When we were thinking about buying a condominium some years ago, we looked at older units and brand new ones, and one of our least favorite features was pebbled or fogged window glass, which was ubiquitous. Early on, when we didn't know any better, we asked a real estate agent how much it would cost to replace a set of pebbled windows with transparent ones, and he said that was not possible. We reacted with confusion, thinking that as owners of the unit, not renters, we could make any changes we liked. But as he explained to us, some of the features of a condominium are considered "communal" (kyōyō-bu), meaning they are owned by all the residents of a building, and one of those common features are windows, which means they can't be changed.
Most communal features and facilities are obvious — building foyers, elevators, hallways, mailbox areas — as are "private" features (senyū-bu), which incorporate almost everything within an individual unit. However, some features exist in a kind of gray area — nominally communal space that is nevertheless used exclusively by a certain owner. Examples include first floor or penthouse gardens attached to individual units, parking spaces, exterior front-door alcoves, trunk rooms and balconies.
Yes, balconies are considered communal features even though each one is attached to a particular unit, and in principle most condominium associations do not allow owners to place items such as sheds on their balconies. Even air conditioning units usually have to be placed in a specific space. One reason balconies are considered communal is that in the case of an emergency they can be used to evacuate residents from one floor to another.