Your Property Tax Assessment Is Probably Wrong. Here's How to Prove It.

What an Assessment Actually Measures

Your property tax bill is not based on what your home would sell for today. It's based on the assessor's opinion of value, which can lag the market by one to three years. That lag works against you in appreciating markets and potentially in your favor in declining ones.

The assessment process works in two steps. First, the assessor assigns an assessed value. Second, that value is multiplied by the local assessment ratio, a percentage that varies by jurisdiction. The result is the taxable value, which is then multiplied by the mill rate to produce your annual bill.

The formula is:

Annual Tax = Assessed Value × Assessment Ratio × Mill Rate

Or simplified:

Annual Tax = Taxable Value × Mill Rate

Most homeowners look only at the final bill. The error sits one step earlier.

The Assessment Ratio: Where the Math Starts

Every jurisdiction publishes a target assessment ratio. Many jurisdictions set this at 100%, meaning assessed value should equal market value. Others assess at 80%, 70%, or lower. The ratio matters because your actual ratio may differ from the published target.

Your Effective Assessment Ratio = Assessed Value / Market Value

If your home would sell for $620,000 and your assessed value is $589,000, your effective ratio is 95%. If your jurisdiction targets 100%, you are assessed above market, and you have grounds to appeal.

Worked Example 1: The Over-Assessed Suburban Home

A homeowner in Cook County, Illinois receives an assessment of $312,000 on a property that recently sold for comparable homes in the neighborhood at an average of $274,500. The county's target ratio is 100%.

Effective Assessment Ratio: $312,000 / $274,500 = 113.7%

The home is assessed 13.7% above market value. Cook County's effective mill rate for this neighborhood is approximately 2.19%.

Excess assessed value: $312,000 - $274,500 = $37,500

Annual over-payment: $37,500 × 2.19% = $821.25 per year

Over five years without correction: $4,106.25 in unnecessary taxes paid.

This is a real, recoverable number. The homeowner can appeal, document the comparable sales, and request a corrected assessment of $274,500.

How to Find Your Own Comparable Sales

The assessor's evidence is comparable sales data. Your counter-evidence is the same thing, sourced independently.

Pull sold listings from the past six to twelve months within one mile of your property. Filter for homes within 15% of your square footage, similar age, similar lot size, and similar condition. Collect at least three sales. Five to eight strengthens the case.

Your target number is the price per square foot. Calculate it for each comparable:

Price Per Square Foot = Sale Price / Gross Living Area

Average those figures. Multiply by your home's gross living area. That is your supportable market value.

If your assessed value sits materially above that figure, you have a quantifiable case.

What "Material" Means in Practice

Minor discrepancies rarely produce refunds. A 2% over-assessment on a $400,000 home equals $8,000 in over-assessed value. At a 1.5% mill rate, that generates $120 per year in excess tax. Most jurisdictions set a minimum threshold for adjustment, typically 5% to 10% variance from market value. Target cases where the gap exceeds 8%.

The Assessment Card: Your Starting Document

Every jurisdiction maintains a property record card for each parcel. Request yours. It documents the assessor's data: square footage, bedroom count, bathroom count, basement finish, garage size, and condition rating.

Errors on this card are common. A finished basement recorded as unfinished suppresses value artificially. Extra bathrooms that don't exist inflate it. Incorrect square footage distorts every calculation downstream.

Verify the card against your closing documents, appraisal, and permit records. A factual error on the card is your strongest appeal argument. It requires no judgment call, just a corrected fact.

Worked Example 2: The Data Error That Cost $2,100

A homeowner in Maricopa County, Arizona discovers her assessment card records 2,840 square feet. Her recorded floor plan from closing documents 2,410 square feet. The assessor used incorrect data from a prior permit application.

The county's assessed value: $487,000. The county mill rate: 0.859%.

Corrected market value using $171.50 per square foot (derived from comparable sales): 2,410 × $171.50 = $413,315.

Corrected assessed value (county targets 100% ratio): $413,315.

Annual tax difference: ($487,000 - $413,315) × 0.00859 = $73,685 × 0.00859 = $632.85 per year.

The assessment went unchallenged for three years before discovery. Total over-payment: approximately $1,898.55. The county granted a retroactive correction for two years, resulting in a $1,265.70 refund.

The documentation required: a copy of the recorded floor plan and a written correction request. No attorney. No hearing.

Filing the Appeal: What You Need and When

Every jurisdiction sets a hard deadline for appeals, typically 30 to 90 days after the assessment notice mails. Missing that window forfeits your right to contest until the next assessment cycle.

Your appeal package should contain:

1. A completed appeal form from your county assessor or board of review.
2. Your documented market value calculation with at least three comparable sales, each showing address, sale date, sale price, and square footage.
3. A copy of your property record card with errors marked and corrected.
4. Any independent appraisal if the disputed amount justifies the cost. An appraisal runs $300 to $600. It is worth commissioning when the annual tax savings exceed $500 and the assessor contests your comparable sales evidence.

Most jurisdictions offer an informal review before a formal hearing. Take it. Informal reviews resolve approximately 60% of appeals without advancing to a board.

When to Hire a Tax Attorney or Consultant

Most straightforward appeals do not require professional representation. When they do:

The disputed annual tax savings exceed $3,000. Attorney fees on contingency typically run 25% to 40% of the first year's savings. That math works when the savings are large enough.

Your property is commercial or mixed-use. Assessors apply income capitalization methods to income-producing properties, not just comparable sales. That analysis requires specialized expertise.

You have already lost an informal review and are proceeding to the state tax tribunal. Procedural rules at that level favor represented parties.

Recurring Reassessments: Build a Monitoring System

A single successful appeal does not protect you permanently. Most jurisdictions reassess every one to four years. Some reassess annually.

Set a calendar reminder for the date your assessment notice typically arrives. Pull three to five recent comparable sales each year in the spring. Maintain a running record of your property's supportable market value. When the assessment arrives, the comparison takes 20 minutes rather than a research project.

Properties in rapidly appreciating neighborhoods face the highest risk of over-assessment because assessors often apply blanket percentage increases rather than sale-specific analysis. Check your assessment ratio every cycle.

Run the Numbers Before You Decide

The decision to appeal is a financial calculation, not a principled stand. Quantify the over-assessment, estimate the recoverable tax, subtract the time cost and any professional fees, and compare the net return.

For most homeowners with assessments more than 8% above documented market value, the return on a self-prepared appeal exceeds $1,000 per year for under ten hours of work. That is a better hourly return than most professional activities.

You Might Also Like

• How to Calculate Monthly Cash Flow on a Rental Property (The Right Way)
• How to Calculate Home Sale Capital Gains Tax Before You Close
• How to Calculate Investment Property Monthly Cash Flow (With Real Numbers)

The CalcMoney mortgage calculator lets you model the full cost of property taxes within your housing cost structure, including what a corrected assessment would do to your monthly effective payment. Run both scenarios before your appeal deadline. The difference between assessed values is money already owed to you. The only question is whether you claim it.