You appeal the assessed value, not the tax bill, and you do it township by township inside a filing window that lasts about 30 days. There are three venues in order: the Cook County Assessor, the Board of Review, and the Illinois Property Tax Appeal Board. Most reductions come from the first two. Filing is free at all three.
1. Find your window
Each township opens once a year at the Assessor and once at the Board of Review. Dates are on our deadlines page. Miss the Assessor and you can still file at the Board; miss both and you wait a year.
2. Check the assessment
Your assessed value is on your reassessment notice and on the Assessor's site. Residential property is assessed at 10% of market value, so an assessed value of $35,000 means the county thinks the home is worth $350,000. If that number is above what similar homes nearby sold for or are assessed at, you have a case. See how the assessment is set and becomes a bill.
3. Gather evidence
Comparable homes (same township, similar age, size and style) with lower assessments per square foot; recent sales; photos of condition problems; an appraisal if you have one. Commercial property adds income and expense statements and vacancy.
4. File
Online with the Assessor during the window; then, or instead, with the Board of Review during its window. A Board decision can be taken to PTAB or the circuit court.
5. Decision
Reduction, no change, or a request for more information. A reduction lowers the bill issued next year, not the one due now. Board reductions in a reassessment year carry into the next two.
Do you need a lawyer?
No. Whether it helps depends on the venue and the evidence; the county's own outcome data by venue is in Cook County Property Tax Attorney: When Representation Matters. Our fee is 25% of the first year's savings and nothing if there is no reduction; start with the free property check.