesTOCSelected.xhtml?tocCode=RTC
  3. California State Board of Equalization. (n.d.). Supplemental assessment. Retrieved April 28, 2026, from https://www.boe.ca.gov/proptaxes/supplemental-assessment/
  4. San Diego County Treasurer-Tax Collector. (n.d.). Secured property taxes. Retrieved April 28, 2026, from https://www.sdttc.com/content/ttc/en/tax-collection/secured-property-taxes.html
  5. San Diego County Assessor/Recorder/County Clerk. (n.d.). Property tax frequently asked questions. Retrieved April 28, 2026, from https://www.sandiegocounty.gov/content/sdc/auditor/ptsfaq.html
  6. San Diego County Assessor. (n.d.). New homeowners property tax guide. Retrieved April 28, 2026, from https://www.sdttc.com/content/dam/ttc/docs/new-homeowners-property-tax-guide.pdf