The Illinois Answers Project and Chicago Tribune explained how they calculated O'Hare data center tax breaks in Cook County’s O'Hare subregion. Their method compared actual 2025 bills with simulated bills built from property tax appeals and county records.
The calculation used records dating back to 2022 for each property and ran three 2025 assessed value scenarios. Reporters then multiplied the differences by Cook County’s 2025 multiplier to estimate additional equalized assessed value before projecting tax rates and bills.
Cook County records and appeals
The reporters said they used third-party databases, county records and municipal sources in Elk Grove Village and Northlake to compile a list of data centers in Cook County’s O'Hare subregion. They requested property tax appeal records from the Cook County assessor's office, Board of Review and the Illinois Property Tax Appeals Board.
The records gave them a starting point for each property’s assessment history. That let them compare what owners actually paid in 2025 with what they would have paid under three separate scenarios.
Three 2025 scenarios
One scenario assumed no property received reductions from the Board of Review and used the final assessed value certified by the assessor's office. A second assumed no property operated under Cook County class 6(b) incentives by dividing the Board of Review’s certified market value by four. A third assumed no property received either benefit by dividing the assessor’s certified market value by four.
The difference between those simulated figures and the actual assessed value was totaled across Elk Grove Village, Northlake, Franklin Park, Des Plaines and Mount Prospect. The reporters then applied Cook County’s 2025 multiplier and, for Elk Grove Village, Northlake and Franklin Park, added the result to the existing equalized assessed value calculated by the Cook County clerk's office for the 2025 tax year.
How the tax rate changed
From there, they divided each taxing body's 2025 levy by the simulated higher equalized assessed value to project a new tax rate. They applied that rate to a home with an assessed market value equal to Zillow’s median home sale price for that town, then compared the simulated bill with a bill using actual 2025 rates.
The method produces an estimate of how much tax liability shifts when reductions and class 6(b) incentives are removed from the model. It also shows why the same data center footprint can change bills in more than one municipality, even before any individual property total is broken out.
What readers still need
The excerpt does not identify which specific data centers drove the largest savings. It also does not give the final savings total for each municipality or property, leaving the math useful for understanding the headline number but not for ranking the properties behind it.
