Assessments

The Cook County's Assessor's Office is responsible for valuing the more than 1.8 million parcels in Cook County. Illinois law requires that the estimated property value and assessed valuation of property be periodically updated for tax purposes.

Cook County is divided into three areas for reassessment: the northern suburbs, southern and western suburbs, and the City of Chicago. Each of these areas is valued once every three years.

Residential properties are assessed as of January 1st of the applicable year, using three to five years of prior sales information—multiple years of data increases the stability of market value estimates.

The Cook County Assessor's Office conducts mass appraisal to produce property values. Instead of valuing properties one-by-one, mass appraisal uses a technique called multiple regression. This technique is based on sales data from throughout the County and models a mathematical relationship between the value of a house and its neighbors. It follows similar steps that an appraiser would take when valuing a single-family home. The key concerns in residential property assessment are uniformity and equity. Similar properties must end up with similar values, and high-value properties must be assessed at the same percentage of their sale price as low-value ones. Mass appraisal ensures this is done in the least biased way possible.