The Illinois Answers Project and Chicago Tribune have laid out how they calculated the property-tax savings tied to data centers in Cook County’s O'Hare subregion, using county records, municipal sources and appraisal files to build a county-by-county picture. The analysis covered data centers in Elk Grove Village, Northlake, Franklin Park, Des Plaines and Mount Prospect.
The reason readers are searching for O'Hare Data Center Tax Breaks now is simple: the reporting turns a broad claim about tax relief into a method people can examine. Records requests to the Cook County assessor's office, the Board of Review and the Illinois Property Tax Appeals Board produced appeal files dating back to 2022 for each property, along with privately commissioned appraisals and appraisals from the assessor's office.
From there, the reporters projected 2025 assessed values in three scenarios. One used the final assessed value certified by the assessor's office and assumed no Board of Review reductions. A second divided the Board of Review's certified market value by four to create a 25% level of assessment and assumed no Cook County class 6(b) incentives. A third divided the assessor's certified market value by four to model the absence of both Board of Review reductions and class 6(b) incentives.
The math then moved from property values to tax bills. The analysis totaled the differences between actual assessed values and simulated assessed values across the five municipalities, multiplied those differences by Cook County's 2025 multiplier to generate additional equalized assessed value, and for Elk Grove Village, Northlake and Franklin Park added that amount to the existing equalized assessed value calculated by the Cook County clerk's office for the 2025 tax year. It then simulated a higher equalized assessed value for each taxing body, divided each body's 2025 levy by that number to project a new tax rate, and applied the new combined rate to a home valued at Zillow's median home sale price for that town.
That is also where the limits of the exercise show up. The method is careful, but the article does not publish the underlying market values for each property or identify which specific data centers generated the biggest savings, even as it says the analysis calculated projected tax savings by multiplying the simulated higher equalized assessed value by the simulated lower rate and subtracting the actual tax bill amount. In other words, the framework is transparent, but the biggest individual drivers remain out of view.
What the reporting does make clear is the scale of the assumptions behind the local tax-break debate. The work focuses on Cook County’s O'Hare subregion and the two county programs at the center of the calculation, Board of Review reductions and Cook County class 6(b) incentives. The next question is not how the math works. It is which properties were doing the heavy lifting behind the nearly $100 million in local tax breaks, and that part still is not spelled out.
