35 ILCS 200/17-10
The Department shall monitor the quality of local assessments by designing, preparing and using ratio studies, and shall use the results as the basis for equalization decisions. In compiling sales ratio studies, the Department shall exclude from the reported sales price of any property any amounts included for personal property and, for sales occurring through December 31, 1999, shall exclude seller paid points. The Department shall not include in its sales ratio studies sales of property which have been platted and for which an increase in the assessed valuation is restricted by Section 10-30. The Department shall not include in its sales ratio studies the initial sale of residential property that has been converted to condominium property. The Department shall include compulsory sales occurring on or after January 1, 2011 in its sales ratio studies. The Department shall also consider whether the compulsory sale would otherwise be considered an arm's length transaction, based on existing sales ratio study standards.
When the declaration required under the Real Estate Transfer Tax Law contains financing information required under Section 31-25, the Department shall adjust sales prices to exclude seller-paid points and shall adjust sales prices to "cash value" when seller related financing is used that is different than the prevailing cost of cash. The prevailing cost of cash for sales occurring on or after January 1, 1992 shall be established as the monthly average 30-year fixed Primary Mortgage Market Survey rate for the North Central Region as published weekly by the Federal Home Loan Mortgage Corporation, as computed by the Department, or such other rate as determined by the Department. This rate shall be known as the survey rate. For sales occurring on or after January 1, 1992, through December 31, 1999, adjustments in the prevailing cost of cash shall be made only after the survey rate has been at or above 13% for 12 consecutive months and will continue until the survey rate has been below 13% for 12 consecutive months. For sales occurring on or after January 1, 2000, adjustments for seller paid points and adjustments in the prevailing cost of cash shall be made only after the survey rate has been at or above 13% for 12 consecutive months and will continue until the survey rate has been below 13% for 12 consecutive months. The Department shall make public its adjustment procedure upon request.
35 ILCS 200/17-10