Single-family house 175 sqm without basement, too big?

do I understand correctly that you don't like all these considerations and would go back to the large hallway?

No, I did not want to comment on a specific attempt to change. Rather, I mean in general: when optimizing the hundredth of a percent, one easily gets into a shiny vicious circle (which in turn is only a mutation of Morbus StanSch). Then the rule is: brake, disengage, take a deep breath, turn around, back to the last safe "restore point" as it is nicely called in computer operating systems. In this sense, also @ all readers: if you get stuck for a moment, take a break - don't bury yourself at full throttle with spinning wheels.

Thank you for the draft, it looks quite good! So you don’t think the light source would be too far away then?

I think a large window there would bring you significantly more light than a frosted glass door. You’d just have to see how to nicely integrate it into the façade. Otherwise, you could also assign that corner to the son’s room. Or make a WC niche. But the bathroom and the room are already plenty big anyway.

A skylight behind the stairs is also conceivable, it brings a lot of light even though I don’t have the cardinal directions in mind right now. And one should consider how to design the railing; if it becomes a half-wall or even an open railing, the light is distributed well.

I really like that, thank you for the suggestion. First of all, I also considered adding the corner to the bathroom, "great for the toilet," but somehow nothing else fit anymore. It's fine the way it is planned now. The fixed glazing somehow gives me a good feeling

Skylights are, of course, the first idea, but we would lose the attic because of that and it would be (in my opinion) incredibly expensive since both the ceiling and the roof would need to be broken through. I would definitely want to install a railing in the hallway, as open as possible, maybe even glass.

I looked again at your first post: I remembered a pitched roof, but with the targeted high knee wall = full upper floor, it naturally makes less sense, but there is nothing against a normal window/fixed glazing then!