Basement windows in Illinois must have a minimum opening width of 20 inches, a minimum opening height of 24 inches, a net clear opening space of 5.7 square feet and a maximum sill height of 44 inches. These measurements follow the standards of the International Code Council, which Illinois follows.
In addition, according to the ICC's standards adopted by Illinois, the windows must be operational from the inside and require no special knowledge, hardware, equipment, tools or keys to open them. In addition, egress windows must have a minimum of 9 square feet of clear area, or escape well, on the outside, with a permanent ladder mounted to the window well if the well is greater than 44 inches below grade.
In Illinois, basements with habitable space are required to have a minimum of at least one egress, or exit, window to allow for escape from the basement in the event of a fire. Habitable space is defined as living, sleeping, eating or cooking space. Each sleeping area in the basement must also have a minimum of one egress window located within the room. The window in the sleeping room fulfills the requirements for the nonsleeping area of the basement.