Assessed value, market value, fair cash value: three different numbers
They are related by a ratio set in statute, and confusing them is the most common reason an appeal goes nowhere.
Fair cash value is what your property would sell for in an arm's-length sale. Illinois law calls this the standard. Market value is the everyday name for the same idea. Assessed value is a legally defined fraction of it.
The fraction
Outside Cook County, Illinois assesses at one third of fair cash value — 33⅓%. So a house the county believes would sell for $300,000 carries an assessed value near $100,000. Cook County works differently, with classes assessed at different percentages.
Why people lose appeals over this
An owner sees $100,000 on the notice, knows the house is worth $300,000, and concludes the county has undervalued it — no appeal. Or the reverse: they see $100,000, think that is the county's opinion of sale price, and file claiming a wild under-assessment they cannot support.
The right comparison is like for like. To argue your assessment is too high, either show that the implied fair cash value (assessed ÷ 0.3333) is above what the property would sell for, or leave value out of it entirely and argue you are assessed unequally against comparable properties.
The arithmetic to do first
Take your assessed value, divide by 0.3333, and ask honestly whether you would list at that price. If the answer is "yes, easily", a market-value appeal is not your argument — but a uniformity appeal still might be, and it does not depend on what your house is worth at all.