Every apartment is flexible and has an isolable room, which can be autonomous or connected to the flat.
This room is accessible from the common spaces and from the flat itself.
Exemple 1A couple with a child move to a 2-room flat.When the child is about to leave the flat, his room can be detached from the flat to be rent or sold.
Exemple 2A young couple buying his first flat can buy an extra room to rent it at first and can decide to extend the flat at a moment.