Nothing herein contained shall be construed
so as to limit the height of any belfry, bulkhead, chimney, church
spire, clock tower, cupola, dome, silo, skylight, ventilator, water
tank and other necessary mechanical appurtenances usually carried
above the roof level. Such features, however, shall be erected to
accomplish the purpose they are required to serve and shall not occupy
in the aggregate more than 25% of the roof area of the main building.
In such cases in residential zones where the frontage on the same side of the street within 500 feet is 50% or more developed, then the required front yard for a new structure may be modified to the average for such existing development. Otherwise, the requirements of §
130-10 shall apply.