The height provisions of this chapter shall not apply to the erection of building appurtenances such as church spires, belfries, towers or flagpoles designed exclusively for ornamental purposes. The height provisions of this chapter shall moreover not apply to chimneys, flues, bulkheads, elevator enclosures, water tanks or similar accessory structures occupying an aggregate of 10% or less of the area of the roof on which they are located and further provided that such structures do not exceed the height limit by more than 10 feet. Nothing in this chapter shall prevent the erection above such height limitation of a parapet wall or cornice extending above such height limit not more than three feet or to the construction of farm buildings of variable heights.