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. Under its own classification ordinance it assesses residential property at 10% of market value and commercial and industrial property at 25%. The same $300,000 house in Cook carries an assessed value near $30,000.
Neither number means Cook homeowners pay a third of what DuPage homeowners pay. The tax rate applied to each is set to fit the base it is applied to, and Illinois additionally applies a state equalisation factor — 3.0355 for Cook in 2024 — precisely to bring the two onto a common footing for overlapping districts. The ratio tells you how to read your notice. It tells you very little about whether your taxes are high.
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 — so they file nothing. 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.
Both are the same error. The right comparison is like for like.
The arithmetic to do first
Divide your assessed value by your jurisdiction's ratio to recover the county's implied opinion of market value.
| Where | Assessed value | Divide by | County thinks it is worth |
|---|---|---|---|
| Cook County | $32,000 | 0.10 | $320,000 |
| DuPage, Lake, Will… | $110,000 | 0.3333 | $330,000 |
| Connecticut | $210,000 | 0.70 | $300,000 |
Now ask honestly: would you list at that price tomorrow?
If the implied value is clearly above what you would list at, you have a market-value argument. Gather evidence of what genuinely comparable properties actually sold for, and be ready for the assessor to point at any sale of your own property if there has been one.
If the implied value is at or below what you would list at, a market-value appeal is not your argument. This is the common case, because assessors tend to be conservative, and it is where most people stop.
The argument that survives a fair valuation
Stopping there is a mistake, because there is a second and entirely separate ground.
A uniformity claim says: whatever my property is worth, comparable properties near me are assessed at a lower level per square foot than I am, and I should be brought into line with them. It is a claim about even-handedness, not about value, and it is available even when the county's opinion of your market value is perfectly reasonable.
Note the arithmetic carefully. It runs on the building assessment divided by the building's floor area, compared against the same figure for comparable properties. Dividing total assessed value — land plus building — by the floor area is a formula for nothing at all, and it penalises every property that happens to sit on a larger lot.
One number that is easy to miss
Your notice usually splits the assessment into land and improvement. Only one of them may be wrong, and the remedy differs. If the land figure looks out of line with neighbouring lots of similar size, that is a separate argument from anything about the building — and because uniformity comparisons use the improvement figure alone, a land error will not show up in them at all.