The city of Roanoke, Virginia, in furtherance of its objective to
establish building lines. and regulate and restrict the construction and
location of buildings within the city, divided the surface area of the
city into "business" and "residential" districts. Another ordinance of
that city created a setback line requiring all buildings subsequently
erected to be located at least as far from the street as were sixty per
cent of the then existing buildings fronting the street on the same
block.