The 33⅓% rule, and the counties it does not apply to
Illinois assesses at one third of fair cash value — except in Cook County, and except for farmland.
Illinois law sets the level of assessment at one third of fair cash value — 33⅓% — for every county in the state. That is 35 ILCS 200/9-145, and it is the rule almost everywhere.
Then there is Cook County, which does not follow it.
Why Cook is different
Cook County is a home rule unit and operates its own classification ordinance. Instead of assessing everything at the same fraction, it assesses by class:
| Property type | Cook level | Rest of Illinois |
|---|---|---|
| Residential (class 2) | 10% | 33⅓% |
| Commercial (class 5a) | 25% | 33⅓% |
| Industrial (class 5b) | 25% | 33⅓% |
The policy intent is to shift a larger share of the burden onto business property and away from homeowners. Whether it achieves that is a live political argument that has run for decades; what matters for your appeal is simply that the fraction is different.
The equaliser, and why it exists
Because Cook assesses residential property far below the statutory third, its assessed values are not comparable to any other county's. That is a problem, because school districts and other taxing bodies sometimes cross county lines, and state aid formulas compare counties to each other.
So the Illinois Department of Revenue calculates an annual equalisation factor for each county — the "multiplier" — designed to bring its aggregate assessments to the statutory third. For Cook County in 2024 that factor was 3.0355. For counties already assessing at a third, it sits at or very near 1.0, because there is nothing to correct.
The consequence people miss
A dollar of assessed reduction is not worth the same everywhere, because the equaliser multiplies it before the tax rate is applied.
Cook County — $10,000 × 3.0355 × 7.5% composite rate ≈ $2,277 a year
Lake County — $10,000 × 1.0 × 9% composite rate ≈ $900 a year
Two and a half times the benefit for the same reduction, entirely because of the equaliser.
This matters if you own property in more than one Illinois county and are deciding where to spend your effort. It also means any online calculator that applies one model to the whole state is wrong by roughly that factor in one direction or the other.
Checking it against reality
You do not have to take the statute's word for it. Many county parcel pages publish both an assessed value and a market or "fair cash" value for each property. Divide one by the other.
We did this across every DuPage County parcel in our own data that carries both figures. The answer comes back at exactly 0.3333 — for example an assessed value of 44,444 against a market value of 133,333. The statute and the roll agree, which is what you would hope and is worth confirming rather than assuming.
What this means for your appeal
Two practical points.
State your argument in the right units. If you are arguing market value, say what you think the property would sell for and let the office apply the ratio. If you are arguing uniformity, the ratio cancels out entirely — you are comparing assessed values against assessed values, so it never enters the arithmetic.
Do not compare across the county line. A neighbour half a mile away in the next county is assessed on a different fraction, so their assessed value tells you nothing about whether yours is fair. Comparables must come from the same assessment jurisdiction, and in practice from the same neighbourhood within it.