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 be: 1. **Arm's-length** — between unrelated parties at market terms (not a foreclosure, family transfer, or distressed sale) 2. **Recent** — ideally within 36 months of the assessment lien date (January 1 of the tax year) 3. **Similar property type** — same general use (retail to retail, office to office) 4. **Similar size** — within 30-40% of the subject property's building square footage 5. **Same general location** — same township or adjacent area 6. **Similar age and condition** — within 10-20 years of construction date 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/SF, but five comparable sales closed at $80-$110/SF, the median comp price of $95/SF 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: - **3 comps**: Minimum for a credible appeal - **5-6 comps**: Strong appeal with good market coverage - **8+ comps**: Very strong, especially if they're tightly clustered in value More isn't always better — **3 excellent comps beat 8 mediocre ones**. Quality over quantity. ## Common Mistakes in Comp Selection ### Using Non-Arm's-Length Sales Foreclosures, bank-owned sales, related-party transactions, and sales with seller concessions don't reflect true market value. The Assessor will discard these. Cook County's SODA database includes sale filter flags that identify non-arm's-length transactions. ### Comparing Different Property Types A warehouse at $45/SF is not comparable to a retail storefront at $150/SF, even if they're the same size and in the same township. Property type (use code) matters as much as class code. ### Ignoring Time Adjustments A sale from 3 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 Presenting only the lowest 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** (datacatalog.cookcountyil.gov). Key datasets: - **Parcel Sales** (wvhk-k5uv) — every recorded sale with price, date, deed type, and arm's-length filters - **Assessed Values** (uzyt-m557) — current and historical assessments for every property - **Commercial Valuation** (csik-bsws) — 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: - **Same use category** (retail, office, industrial) — highest weight - **Same neighborhood** — geographic proximity - **Building size** — within a dynamic tolerance (tighter for larger properties) - **Age** — construction year similarity - **Sale recency** — more recent sales weighted higher - **Distance** — physical proximity to the subject We then apply statistical checks: removing outliers, verifying the comp set is tightly clustered (low coefficient of variation), and time-adjusting older sales. The result is a defensible, data-driven set of comparables that the Assessor's office will take seriously. ## The Bottom Line Comparable sales evidence is the foundation of a successful commercial property tax appeal. The key is finding sales that are genuinely similar to your property, presenting them clearly, and letting the market data make the argument. If you'd like to see what comparable sales exist for your property, enter your PIN on our homepage. We'll show you instantly whether there's evidence to support a reduction.