65 ILCS 5/2-2-5
Whenever any area of contiguous territory, not exceeding 4 square miles, and not already included within the corporate limits of any municipality has residing thereon a population of not less than 2,500 persons, including 2,000 living in immobile dwellings, it may be incorporated as a city as follows. Whenever in any county with more than 1,000,000 inhabitants any area of contiguous territory not exceeding 4 square miles and not already included within the corporate limits of any municipality, has residing within the area all of the registered voters of a township who are not already included within the corporate limits of any municipality, is wholly bounded by a single municipality, and contains more than 1,200 residents, it may be incorporated as follows. If such area contains fewer than 7,500 residents and lies within 1 1/2 miles of the boundary line of any existing municipality, the consent of such existing municipality must be obtained before such area may be incorporated.
65 ILCS 5/2-2-5