No building or part thereof shall be erected or altered that is nearer the street line upon which it fronts than the average setback of the buildings on the same side of the street within the same block, but no street wall of a building shall be set back less than 30 feet. When all lots within the same block are vacant, no street wall of a building shall be set nearer than 30 feet to the street line. On corner lots, the setback distance from the street line, other than the street upon which the building fronts, shall not be less than 25 feet.