How the checker works, and what it can't do

Data

Everything comes from the Cook County Assessor's open data: assessed values, building characteristics, parcel addresses and parcel sales, read through the county's public API and cached for a day. Appeal dates are copied by hand from the Assessor's and Board of Review's calendars and dated. Nothing is scraped from the Assessor's or Board's portals.

Method

Cook County assesses homes at 10% of market value. A "lack of uniformity" appeal argues that similar homes nearby are assessed at a lower rate. The checker finds 5 to 10 homes in your assessment neighborhood of the same class group with similar size, age and lot, takes the median of their assessed value per square foot, and applies it to your square footage. The gap, if any, is the estimated over-assessment. Savings use the state equalizer (3.03) and either your tax rate or a typical range.

Limits

It compares building square footage only. Condition, renovations, basements, garages and views are ignored.
The Assessor's square footage is sometimes wrong. If it is, that is its own ground for appeal.
Condos have no published unit size, so they can't be checked this way yet.
Values at the "mailed" stage change after certification and Board action.
Default tax rates are rough; the rate on your bill is exact.

The legal line

Under In re Yamaguchi and Board of Review Rule 1, only the owner or an Illinois attorney may prepare or file a residential appeal. This site gives information and evidence; it never fills in or files a complaint, never gives advice on your specific case, and never takes a share of savings. Entity-owned property (an LLC or trust) must use an attorney.

All figures are estimates, not legal or tax advice, and not an appraisal. Check every figure against the official records before relying on it.