How Comparable Sales Evidence Wins Property Tax Appeals

The single most important factor in winning a commercial property tax appeal in Cook County is the quality of your comparable sales evidence. The right comps can reduce your tax bill by thousands of dollars. The wrong ones, or no comps at all, almost guarantee a denial.

Here's how comparable sales work in practice and how to select the strongest ones for your appeal.

What Makes a Sale "Comparable"?

A comparable sale is a recent transaction of a property similar to yours that provides evidence of market value. For a sale to be useful in a Cook County appeal, it should meet several criteria. It must be an arm's-length transaction between unrelated parties at market terms, not a foreclosure, family transfer, or distressed sale. It should be recent, ideally within 36 months of the assessment lien date (January 1 of the tax year). The property type should match — retail compared to retail, office to office. Building size should be within 30-40% of the subject. The property should be in the same general location, ideally the same township or adjacent area. And construction age should be similar, within 10-20 years.

No comp will be identical to your property. The goal is to find the most similar sales available and present them credibly.

The Key Metric: Price Per Square Foot

The most persuasive way to present comparable sales is on a price per square foot basis. This normalizes for size differences and makes direct comparison straightforward. For example, if your property is assessed at an implied FMV of $150 per square foot but five comparable sales closed at $80-$110 per square foot, the median comp price of $95 per square foot provides strong evidence that the Assessor's value is too high.

How Many Comps Do You Need?

The Cook County Assessor and Board of Review have not published a specific minimum, but in practice three comps is the minimum for a credible appeal. Five to six comps represent a strong appeal with good market coverage. Eight or more comps make for a very strong case, especially if they're tightly clustered in value. However, more isn't always better. Three excellent comps beat eight mediocre ones. Quality matters more than quantity.

Common Mistakes in Comp Selection

Using non-arm's-length sales is the most frequent error. Foreclosures, bank-owned sales, related-party transactions, and sales with seller concessions don't reflect true market value. The Assessor will discard these.

Comparing different property types undermines the entire analysis. A warehouse at $45 per square foot is not comparable to a retail storefront at $150 per square foot, even if they're the same size and in the same township. Property type matters as much as class code.

Ignoring time adjustments weakens older comps. A sale from three years ago may not reflect today's market. If the market has appreciated 3% annually, a 2023 sale price should be adjusted upward by approximately 9% to be comparable to a 2026 assessment.

Cherry-picking low sales while ignoring higher ones that are equally comparable will undermine your credibility. The Assessor and BOR analysts will look at the full market picture. Present a representative sample and let the median speak for itself.

Where to Find Comparable Sales Data

Cook County makes all recorded property sales available through their Open Data Portal at datacatalog.cookcountyil.gov. The Parcel Sales dataset contains every recorded sale with price, date, deed type, and arm's-length filters. The Assessed Values dataset provides current and historical assessments for every property. The Commercial Valuation dataset includes the Assessor's own valuation model inputs.

You can also use commercial real estate databases like CoStar, LoopNet, or local MLS services for additional market context.

How TaxRival Selects Comps

Our algorithm scores every potential comparable sale on multiple factors. We prioritize properties of the same use type, such as retail matching to retail, never to industrial. We weight geographic proximity, building size similarity, construction age, and sale recency. We apply statistical checks to remove outliers, verify the comp set is tightly clustered, and time-adjust older sales to current values.

The result is a defensible, data-driven set of comparables that the Assessor's office will take seriously. If you'd like to see what comparable sales exist for your property, enter your PIN on our homepage and we'll show you instantly whether there's evidence to support a reduction.