The provisions of this chapter relating to multiple dwellings erected after April eighteenth, nineteen hundred twenty-nine, shall not apply to any fireproof building structurally enclosed but uncompleted on April first, nineteen hundred forty-five, conforming to the provisions of the local zoning law or resolution applicable to an apartment hotel, in effect at the time of the approval of the original plans filed for the construction of such building, and which building shall thereafter be altered and completed as a class A multiple dwelling, in accordance with plans and amendments thereto approved by the department.  Such building, if so altered and completed on or before July first, nineteen hundred forty-eight, may be lawfully occupied thereafter as a class A multiple dwelling if it conforms with the provisions of section twenty-five applicable to fireproof class A multiple dwellings occupied as such before April eighteenth, nineteen hundred twenty-nine.