For California Home Buyers

Supplemental Tax Calculator

Estimate the one-time supplemental property tax bills you'll receive after closing — the bills that catch most new Peninsula buyers off guard.

California Supplemental Property Tax

Estimate your supplemental bills

Supplemental Tax Estimate
Increase in Assessed Value
Full-Year Supplemental Tax (before proration)
First Supplemental Bill (months in current FY)
Second Supplemental Bill (if applicable)
Total Supplemental Bills Combined
Estimates only. Actual supplemental bills are issued by the county assessor and may differ based on the precise assessment date, county processing timing, and any adjustments for property improvements or exemptions. Prior owner's assessed value shown on title prelim or the most recent tax bill — not the asking price. These bills are not collected through your mortgage escrow; plan for a direct cash payment. Lisa M. Lum and Coldwell Banker Realty do not provide tax advice.

California Supplemental Property Tax — What to Know

What is the California supplemental property tax?

California's supplemental property tax bridges the gap between the seller's old Prop 13 assessed value and your new purchase price. When you buy a home, your assessed value jumps to the purchase price under Prop 13 reassessment. The county issues separate supplemental bills to capture the additional tax for the portion of the fiscal year you own the home at the new value. These bills arrive 6-18 months after closing and surprise many first-time Peninsula buyers who aren't expecting them. On a $3M purchase where the seller's assessed value was $500,000, the supplemental bills together can easily total $25,000 to $35,000.

Why do I get two supplemental tax bills?

California property tax runs on a fiscal year from July 1 to June 30. If you close before July 1, you receive two supplemental bills: one for the partial year remaining in the current fiscal year (prorated by months remaining), and one for the full following fiscal year calculated before the new regular annual bill takes over. If you close on or after July 1, you receive only one supplemental bill. The math: supplemental bill = (new AV − prior AV) × tax rate × (months remaining ÷ 12).

When do supplemental tax bills arrive?

Supplemental bills typically arrive 3 to 9 months after closing. San Mateo County and Santa Clara County typically process within 3 to 6 months of the change of ownership. The bills are mailed directly to the new owner and are not managed through your mortgage escrow account — they are your direct cash responsibility. Plan for this payment in your post-closing budget. Missing a supplemental bill results in penalties (typically 10% after the delinquency date).

Where do I find the prior owner's assessed value?

Three sources: (1) The title preliminary report your escrow officer pulls early in the transaction — it usually lists the current assessed value; (2) The county assessor's parcel viewer online — both San Mateo County and Santa Clara County have searchable parcel databases with current assessed value shown; (3) The most recent property tax bill, which the seller's disclosures may include. Do not use the Zillow "property tax" field or the MLS listing, as these are often outdated or incorrect. Always pull the actual assessor record.

Preparing to close on a Peninsula home?

Lisa M. Lum walks every buyer through the post-closing tax timeline so nothing arrives by surprise. The supplemental bills, the regular annual impound, and the exact timing — she'll explain it all during the escrow process.

Talk to Lisa Before You Close