Cook County: why advice written for Illinois usually does not apply
Different ratios, a different cycle, an extra appeal stage and a very large multiplier.
Almost every general guide to Illinois property tax describes the 101 counties that are not Cook. If you are in Cook, four things change.
1. Classification
The rest of Illinois assesses everything at 33⅓% of fair cash value. Cook classifies property and assesses classes at different levels, with residential assessed lower than commercial and industrial. Arithmetic borrowed from a downstate guide will be wrong.
2. A three-year cycle
Cook reassesses on a triennial cycle by triad — City, North, South — rather than the quadrennial used elsewhere.
3. Two bites at the appeal
Cook owners can appeal first to the Cook County Assessor and then, separately, to the Board of Review. These are distinct filings with distinct windows. Elsewhere in Illinois the Board of Review is the first formal forum.
4. A large equalization factor
Because Cook's statutory ratios are below 33⅓%, the state multiplier is correspondingly large — it has historically run near 3.0. That is normal and is not evidence of anything about your property.
What does not change
Uniformity is still a ground. Factual errors are still fixable. Exemptions still work the same way. The reasoning is identical; the numbers and the forums are not.