160m2 detached house in timber frame construction on the north slope with basement

This knee wall holds up when you position it so that the rooms can be used; the usable living area is smaller, but not the exterior area to be built.

What I think of the children's rooms in the basement, I have already said.

The house won't get any smaller now, the exterior dimensions are still the same if I looked correctly.

Instead of >1.80m knee wall in the attic, it's about 50 cm. That should already bring a considerable saving and is also smaller in terms of living space.

Your external dimensions are identical to the original design except for the little knee wall. What exactly should have changed due to the little unusable space in the attic? A carport has been added. Excavation remains, garden design remains, the roof area is probably slightly larger since it's steeper, so a bit more gable height will have to be built. Let's get an architect involved who can actually do the calculations.

Instead of >1.80m knee wall in the attic, it's about 50 cm. That should bring a decent saving and is also smaller in terms of living space.

I fear I understand: the father of the naive idea is, "knee wall equals surcharge, less knee wall equals less surcharge" (?) No, that does not bring a saving, because the less usable space has to be compensated elsewhere. He certainly avoids a full story that way, yes. But the knee wall is not only supposed to create working height for vacuum cleaners. It is also supposed to provide space for beds and dressers. And it does not do that at this height. A dwarf knee wall is worse than no knee wall at all: simply because it forces the nonsensical combination "knee wall and knee high wall". Even fools only do that in their first year of apprenticeship.

And he doesn’t really save anything worth mentioning. During the planning phase, we turned our barely one-story house into a real two-story house (completely without sloping roofs), which cost about 12k (gross!). Sure, that’s money, but it’s not decisive for the overall outcome. If we wanted to, we could save that again during the fittings.

I don't think the basic idea is that bad. But trying to save money with it is rather nonsense. You have to slim down the footprint. This bedroom on the ground floor, can that be removed? Closet gone, WC smaller, pantry relocated, squeeze everything together, the entrance area is gone anyway. Then it will slowly work.