Cook County's class codes, and what they do to your assessment
The class sets the assessment level. It also sets who you are allowed to compare yourself to.
Cook County does not assess everything at the same fraction. Its classification ordinance sets a different level for each type of property, and your class code determines both what fraction applies and who you are compared against.
The main levels
| Class | Property | Assessment level |
|---|---|---|
| 1 | Vacant land | 10% |
| 2 | Residential, up to six units | 10% |
| 3 | Residential, seven or more units | 10% |
| 4 | Not-for-profit | 25% |
| 5a | Commercial | 25% |
| 5b | Industrial | 25% |
The gap between 10% and 25% is why classification errors on mixed-use property are worth so much.
The residential sub-classes
Within class 2, the code identifies the building type and size band:
- 202 — one storey, under 1,000 sq ft
- 203 — one storey, 1,000 to 1,800 sq ft
- 204 — one storey, over 1,800 sq ft
- 205 — two or more storeys, over 62 years old, up to 2,200 sq ft
- 206 — two or more storeys, over 62 years old, 2,201 to 4,999 sq ft
- 207 to 209 — two or more storeys, newer, by size band
- 211 — apartment building, two to six units
- 299 — condominium
Why the sub-class matters as much as the level
The level decides your fraction. The sub-class decides your comparison group — and for a uniformity argument that is the whole ball game. A comparable in a different sub-class is a weaker comparable however similar the house looks, because the model priced it differently.
The size bands are where errors compound. A house recorded at 1,850 square feet sits in a different class from the same house recorded at 1,750 — so a floor-area error produces a classification error, and the classification error puts you against the wrong peers.
The errors worth finding
- A two-flat coded commercial. The difference between 10% and 25% on a residential building.
- Mixed use coded entirely commercial — a shop below, a flat above.
- A conversion never updated — a single-family house divided into flats, or flats converted back.
- Vacant land still coded as improved after a demolition, or the reverse.
- A size band crossed because the recorded floor area is wrong.
How to check
Find the class code on your notice or parcel page, look up its definition on the Assessor's site, and read the description against your actual property: storeys, age, floor area band, number of units.
If it does not match, that is a factual argument rather than a matter of opinion — the strongest kind available, and one an office can verify by looking rather than by judging.
Incentive classes
Cook also operates incentive classifications — 6b for industrial development, 7a and 7b for commercial development in defined areas, 8 for particular communities — which assess qualifying property at a reduced level for a period, typically to encourage investment where it would not otherwise happen.
These are applied for, granted by the county board, and time limited. If you own commercial or industrial property in an area that qualifies, it is worth asking about — and if you hold one already, worth diarising when it expires, because the increase at the end of the period is substantial and entirely foreseeable.