The governing body shall require the city engineer to make a detailed estimate of the cost of such sidewalk or sidewalks, curbs, gutters, curb cuts, curb cutbacks and driveways under oath, and file said estimate with the city clerk, which estimate shall be sealed and remain in the custody of the city clerk, and shall be submitted to the bid board and opened prior to the opening of bids for the awarding of a contract for the construction, reconstruction or repair of such sidewalk, sidewalks, curbs, gutters, curb cuts, curb cutbacks and driveways. The engineer’s detailed estimate shall be approved by the commission [council] prior to the awarding of any construction contract. (C.O. 17 § 7, 1-30-71.)