Except as herein provided:

   1.   No building or structure shall be erected, converted, enlarged, reconstructed, or structurally altered, nor shall any building or land be used, nor shall any use of the land be changed, which does not comply with all of the district regulations for the district in which the building or land is located.

   2.   No yard or lot existing at the time of passage of the Zoning Code shall be reduced in dimension or area below the minimum required. No part of a yard or other open space, or off-street parking or loading space provided about any building, structure, or use for the purpose of complying with the provisions of this chapter shall be included as part of a yard, open space, or off-street parking or loading space required under this chapter for another building, structure, or use.

   3.   Every building hereafter erected or subject to structural alteration shall be located on a lot as herein defined and in no case shall there be more than one main building on one lot unless otherwise provided in this chapter.