Prop 13 does not protect you from the supplemental bill. It arrives months after closing, is not escrowed by your lender, and can run $2,000–$5,000+ depending on when and where you buy. Here is the math.
California's Proposition 13 caps annual increases in assessed value at 2% per year for existing owners. It is one of the most powerful homeowner protections in the country — once you own a California home, your tax bill grows slowly and predictably.
What Prop 13 does not do: protect new buyers from a one-time supplemental assessment at purchase. When you buy a California home, the county assessor resets your assessed value to the purchase price. If the seller has owned the home for 15 years and their assessed value is $280,000 but you paid $640,000, the county must collect the difference in taxes for the remaining portion of the current fiscal year. That difference — assessed at the full applicable tax rate — arrives as a separate "supplemental" bill, mailed directly to you, typically 3–6 months after closing. Your lender does not escrow for it. Most buyers are not warned about it. It is one of the most common financial surprises in California real estate.
Standard mortgage escrow accounts collect for your regular property tax installments. They do not collect for the supplemental tax bill because the amount is unknown at loan origination. The supplemental bill is mailed directly to you — not your lender, not your escrow company — typically 3–6 months after closing. If you do not have cash set aside, it becomes an unexpected and urgent expense. Set aside the estimated amount at closing. Use the county's supplemental tax estimator to calculate it before you sign.
The formula is simple. The county applies a proration factor based on how many months remain in the current fiscal year, then multiplies by the difference in assessed value and the applicable tax rate.
If you close between January and May, you may receive two supplemental bills: one for the partial current fiscal year, and one for the entire following year (because the assessment rolls had already been locked when you bought).
When you buy a new construction home, the assessed value before your purchase was based on raw land value — often $50,000–$120,000. Your purchase price is $500,000–$700,000+. The gap that triggers the supplemental bill is enormous. Four Seasons at the Ranch in Rancho Cordova, Esplanade at Turkey Creek in Lincoln, and Heritage Carson Creek in El Dorado Hills buyers should expect supplemental bills in the $4,000–$8,000 range depending on purchase price and closing date. This is not optional, and it is not tax-deductible in the year you pay it (supplemental bills are not deductible as property taxes in the same way regular installments are — consult a tax advisor).
| County | Effective Rate | Supplemental on $250K Basis Gap | Online Estimator Available |
|---|---|---|---|
| Placer County (Roseville, Lincoln, Rocklin) | ~1.12% | ~$2,800 (full year) | Yes — placer.ca.gov |
| Sacramento County (most areas) | ~1.19% | ~$2,975 (full year) | Yes — assessor.saccounty.gov |
| Sacramento County (Rancho Cordova) | ~1.23% | ~$3,075 (full year) | Yes — assessor.saccounty.gov |
| El Dorado County (El Dorado Hills) | ~1.15% | ~$2,875 (full year) | Yes — edcgov.us/assessor |
Understanding the sequence matters for cash flow planning.
Your regular property tax is prorated between buyer and seller on the HUD-1. Your first regular tax installment is collected through escrow. No supplemental bill exists yet.
The county assessor processes the ownership change and issues a Notice of Supplemental Assessment. This tells you the new assessed value and the supplemental amount owed.
The supplemental tax bill arrives by mail, addressed directly to you (not your lender). It has its own due dates — typically 30 days from issuance. Late payments incur 10% penalties.
A second supplemental bill covering the full following fiscal year may arrive separately. Two bills, two due dates. Both are your direct responsibility.
Your regular annual property tax is assessed at your new purchase-price basis. The supplemental bill process is complete. From here, Prop 13's 2% cap applies.
Placer County operates a Supplemental Tax Estimator at placer.ca.gov that lets you enter your purchase price, the seller's current assessed value, and your expected closing date to get a projected supplemental bill amount. Run it before your offer is accepted — not after. Sacramento County and El Dorado County have similar tools. Building the supplemental bill into your closing cash requirements is the single most important financial planning step California homebuyers overlook.
Tell us the community, purchase price, and expected closing date and we can walk you through the supplemental bill calculation for your specific situation.
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