Required parking areas for all uses or buildings in a Residence A-1, A-2, A-3, B-1 or B-2 District and for all dwellings or multifamily houses in all districts shall be upon the same lot with the main building, except that off-street parking space required for structures and uses on two adjoining lots may be provided in a single common facility on one or both of such lots in accordance with an agreement between the owners of such adjoining lots, which agreement is to be filed with the Building Committee. In all business districts, required parking areas for all uses other than dwellings or multifamily houses shall be either upon the same lot or, if on a nonadjoining lot, in the same or another business district or Residence M or M-1 District within 300 feet of the property to which they are accessory, measured in a straight line between the nearest point of the parking area and the nearest point of the property to be served. Any accessory parking area which is not upon the same lot as the main building or use, and its necessary accessways, shall be reserved for off-street parking use, under conditions and safeguards approved by the Village Board of Trustees and designed to assure the continuing availability of such parking area to serve the use to which it is accessory for as long as such use continues in existence, unless the Board of Trustees finds that adequate substitute public parking facilities have been provided.