In the case of a designed residential subdivision or group housing of two or more buildings to be constructed on a plot of ground, not subdivided into the customary streets and lots, and which will not be so subdivided or where the existing or contemplated street and lot layout make it impracticable to apply the requirements of this chapter to the individual building units in such group housing, the application of the terms of this chapter may be varied by the Board of Adjustment in a manner which will be in harmony with the character of the neighborhood; however, in no case shall the Board of Adjustment authorize a use prohibited in the district in which the housing is to be located, or a smaller area per dwelling unit than the minimum required in such district or a greater height than the requirements of this chapter permit in such a district.