I’m not so sure it’s common property because it depends if the balcony door is considered part of the balcony or part of the building.
(b) any part of a balcony on the building. (common property)
The door is def not load bearing and so it could be considered part of the lot and owners responsibility. The Act doesn’t mention doors or windows as being defined parts and this omission implies that it could be owners responsibility if it is not considered part of the balcony.