No building shall be erected, converted, enlarged, reconstructed, moved or structurally altered, nor shall any building be used, except for a use permitted in the district in which such building is located, or to exceed the height limit herein established for the district in which such building is located, except in conformance with the off-street parking and loading regulations of the district in which such building is located, and in no case shall there be more than one main building on one lot except as specifically provided hereinafter.