The Illinois Answers Project and Chicago Tribune 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. Through records requests to the Cook County assessor’s office, Board of Review and the Illinois Property Tax Appeals Board, we obtained property tax appeal records dating back to 2022 for each property, including all privately commissioned appraisals and appraisals produced by the assessor’s office.
To calculate the property tax savings data centers achieved through county programs, we began by projecting 2025 assessed values (AVs) for each property in three scenarios: One in which no properties received reductions from the Board of Review by using the final AV certified by the assessor’s office; two, in which no properties operated under Cook County class 6(b) incentives by dividing the Board of Review’s certified market value (MV) by four, creating a 25% level of assessment; and finally, a scenario in which no property received either benefit by dividing the assessor’s certified MV by four.
For each scenario, we totaled the differences between the actual AV and simulated AV for data centers across each of five municipalities — Elk Grove Village, Northlake, Franklin Park, Des Plaines and Mount Prospect — and multiplied them by Cook County’s 2025 multiplier to generate an “additional” equalized assessed value (EAV). For Elk Grove Village, Northlake and Franklin Park, which each have multiple data centers of significant value, we added the additional EAV to the existing EAV that was calculated by the Cook County clerk’s office for the 2025 tax year for taxing bodies across all three towns, to simulate a new, higher EAV for each taxing body in a world without reductions and/or incentives. We then divided each taxing body’s 2025 levy by the simulated, higher EAV to project a new, simulated tax rate.
For suburbs with several data centers, we generated a new combined tax rate by adding the simulated rates for each taxing body. We then applied the new combined rate to a home with an assessed market value equal to Zillow’s median home sale price for that town to generate a simulated tax bill. We compared that simulated bill to a comparable bill using actual 2025 rates.
To calculate the property tax savings each data center received from incentives and/or reductions in a given municipality, we took the simulated higher EAV in each town and multiplied it by the simulated lower rate to project a new bill. We subtracted from that number the product of the data centers’ combined 2025 EAV and the actual combined rates applied to their 2025 tax bills, yielding each property’s projected tax savings.