The requirements of this chapter, except those relating to front and rear setback and percentage of lot occupancy by buildings and accessory buildings, shall not prevent the erection of a duplex or double house on any parcel of land in Districts 3, 5 or 6, less than 7,200 square feet in area, but having two side yards, each at least five feet in width and restricted by covenant or other restriction duly recorded in the office of the Register of Deeds of Milwaukee County, Wisconsin, prior to August 4, 1927, exclusively to duplexes or double houses, respectively; provided, however, that the health, safety and general welfare of the district in which said lot is located is not impaired by such erection, which fact shall be determined by the Board of Appeals.