The owners of all properties within commercial zones of the city, properties abutting arterial streets or collector streets and properties upon which there are to be public buildings, shall, in conjunction with new construction on such properties or alterations or improvements to existing structures on such properties where the estimated cost of the alterations or improvements constitute twenty-five percent or more of the value of the existing structures on the property, construct sidewalks along abutting streets; provided, however, that clearly marked walkways may be allowed by the director of public works along those streets on an interim basis where the plans of the city call for the future widening of the adjoining roadway surface. (Ord. 530 §1, 1979; Ord. 520 §5, 1979; Ord. 380 (part), 1975).