The gross area of the tract to be developed under the planned unit development approach shall comprise not less than ten acres. The minimum lot size shall be not less than seventy percent of the lot area per family or use which would be otherwise required. A minimum of ten percent of the land developed in a planned unit development project shall be reserved for open space and similar uses. Lot widths and required yards may be reduced to eighty percent of the requirement of this Zoning Code. The minimum tract size to be developed under the planned unit development may be reduced fifty percent where the proposed development is to contain only residential, commercial or industrial development, not a mixture of uses.