The County Assessor’s office multiplies a fair "Market Value" (MV) of your property by an assessment level for your property class to calculate your total property assessment. In Cook County, IL, the assessment level for residential properties is 10% of the MV, whereas the assessment level for commercial and industrial properties is 25% of the MV. Simply stated, your total property assessment is a percentage, based on property class, of your fair Market Value, on which your property taxes are calculated.