San Diego Supplemental Tax Bill: Calculation, Deadlines, and Appeals

A San Diego supplemental tax bill is a one-time charge the county sends after a sale or new construction resets your property’s assessed value partway through the fiscal year. It covers the gap between the old assessed value and the new one, prorated for the months remaining between the triggering event and the following June 30. It arrives separately from your regular annual property tax bill, and your mortgage lender almost certainly won’t pay it for you.

What Triggers the Bill

Under California Revenue and Taxation Code Section 75.10, the county assessor must reappraise a property at full market value whenever a change in ownership occurs or new construction is completed.¹ For most sales, the trigger date is the close of escrow rather than the date the deed is recorded.

Qualifying new construction includes adding living space, building an accessory dwelling unit, or installing a permanent structure like a pool. Routine maintenance and cosmetic updates don’t count because they don’t add structural value. The assessor tracks building permits and recorded deeds to catch these events.

Several transfers and improvements are excluded by statute, including transfers between spouses or registered domestic partners, qualifying parent-child or grandparent-grandchild transfers of a family home under Proposition 19, and the installation of an active solar energy system. If you think your situation fits one of these and you’ve still received a bill, that’s grounds to contact the assessor rather than simply pay.

How the Amount Is Calculated

The San Diego County Auditor-Controller runs the calculation in three steps.

First, the existing assessed value on the current tax roll is subtracted from the new market value set by the assessor. A home previously assessed at $500,000 that sells for $800,000 produces a supplemental assessment on the $300,000 difference.

Second, that difference is prorated by the number of months left in the fiscal year, which runs July 1 through June 30. The Board of Equalization publishes a proration table with a factor for each month, ranging from 1.00 for events in July down to 0.08 for events in June.² A January close carries a factor of 0.50 (six months). An October close carries 0.75 (nine months).

Third, the prorated difference is multiplied by your tax rate. Under Proposition 13 that starts at one percent of assessed value, plus local voter-approved bonds and special assessments tied to your tax rate area, so the effective rate in San Diego County usually runs slightly above one percent.³

Why You Might Get Two Bills From One Purchase

If your change in ownership or new construction happens between January and May, expect two separate supplemental bills: one covering the remainder of the current fiscal year, and a second covering the entire next fiscal year starting July 1.² The second bill exists because the regular tax roll for the upcoming fiscal year was already prepared using the old assessed value and can’t be rewritten in time. Events between June and December produce only one supplemental bill, since the new value makes it onto the next regular roll.

When the Bill Goes the Other Way

A supplemental bill doesn’t always mean you owe. If the new assessed value is lower than what the prior owner was paying, you get a negative supplemental assessment and a refund check from the county.⁴ This can happen with family transfers that carry forward a Proposition 13 base-year value, or when market value has genuinely dropped since the last assessment. You can take the refund as a check or apply it to other open property tax bills on the same parcel. Either way, you still owe your regular annual property taxes.

Payment Deadlines

The San Diego County Treasurer-Tax Collector mails supplemental bills directly to the owner’s address on file. Your mortgage lender will not receive or pay the bill, even if you have an impound or escrow account for regular property taxes.⁴ You can ask your lender whether they’ll reimburse you from escrow, but don’t assume it will happen on its own.

Delinquency dates depend on when the bill is mailed, not when the triggering event happened. Revenue and Taxation Code Section 75.52 sets the schedule:⁵

If the bill is mailed July through October, the first installment becomes delinquent at 5 p.m. on December 10 of that year, and the second installment at 5 p.m. on April 10 of the following year.
If the bill is mailed November through June, the first installment becomes delinquent at 5 p.m. on the last day of the month following the mailing month. The second installment becomes delinquent at 5 p.m. on the last day of the fourth calendar month after the first installment deadline.

If a delinquency date lands on a weekend or legal holiday, the deadline shifts to 5 p.m. on the next business day.⁵

Penalties

Missing a deadline triggers an automatic 10 percent penalty on the delinquent installment.⁵ A delinquent second installment adds an extra $10 charge.⁶ Penalties can be canceled if you can show the delinquency was caused by the tax collector’s failure to mail the bill to the correct address on file. Unpaid taxes become tax-defaulted on July 1 of the following fiscal year, which affects your ability to sell or refinance long before any other collection action occurs.

How to Pay

The Treasurer-Tax Collector’s website lets you search for your bill by parcel number, bill number, or mailing address and pay online. E-check payments are free; credit card payments carry a convenience fee. You can also pay by phone at 1-855-829-3773, by mailed check, or in person at the County Administration Center at 1600 Pacific Highway, Room 162, San Diego, CA 92101.

If You Think the Value Is Too High

If the market value the assessor used looks too high, you can challenge it through the San Diego County Assessment Appeals Board, which is independent of the assessor’s office.⁷ You have 60 days from the mailing date printed on the supplemental tax bill to file an Application for Changed Assessment, or 60 days from the date the assessor’s notice was mailed, whichever is later.⁸ Miss that window and you generally lose the right to contest that specific assessment. The strongest appeals rest on comparable sales showing that similar nearby properties sold for less than the assessor’s figure around your purchase date.

Filing an appeal does not pause your obligation to pay. You still have to pay the full amount by the delinquency deadlines to avoid penalties; if the board reduces the value, the county refunds the overpayment. Forms and hearing schedules come through the Clerk of the Board of Supervisors.⁹

1
California Legislative Information. California Code Revenue and Taxation Code 75.10 – Assessments on the Supplemental Roll
2
Property Tax – California State Board of Equalization. Supplemental Assessment
3
San Diego County Treasurer-Tax Collector. Secured Property Taxes
4
San Diego County Treasurer-Tax Collector. Supplemental Tax FAQs
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California Legislative Information. California Code Revenue and Taxation Code 75.52
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San Diego County Treasurer-Tax Collector. Supplemental Property Taxes
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San Diego County. A Guide to Filing Your Assessment Appeal
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San Diego County. Changed Assessment Application Instructions
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San Diego County Assessor | Recorder | County Clerk. Assessment Appeals