815 ILCS 670/15
A contract to build new residential construction in any non-building code jurisdiction must adopt as part of the construction contract the applicability of a residential building code that is agreed to by the home builder and the home purchaser as provided in this Section. The home builder and the home purchaser may agree to adopt the International Residential Code or any municipal residential building code or county residential building code that is in effect on the first day of construction in any county or municipality that is within 100 miles of the location of the new home. If the home builder and the home purchaser fail to agree to a residential building code or if no residential building code is stated in the contract, the code adopted under Section 15 of the Energy Efficient Building Act, the plumbing code promulgated by the Illinois Department of Public Health under Section 35 of the Illinois Plumbing License Law, and the current edition of the International Residential Code shall, by law, be adopted as part of the construction contract.
815 ILCS 670/15