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 assigns every parcel a class code that determines both the assessment level applied and the pool of comparable properties.
The broad shape
Residential classes — single family, two to six units, condominiums — are assessed at a lower percentage of market value than commercial and industrial classes. Vacant land, not-for-profit and incentive classes have their own treatment.
Incentive classes
Cook offers reduced assessment levels for qualifying industrial and commercial redevelopment. These are applied for, time-limited, and conditional. A neighbouring property carrying an incentive class is not a comparable for you, and mistaking one for a comparable is a common own goal.
Why the code matters to an appeal
A comparable in a different class is not a comparable, because it is assessed at a different level by design. Filter on class before anything else. And check your own: a converted building or a newly built home can carry a stale code for a cycle.
Where to find it
The Assessor's property search shows the class for every PIN, along with characteristics and assessment history.