What I would find funny is a window in the wall to the stairs, so you can call down to the basement from the kitchen. That way you also get some natural light down there. Alternatively, you could integrate the stairs into the living area like that. It can look very stylish. How it affects energy efficiency, I can't say. What else comes to mind: Consider now whether you want a fireplace. They aren't that freely placeable. We ran into some trouble with that in the planning. The shaft is enough as preparation. The fireplace can then be connected later when you have a bit more cash.