Elevators, bulkheads, water tanks, air conditioning equipment and similar structures and appurtenances may be erected on a building to a height greater than the limit for the district in which the building is located; provided that such structures and appurtenances are screened on all sides; and provided further that no such exception shall cover at any level more than 25% of the area of the roof on which it is located; and provided further that no such exception shall be used for occupancy of any kind.