You've closed escrow, the moving boxes are finally broken down, and the new place is starting to feel like home. Maybe it's morning coffee on the patio in Carlsbad or an evening walk to the beach in Encinitas. Then an envelope from the County of San Diego shows up with a tax bill you weren't expecting.
Take a breath. It's not a mistake, and it's one of the most common questions we get from buyers after closing. It's called a supplemental tax bill, and once you know how it works, it's easy to plan for. Here's a quick explanation.
What Is a Supplemental Tax Bill?
When a property in San Diego County sells or new construction is completed, state law requires the County to reassess it. If the new assessed value is different from the old one, the County recalculates your property taxes for the rest of the fiscal year and sends a supplemental tax bill to cover the difference.
The bill covers the period from your change in ownership through the end of the fiscal year on June 30. It's separate from, and in addition to, your regular annual property tax bill.
If your new assessed value is lower than the prior value, you'll receive a negative supplemental bill along with a refund check instead.
Why Did I Receive One?
California property taxes are based on a property's assessed value as of January 1 each year. Under Prop 13, that assessed value can only rise by up to 2% a year until the property changes hands. So if the previous owners bought the home years ago, the annual tax bill in place when you closed was likely based on their much lower value.
The supplemental bill catches your taxes up to the new value for the months you've owned the home. In coastal North County, where many homes haven't sold in years, that gap can be significant.
What Happens During Reassessment?
The Assessor replaces the prior assessed value with a new one based on the property's market value at the time of the ownership change, which is usually your purchase price. You'll receive a Notice of Supplemental Assessment showing the old value, the new value, and the difference. The supplemental tax bill follows from the Treasurer-Tax Collector.
One bill or two? If you close between June 1 and December 31, expect one supplemental bill. If you close between January 1 and May 31, expect two. That's because the value for the upcoming fiscal year was already set on January 1 at the old amount, so a second bill covers that full next fiscal year.
How Is a Supplemental Tax Bill Calculated?
The math follows three steps:
- Subtract the old assessed value from the new assessed value.
- Prorate that difference by the number of months left in the fiscal year.
- Apply the tax rate to get your total.
Here's a hypothetical example. Say you buy a home in Del Mar for $2,500,000 and close in late August. The previous owners' assessed value was $900,000, so the difference is $1,600,000. At the 1% base tax rate, that's $16,000 for a full year. With about 10 months left in the fiscal year, your supplemental bill would come in around $13,300.
Keep in mind your actual bill may be a bit higher, since voter-approved local bonds add to the 1% base rate depending on where the property is.
When Is It Due?
Supplemental bills are mailed throughout the year, and due dates vary, so check the delinquency dates printed on your bill. Bills are typically split into two installments.
If you miss a due date, the County adds a 10% penalty, plus a $10 charge if the second installment is late. If neither installment is paid by June 30, the property goes into tax default, with a 1.5% monthly penalty and a redemption fee on top.
The big one to know: supplemental bills are mailed only to the property owner, not to your lender. Even if your regular taxes are paid through an impound account, you're responsible for making sure this bill gets paid. Call your lender to ask whether they'll cover it.
What Counts as a Change in Ownership?
Most sales and transfers trigger a reassessment. Some common exclusions include:
- Transfers between spouses
- Transfers between registered domestic partners
- Moving a property into a revocable living trust for your own benefit
- Adding a joint tenant to title while the original owner stays on
- Certain parent-child and grandparent-grandchild transfers of a family home under Prop 19, which took effect in 2021. The home generally has to become the new owner's primary residence, value limits apply, and a claim must be filed with the Assessor.
If you think your transfer might qualify, check with the Assessor's Office before you assume it does.
Can I Appeal It?
Yes. Appeals must be filed with the Assessment Appeals Board within 60 days of the mailing date on the bill. You'll still need to pay on time while the appeal is pending to avoid penalties.
Our Advice: Plan for It From Day One
The easiest way to handle a supplemental tax bill is to know it's coming. When we work with buyers, we help estimate the supplemental amount before closing so it's part of the budget from the start, not a surprise months later. With 40+ years of combined experience in North County's coastal markets, it's one of those small details we make sure our clients never have to scramble over.
Questions about your own property? Reach out anytime. We're always happy to run the numbers with you.
