70 ILCS 810/26
Any territory adjoining any forest preserve district organized hereunder and situated in the same county as such district may become a part of such district in the following manner: Upon the filing with the circuit court of the county in which such district is located, of a petition signed by not less than 10% of the legal voters residing within the territory proposed to be annexed, such circuit court shall certify the question to the proper election officials, who shall submit the question of such annexation to the voters at an election held in the territory so proposed to be annexed in accordance with the general election law, and if a majority of the votes cast upon such question shall be in favor of such annexation, such territory so proposed to be annexed shall become and be a part of such forest preserve district.
70 ILCS 810/26