For any development, the City Planning Commission may grant special permits to allow semi-detached or attached single-family residences in R2 Districts and attached single- or two-family residences in R3-1 Districts. As a condition for granting such special permits, the Commission shall find that:

(a)        only by modifying the residential building type, the preservation of trees over six-inch caliper and of existing topography can be assured;

(b)        by inclusion of the proposed residential building types, better standards of privacy and usable open space can be achieved; and

(c)        if the zoning lot is located in the designated open space, the prohibition of development on the designated open space requires the permitted development to be concentrated in the remaining portion of the tract.