The council shall prescribe by general ordinance the complete special assessment procedure to be used, including the preparing of plans and specifications, estimated costs, the preparation, hearing and correction of the special assessment roll, the collection of special assessments, and the assessment of single lots or parcels. Said ordinance shall provide for a public hearing by the council on the necessity of making such public improvement, and shall provide for a public hearing by the council on the apportionment of the roll, as well as any other matters concerning the making of improvements by the special assessment method. It shall authorize additional assessments if the prior assessment proves insufficient to pay for the improvement and costs incident thereto or in case of invalidity in whole or in part, and it shall also provide for the refund of excessive assessment but if the excess is less than 5% of the total cost it may be placed in the general fund of the city.