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Supplemental Assessment Calculator
In California (Prop 13) and some other states, buying a property triggers reassessment to the new purchase price. The county issues a one-time 'supplemental' tax bill covering the difference between the old and new assessed value — prorated from purchase date to the end of the fiscal year, and again for the following year.
Total supplemental tax
$5,838
Current-FY supplemental
$1,928
Next-FY prorated supplemental
$3,910
Annual tax lift at full year
$3,910
Prior annual tax
$4,715
New annual tax (post-reassessment)
$8,625
Monthly impound increase
$326
How the math works
On a $750K purchase with $410K prior assessment at 1.15%: value lift is $340K, annual tax lift is $3,910. Supplemental covers 180 days of current FY ($1,928) plus full next FY ($3,910 if scheduled) = $5,838 one-time bill. Plus monthly impound jumps ~$326 once the new base is set.
Budget this cash flow carefully. Many California buyers take their first year's mortgage payment at the low pre-supplemental impound, then face a $4K-$8K supplemental bill plus a 25%+ monthly payment jump in year two. Reserve 3-6 months of the new payment at closing.
How to Use
- Enter purchase price and the property's prior assessed value.
- Enter the tax rate and days remaining in the fiscal year at close.
- See the one-time supplemental bill and the increase in next year's regular assessment.
Frequently Asked Questions
What's Prop 13?
California constitutional amendment (1978) capping annual property tax growth at 2% of assessed value — until sale, when assessment resets to purchase price. Supplemental assessments are the mechanism for catching up from the moment the prior assessment was set to the purchase price.
When do I pay the supplemental?
Usually 6-14 months after close in CA. County mails the bill; it's NOT paid through escrow at closing. Budget separately — many new homeowners are blindsided by a $3K-$15K supplemental bill a year after moving in.
Can escrow collect for it?
Yes — some lenders ask escrow to fund it. If yours doesn't, expect a real bill and set aside funds. Even if escrow does fund it, your monthly mortgage payment may rise substantially to cover the higher ongoing tax.
Does this apply in other states?
Not universally. States without acquisition-value caps (most) simply update assessment at next scheduled reassessment. FL has 'Save Our Homes' cap with similar mechanics for long-held homes. Check your state's reassessment rules before purchase.
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