Illinois homeowners pay an effective rate around 2.1% of home value — roughly double the national average. But Illinois also gives you more free appeal opportunities than almost any state: in Cook County you can appeal to the Assessor AND the Board of Review, every single year, for free.
Cook County opens appeals township by township for roughly 30 days each. Outside Cook, you have 30 days from publication of the assessment list. Find your window on your assessor's calendar.
Cook County publishes every parcel's assessment and sales history through its open data API. Our free scraper pulls uniformity and sales comps into a CSV — no dataset hosting needed.
File with the Cook County Assessor (cookcountyassessor.com) during your township window, and again with the Board of Review when it opens. Outside Cook, file with your county Board of Review.
Not satisfied? Take it to the Illinois Property Tax Appeal Board (PTAB) within 30 days of the Board of Review decision, or to circuit court.
Cook County reassesses one-third of the county each year. In 2026 the south and west suburbs get new values. If that's you, appeal season is critical this year — a reduction can hold until the next triennial reassessment.
Everything runs in your browser — we never see or store your property data.
Cook County assesses residential property at 10% of market value (33.33% elsewhere in Illinois), then applies a state equalizer and local rates. The 2.11% default approximates the statewide average effective rate — enter your own for a tighter estimate. Estimates only; not tax advice.
Enter recent sales of similar homes (or import a CSV from our free Cook County open-data scraper). The worksheet computes price per square foot and a suggested value for your property.
| Address | Sale Date | Sale Price ($) | Sq Ft | $/Sq Ft |
|---|
| Item | Illinois Rule |
|---|---|
| Cook County — Assessor | File free online during your township's ~30-day window (see the Assessor's Assessment & Appeal Calendar) |
| Cook County — Board of Review | Second free appeal after the Assessor level — file when your township opens at the BOR |
| Other 101 counties | 30 days from publication of the assessment list — file with your county Board of Review |
| Next level | Illinois PTAB within 30 days of the BOR decision, or circuit court (specific objection) |
| Assessment level | 10% of market value (Cook residential) · 33.33% (rest of state) |
| Best evidence | Lack of uniformity vs. similar neighbors + recent sales — both available via Cook's open data API |
| Records | datacatalog.cookcountyil.gov — free public API (see our scraper) |