Front yard. There shall be a front setback line determined as follows: Where buildings exist in the same block on either side of the street, the setback line of the building to be constructed shall be at least the average of buildings constructed within the same block. In measuring to determine said building line, open porches shall not be construed as part of the building. In blocks where no buildings exist on either side of the street, the setback line of the building shall be 25 feet. In the case of a corner lot, there shall be two front yards.