The City Commission 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, the assessment of single lots or parcels, and any other matters concerning the making of improvements by the special assessment method. Said ordinance 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 assessments provided that if the excess is less than 5% of the total cost it may be placed in the General Fund of the City.