With us, the interior walls have a different strength structure than the exterior walls. While the exterior walls are truly optimized for heat and therefore even lighter, the interior walls are somewhat firmer. The railing-supporting bolt staircase was a bit difficult for us. Special core drillings had to be made there, which were poured with concrete.
If you have a problem with 11cm walls, you simply go to 17cm. It just comes at the expense of the room volume.