65 ILCS 5/8-10-24

All specifications pertaining to the construction, alteration, rehabilitation or repair of any real property of such municipality shall be prepared by the engineering agency engaged in the design of such construction, alteration, rehabilitation or repair, prior to approval by the purchasing agent, and any such specification shall form a part of any such purchase order or contract, and the performance, inspection and testing of all such contracts shall be supervised by the engineering agency designated in such contracts. If after award of such contracts changes or modifications are necessitated therein, such changes or modifications may be accomplished or ordered in writing by the engineering agency, but if the costs thereof are estimated to exceed $5,000 written approval of the purchasing agent must be first obtained. A modification agreement therefor shall thereafter be executed by the contractor, the mayor or his duly designated agent, by the comptroller and by the purchasing agent.