Off-street loading and unloading space. On the same premises with every building or part thereof hereafter erected to be occupied for the purpose of manufacturing, storage warehouse, retail store, wholesale store, market, hotel, hospital or other use similarly involving large-volume receipt of or distribution of materials or merchandise by motor vehicle, there shall be provided and maintained adequate space for loading and unloading services, so placed and arranged as not to interfere with public use of the sidewalks, street or alley. There shall be not less than one such loading and unloading space for every 20,000 square feet of building floor area or fraction thereof, in excess of 6,000 square feet, used for any of the above-mentioned purposes. To satisfy the above requirement, a loading and unloading space shall be not less than 10 feet by 60 feet within the property lines and have a height clearance of not less than 14 feet.