No land shall be used or occupied and no structure shall be used or erected for any nonresidential purpose which requires provision for the loading and unloading of goods and equipment unless off-street loading space is provided in accordance with this article. Such space shall also be provided for any enlargement or alteration of an existing nonresidential use. Off-street loading space required under this article shall, in all cases, be on the same site or on a contiguous site to the principal use.