That no nonconforming building shall be enlarged, extended or increased unless such enlargements would tend to reduce the degree of nonconformance except that single-family residences in residential zones may be enlarged, providing that such enlargement shall not encroach beyond the existing building lines of any existing principal structure New conforming single-family dwellings shall be permitted as long as all of the requirements of Subsection 23-3.4A Schedule of Yard, Lot and Building Regulations, are met for the zone in which they are located.