This procedure shall be applicable only to zones that permit residential uses, including the PD Districts, and its application shall result in a permitted number of building lots or dwelling units which shall in no case exceed the number which could be permitted, in the Planning Board's judgment, if the land were subdivided into lots conforming to the minimum lot size and density requirements applicable to the district or districts in which such land is situated and conforming to all other applicable requirements of this chapter, including the provisions of § 120-18; provided, however, that where the plat falls within two or more districts with differing density requirements, the Planning Board may approve in any one such district a cluster development representing the cumulative density as derived from the summing of all units allowed in all such districts. When laying out the location of the dwelling units, said units shall not be situated in areas constrained by:
(a)
The one-hundred-year floodplain.