When the said plans, profiles and specifications have been completed, the City Manager shall return the same to the Council and the Council shall, if the work is to be done by independent contractors, order the advertisement for sealed bids for the work in accordance with the purchasing procedure. In addition, such advertisement shall specify that no contract shall be let until the special assessment roll has been confirmed and no bid may be withdrawn for forty-five days after the opening thereof. In the event the work is to be done by the City, the Manager shall present a revised estimate of costs at the time the said plans, profiles and specifications are returned to the Council.