Outdoor Storage of Materials. The outdoor storage of materials, including but not limited to trash receptacles, lumber, auto parts, household appliances, pipe, drums, machinery or furniture, is permitted as an incidental or accessory activity of a permitted use or the principal feature of a conditionally permitted use; provided, that such storage shall be screened by a wall, solid fence, structure, or landscaping from surrounding properties and streets. No more than 40 percent of the lot area shall be devoted to outdoor storage.