Front yard: Each lot shall have a front yard 35 feet, except that if there are principal structures fronting on the same street within 300 feet of either or both of the side lines of the lot, the minimum front yard depth shall be equal to the average of the setbacks of the nearest of such structures on each side or to the average of such setback and 35 feet if there is such a structure only on one side, but not less than 25 feet in any case.