Every part of a required yard shall be open and unobstructed from the lowest level to the sky, except for the ordinary projection of sills, belt courses, chimneys, flues, buttresses, ornamental features and eaves, provided however that none of the enforced projections shall project into the minimum side yards more than 24 inches. Unroofed entrance porches or terraces which do not rise above the height of the floor level of the ground floor that extend into any yard providing the total area of all such porches which extend into such yards does not exceed 200 square feet, are also expected.