Assessed Value vs Market Value: What Cook County Commercial Property Owners Need to Know

Understanding the difference between assessed value and market value is essential for any Cook County commercial property owner who wants to ensure they're not overpaying on taxes. These two numbers are related — the assessed value is derived from market value — but they are not the same thing, and when they diverge, you end up paying taxes on a property worth more than what you actually own.

What Is Market Value (Fair Market Value)?

Fair market value (FMV) is the price a willing buyer would pay a willing seller in an arm's-length transaction, with both parties having reasonable knowledge of the relevant facts. It's not what you hope your property is worth, and it's not the replacement cost. It's what the market would actually bear if you listed the property today.

For commercial property, FMV is typically established through recent comparable sales of similar properties, the income approach (capitalizing the property's net operating income at an appropriate cap rate), or a combination of both. For a deeper look at how these methods work in the Cook County context, see our guide on how Cook County assesses commercial property.

What Is Assessed Value?

Assessed value (AV) is the Cook County Assessor's estimate of your property's fair market value, multiplied by the assessment level for your property classification. For Class 5 commercial property, that assessment level is 25%. So if the Assessor believes your property has a fair market value of $2 million, your assessed value will be $500,000.

The assessed value is the starting point for your entire tax bill. It gets multiplied by the equalization factor (approximately 3.0), and then the composite tax rate is applied to produce your annual tax.

Why Assessed Value and Market Value Diverge

In a perfect system, your assessed value would always equal exactly 25% of your property's true market value. In reality, the two diverge regularly — and usually in the direction of over-assessment. Here's why:

The Assessor uses mass appraisal models, not individual inspections. With over 30,000 commercial parcels in Cook County, the Assessor's Office relies on statistical models to estimate values. These models work reasonably well in aggregate but can miss the specifics of your property — deferred maintenance, unfavorable lease terms, functional obsolescence, or localized market conditions.

The market moves faster than triennial reassessments. Cook County reassesses property on a three-year cycle. If market conditions deteriorate between reassessments — a major tenant leaves, vacancy rates climb, or the submarket softens — your assessment may still reflect the higher values from the prior cycle.

Data errors compound the problem. If the Assessor's records contain incorrect square footage, the wrong building class, an inaccurate age, or an outdated condition rating, the model will produce an inflated value. These errors are more common than most owners realize.

How to Calculate Your Implied Fair Market Value

You can reverse-engineer the market value that the Assessor is implying for your property with a simple calculation. Take your assessed value and divide by the assessment level:

Implied FMV = Assessed Value / 0.25

For example, if your assessed value is $375,000, the Assessor is saying your property is worth $1,500,000. If your assessed value is $625,000, the implied FMV is $2,500,000.

Write that number down. That's the number you need to test against reality.

How to Check If Your Implied FMV Is Reasonable

Compare to recent sales. Look at arm's-length sales of similar commercial properties in your area over the past 1-3 years. Calculate the price per square foot from those sales and apply it to your property's square footage. If the result is materially lower than the Assessor's implied FMV, your property is likely over-assessed. For guidance on finding and using comparable sales, see our guide on how comparable sales evidence wins property tax appeals.

Compare to what you'd sell for. This is the gut check. If you listed your property tomorrow, what would a reasonable buyer offer? If that number is significantly below the Assessor's implied FMV, you have a disconnect worth pursuing.

Run the income analysis. For income-producing properties, divide your actual Net Operating Income by a market-appropriate cap rate. If the resulting value is below the Assessor's implied FMV, your property is over-assessed based on its income-generating capacity.

When the Numbers Don't Match: You're Over-Assessed

If your implied FMV exceeds what the market evidence supports — whether through comparable sales, income analysis, or your own reasonable estimate — then by definition, your assessed value is too high. That means you're paying property taxes on value that doesn't exist.

This is exactly what a property tax appeal is designed to correct. You present market evidence showing that the Assessor's implied FMV is higher than your property's actual FMV, and request that the assessed value be reduced to the level supported by the evidence.

The appeal process is free to file, your assessment cannot increase as a result, and for a complete walkthrough, see our complete guide to Cook County commercial property tax appeals.

How TaxRival Can Help

TaxRival runs this analysis automatically for every commercial parcel in Cook County. We compare your assessed value against real comparable sales data to determine whether your implied FMV is higher than what the market supports.

Enter your 14-digit PIN on our homepage and we'll show you the gap — if one exists — in 30 seconds. Our fee is 25% of first-year tax savings, and if we don't reduce your assessment, you pay nothing.