A dwelling of which the exterior walls or any structural parts of such walls are of wood. A dwelling which would not otherwise be a frame dwelling shall not be deemed a frame dwelling by reason of the existence on such dwelling of frame oriel, bay or dormer windows, frame porches not more than one STORY in height, or frame extensions not more than one STORY in height and fifty-nine square feet in area if such windows, porches or extensions were erected prior to 4/13/1940.