We proposed to the client to keep this small unused area in the staircase instead of giving it to the adjacent room, to create a nice wide volume in the staircase. We also proposed to created a playhouse for the 2 young kids to give an original use of this small residual space.

By the way Bipede, thanks for the inspiration. I might make something like this that wraps around our basement stairs and provides a cool play space for my son.

How much?