Except as specifically provided otherwise in the “B-1” – General Business District and in all residential zoning districts where lots having fifty percent (50%) or more of the frontage on one side of a street between intersections (that is, in one block) are developed with buildings, and the front setbacks of those lots do not differ by more than ten (10) feet, the minimum required front setback on that block shall be the average of the existing front setbacks, but no less than five (5) feet, provided, however, that in any built-up area, no front setback greater than fifty (50) feet shall be required.