You're over-assessed when the County's value is higher than what your home would actually sell for — and then you're over-paying every year until you appeal. Cook County appeals run on a ROLLING township-by-township schedule: the Assessor opens each township for ~30 days, and the Board of Review re-opens it later for a second 30-day window. There is no single county-wide date — find YOUR township's open/close dates on the Assessor and Board of Review sites. Cook assesses residential at 10% of market value; over-assessment is checkable against recent sales (Illinois publishes sale price).
Live Cook County parcels whose assessed market value exceeds their recent arm's-length sale price — a real over-assessment signal pulled from the public assessor roll. If your parcel looks like these, you have a strong appeal:
| PIN | Use | Recent sale | County says mkt | Over | Indic. overpay |
|---|---|---|---|---|---|
| 1816••• | Improved commercial | $750,000 | $2,037,196 | 2.72× | ~$8,045/yr |
| 0915••• | Commercial mixed-use / large | $700,000 | $1,458,204 | 2.08× | ~$4,739/yr |
| 1603••• | Commercial mixed-use / large | $1,100,000 | $1,655,004 | 1.5× | ~$3,469/yr |
| 1708••• | Commercial mixed-use / large | $280,000 | $795,000 | 2.84× | ~$3,219/yr |
| 0823••• | Commercial improvement | $1,937,500 | $2,347,028 | 1.21× | ~$2,560/yr |
| 2334••• | Commercial mixed-use / large | $890,000 | $1,268,000 | 1.42× | ~$2,363/yr |
| 0826••• | Commercial mixed-use / large | $545,000 | $850,640 | 1.56× | ~$1,910/yr |
| 1603••• | Commercial mixed-use / large | $925,000 | $1,126,004 | 1.22× | ~$1,256/yr |
Most owners who win use a contingency property-tax appeal firm — it costs nothing unless they cut your bill, then takes a share of the first-year saving. About 95% of over-assessed owners never appeal, so the saving is usually left on the table.
Open the official Cook County appeal page → Confirm the official deadline →against the official Cook County / IL deadline (cookcountyboardofreview.com · 35 ILCS 200/16-95 (Board of Review); 16-55). These are public deadline facts, not legal or tax advice — confirm your exact date with the Cook County Board of Review (and the Assessor's office, earlier) and consult a professional before filing.
More for Cook County: how to lower Cook taxes · Cook property tax appeal.