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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.

Purchase price

Prior assessed value

Effective tax rate

Days remaining in fiscal year at close

Include next-year prorated supplemental

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.

Editorial noteMaintained by EveryCalc - Reviewed June 2026

EveryCalc calculators are designed for fast, practical estimates with transparent inputs and no required account. We use plain formulas, visible assumptions, and related tools so visitors can check the result from more than one angle.

Results are informational only. For financial, tax, legal, medical, construction, or other high-impact decisions, verify the output against primary sources or a qualified professional.

Learn more about our review process on the EveryCalc methodology page.

How this calculator works

What this page estimates

This Supplemental Assessment Calculator is built to give a quick, browser-based estimate for supplemental assessment. 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. The inputs stay on the page during normal use, and the result should be treated as an estimate for planning, comparison, or education rather than professional advice.

Calculation approach

The calculator applies the standard relationship implied by the inputs, then formats the answer so it can be checked and reused. For finance tools, the most important step is using consistent units, rates, time periods, and assumptions before comparing the result with another calculator or outside quote.

Example workflow

For example, start with a realistic value you already know, change one input at a time, and watch how the answer moves. That makes it easier to tell whether the result is being driven by the main amount, the rate, the time period, or a unit conversion.

Practical checks

Use current, real-world numbers when the result affects money, health, tax, or legal decisions.
Run a low, base, and high case when the inputs are estimates.
Check the related calculators below when the next decision depends on a different assumption.

How to interpret the supplemental assessment result

Best use

Use the result as a planning number for comparing payments, rates, returns, tax reserves, or cash-flow choices before you request a quote or make a commitment.

Cross-check

Compare the answer with the contract, lender estimate, tax form, brokerage statement, payroll record, or invoice that will control the real-world outcome.

Watch for

Do not rely on a single optimistic rate, return, or fee assumption. Money pages work best when you run low, base, and high cases and keep professional advice separate from the estimate.

This page belongs to the Finance calculator library, so the answer should be read in the context of the decision you are modeling rather than as a universal rule.

Before relying on this supplemental assessment estimate

Most calculator mistakes come from the inputs, not the arithmetic. Use this short audit before you reuse the answer in a spreadsheet, quote, application, or important conversation.

Confirm source numbers

Match balances, rates, fees, taxes, income, and payment dates against the lender quote, payroll record, tax form, statement, invoice, or contract.

Separate cash flow from total cost

A lower monthly payment can still cost more over time if fees, interest, taxes, or a longer term are hidden in the structure.

Run conservative cases

Test at least one higher-cost or lower-return case before using the output for a purchase, refinance, investment, loan, or tax decision.

Rerun this page when the rate, price, term, fee, tax rule, income, expense, or expected holding period changes.

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.