A contract let under the provisions of this chapter shall require the work to be done pursuant to the plans and specifications on file in the office of the city auditor, subject to the approval of the engineer acting for the municipality, and shall provide further:
The engineer acting for the municipality shall supervise and inspect the work during its progress. In addition to any rights which a municipality may have under its contract for construction of part or all of an improvement after a contract has been awarded and before work thereunder has been completed a municipality may, with the consent of the contractor and without advertising for bids, order additional work done by that contractor of the same character as that which was contracted for, whether within or without the improvement district for which the original contract was made, and upon the same terms and conditions specified in the original contract except as to time of performance, and at the same prices for the additional work provided that the total price payable to the contractor for such additional work shall not exceed twenty percent of the amount estimated by the engineer for the municipality to be payable for that character of work under the original contract.
N.D.C.C. § 40-22-36