For any lot or any portion of a lot, those areas characterized by slopes in excess of 25% shall not be developed, regraded or stripped of vegetation, except that up to 2,500 square feet of slopes greater than 25% may be disturbed except for construction of a principal structure on any lot created before April 9, 1996. The Planning Board may, in its discretion, permit the development of roads through areas where the slope exceeds 25% in order to provide access to areas of a site which would otherwise be inaccessible.