The California Supplemental Tax Bill

Why the Supplemental Bill Exists

Under California's Prop 13, property is assessed at time of purchase and can only increase 2% per year thereafter. When you buy a home, the county assessor must reset your assessment from whatever the previous owner's assessed value was to your purchase price. This reassessment happens after closing — not at closing.

The problem: there's a lag. Property tax bills are issued semi-annually in California (November and February), based on the assessment that was in place at the start of each fiscal year (July 1). When you close escrow, you may have months of ownership where the county is still billing based on the previous owner's lower assessed value — which may be far below your purchase price.

The supplemental tax bill is how the county collects the difference. It bills you for the gap between what the previous owner's assessment would have generated and what your new higher assessment generates, prorated from the day you closed.

How Supplemental Tax Is Calculated

The formula has three components:

• New assessed value: Your purchase price
• Old assessed value: Whatever the prior owner was assessed at (may be much lower due to Prop 13 freeze)
• Proration factor: The fraction of the tax year remaining from your close date to June 30

The Double-Bill Year: What You Actually Pay in Year One

In your first year of California homeownership, you receive three tax bills where normally there would be two:

1. Regular November bill: Based on the previous owner's assessed value (already prorated and settled through escrow at closing). You may or may not owe this depending on your close date.
2. Supplemental bill: The catch-up bill for the assessment increase. Arrives separately, 3–12 months after closing. Must be paid — ignoring it results in penalties.
3. Regular bill for the following fiscal year: Based on your new assessed value. This is your ongoing permanent tax burden.

Most buyers budget for the regular tax bill (which their lender impounds into monthly mortgage payments). They don't budget for the supplemental bill, which arrives unexpectedly and must be paid directly — lenders do not typically impound supplemental taxes.

Timing: When Does the Bill Arrive?

The county assessor typically completes reassessment 3–12 months after close of escrow. There is no fixed timeline — it depends on county workload and your close date relative to assessment cycles.

• Early in fiscal year (July–October close): Bill may arrive 6–12 months after closing. Often highest proration factor.
• Mid-year (November–January close): Bill often arrives 4–8 months after closing.
• Late fiscal year (February–June close): Bill arrives 2–4 months after closing. Lowest proration factor (fewest months remaining).

San Diego County mails supplemental bills to the property address, not necessarily your forwarding address. If you've moved from out of state, ensure your mail is forwarded or check the San Diego County Treasurer-Tax Collector website (sdttc.com) directly for bills.

What If You Have Prop 19 Basis Transfer?

Prop 19 transfers your old assessed value to your new home, which dramatically reduces or eliminates the supplemental bill.

Important: Prop 19 only eliminates the supplemental bill if your transferred basis is equal to or less than the previous owner's assessed value. If your transferred basis is significantly higher, you may still have a supplemental bill for the difference.

Supplemental Tax at Each San Diego Price Point

The "prior assessment" estimate assumes homes that have been owned 15–25 years. Homes sold recently before your purchase have less Prop 13 advantage — their prior assessment will be closer to market value, and your supplemental bill will be smaller.

How to Budget for the Supplemental Bill

The practical approach: at closing, set aside 0.5–0.75% of your purchase price as a supplemental tax reserve.

• $500,000 purchase: Reserve $2,500–$3,750
• $750,000 purchase: Reserve $3,750–$5,625
• $1,000,000 purchase: Reserve $5,000–$7,500
• $1,200,000 purchase: Reserve $6,000–$9,000

This reserve covers the worst-case scenario (closing early in fiscal year, large assessment jump). If the bill comes in lower, you keep the difference.

Do not spend your closing cost remainder immediately after closing. Keep a buffer for the supplemental bill, which arrives when the reserve is most likely depleted.

What Happens If You Don't Pay

The supplemental tax bill has the same legal status as your regular property tax bill. Failure to pay results in:

• 10% penalty after the delinquency date
• Potential tax lien on the property
• Additional penalties accumulating over time

The bill goes to the property address. If you closed and the bill went to the previous owner's mail forwarding, it may be delayed. Check sdttc.com 6 months after closing if you haven't received a supplemental bill — don't assume no bill means no obligation.

Asking the Right Questions at Closing

Ask your title or escrow officer:

• "What is the current assessed value on this property?"
• "What will my reassessed value be after purchase?"
• "Can you estimate the supplemental tax I'll owe and the likely timing?"
• "Is my lender's impound account set up to handle the supplemental bill or only regular taxes?"

A knowledgeable escrow officer should be able to give you a rough estimate. The county assessor's office can also confirm current assessed values for any parcel at assessor.sandiegocounty.gov.

Next Steps

Budget your supplemental reserve before close, not after. Verify it with your escrow officer. Monitor sdttc.com starting 3 months post-close. If you have Prop 19 basis transfer, confirm with your CPA whether the transfer is properly filed — it affects your supplemental bill calculation directly.