What the county's own roll says, and how many parcels sit above their neighbours.
30 days from the publication of your township's assessment roll in the local newspaper. The window is short and it is not the same date across the county.
Statutory basis: 35 ILCS 200/16-55. A late filing cannot be cured, so confirm the date on your own notice — it is not the same across the county.
Illinois assesses at 33.33% of market value. The assessor estimates market value by mass appraisal — a model over recent sales, adjusted by the characteristics they hold on file for your property. If those characteristics are wrong, the number is wrong. The full anatomy is here.
Beyond “my house is worth less than they say”, the Illinois Constitution requires uniform taxation. If comparable property near you is assessed at a lower rate per square foot, that is a ground for relief in its own right — no appraisal needed, and built from figures the assessor published themselves.