The principal use of a hospital shall be buildings and structures for the purpose of serving general medical, surgical, psychiatric, physical therapy and rehabilitation purposes. Accessory uses may include outpatient clinics or facilities, or facilities for nursing, teaching, medical research, convalescent or extended care when sponsored by or affiliated with such hospital; nursing homes; residential facilities for staff personnel, including volunteers, nuns or students in hospital training programs; accessory storage facilities with a gross floor area not in excess of 20% of the gross floor area of the principal building; places of worship; accessory parking facilities, vehicle maintenance facilities, recreation facilities, eating and drinking facilities, gift shops, flower shops and newsstands for the exclusive use of the staff, administration, patients and visitors; and laundries, food preparation facilities, power plants or other similar facilities customarily incidental to a hospital.