Front yards. Each lot shall have a front yard with a minimum depth of 40 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 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 50 feet if there is such a structure on one side only, but not less than 20 feet in any case. In the case of a corner lot, there shall be provided on each street a yard equal in depth to the required front yard depth on such street.