Storage of materials and supplies used in the building trade industry by the occupant of the principal building, such as lumber, forms, masonry materials, etc., provided that:
(a) The space allotted to such storage shall not exceed 25% of the area of the lot.
(b) The materials and supplies are appropriately screened from adjacent residences and from street view by means of a wooden, masonry or evergreen fence.
(c) The height of the stored materials and supplies shall not exceed three feet above the established street grade.