Quadrennial reassessment: the year your number actually gets rebuilt
Most of Illinois reassesses every four years. Cook works on a three-year triad. Knowing your year changes your timing.
Outside Cook County, Illinois townships conduct a general assessment every four years — the quadrennial. In that year every parcel in the township is re-examined and values are rebuilt from scratch. In the three years between, assessments generally move only for equalization, new construction, or an individual correction.
Cook County: three-year triads
Cook divides into three triads — City, North and South — and reassesses one each year, so every property is reassessed every third year.
Why the year matters to you
In a reassessment year, the largest changes happen and the largest errors are introduced. It is also the year the assessor is most willing to correct something, because they are already in the file. If your township is being reassessed, read the notice carefully the moment it arrives.
Appealing in a non-reassessment year
You can. The right to appeal is annual and does not depend on whether your value changed. A uniformity argument in particular does not care: if identical parcels are assessed differently, that is true in year two of the cycle as much as year one.
Finding your year
Your township assessor publishes it, and the county's Supervisor of Assessments can tell you. It is worth knowing simply so an increase in the quadrennial year does not arrive as a shock.