Cook County Property Tax Reform Group Announces New, Unified Method of Tax Rate Calculation between the Cook County Assessor and Cook County Board of Review

(COOK COUNTY, IL) – The Cook County Property Tax Reform Group today announced the implementation of a new unified, tax-rate calculation methodology to be jointly implemented for the 2026 Tax Year by the Cook County Assessor’s Office (CCAO) and the Cook County Board of Review (BOR) — a major step forward in aligning valuation practices, improving consistency across reassessment cycles and strengthening transparency for taxpayers. 

The methodology is the result of reform work launched in 2024 by the Cook County Property Tax Reform Group, led by Cook County Board President Toni Preckwinkle. The group commissioned an independent audit of the County’s commercial valuation methodologies. The audit identifieddifferences in methodology between the Assessor and Board of Review and recommended updates to both agencies’ tax rate and capitalization rate practices. 

“This agreement reflects what reform looks like in practice: collaboration, transparency, and a commitment to getting the fundamentals right,” Cook County Board President Toni Preckwinkle said. “For too long, differences between agencies created confusion for taxpayers and undermined confidence in the property tax system. By aligning the offices’ tax-rate methodologies, we are taking a meaningful step toward a more fair, predictable and accountable process.” 

The shared methodology is intended to reduce discrepancies in valuation assumptions, improve predictability for property owners and taxing bodies, and advance ongoing modernization of Cook County’s property tax system. 

Following the County’s commercial valuation audit, the President’s Office, CCAO, and BOR partnered with the Civic Consulting Alliance to translate the study’s recommendations into more than 40 detailed workstreams focused on improving collaboration, aligning valuation practices and increasing transparency. A key priority was establishing a unified approach for calculating tax rates and capitalization rates. Subcommittees that included commercial valuation analysts from both offices conducted a series of technical workshops to reconcile methodological differences, test alternative models and develop a shared approach that both offices agreed to adopt.  

Both offices concurred that the first year of the reassessment cycle requires a distinct approach. A new, unified framework for determining tax rates will be implemented for the reassessment of the south and west suburbs in Tax Year 2026, which is the timing of the regional triad. This is significant because the south and west suburbs have a higher fluctuation of tax rates than other triads. 

Under this new framework, the CCAO will estimate 2026 tax rates by applying historical rate-change patterns from the previous reassessment of the south and west suburbs in 2023. This same method will be used by the Board of Review when deciding appeals in the first year of the 2026 reassessment cycle. 

For years two and three of the reassessment cycle, the offices’ methodologies will vary slightly due to the availability of data when they perform their respective duties, though they will approximate the previous year’s tax rate. 

Both agencies have committed to updating their internal models and publicly releasing more information on their revised methodologies, marking one of the first coordinated changes stemming from the Property Tax Reform Group’s audit recommendations.