Within the Subdistrict, single- and two-family detached and semi-detached residences and uses listed in Use Groups 3 or 4 are the only permitted uses. Non-conforming single- or two-family residences may be enlarged or extended pursuant to the provisions of the Subdistrict provided that a 30 foot rear yard is maintained. All other non-conforming uses shall be subject to the provisions of Article V, Chapter 2 (Non-conforming Uses).