In any residential district, where the average setback distance for existing buildings on all lots located wholly or partly within 200 feet of any lot, and within the same zoning district and fronting on the same side of the same street as such a lot is less than the minimum setback required in such zoning district, the setback on such lot may be less than the required setback but not less than the existing average setback distance for all lots within the 200 feet, and in no instance shall the setback be reduced to less than 15 feet. When lots within the 200 feet are vacant, such vacant lots shall be considered as having the minimum required setback for the purpose of computing an average setback distance. (11-20-75, art. 3, 3.12; 2-6-78)