Is staircase DIN 18065 mandatory or not?

There was nothing derogatory. I described an impression and the question at the end, whether your very special planning can be explained by a correspondingly unusual store of experience and habits, is absolutely serious.

I would not compare a stair rise to a ladder angle of inclination, that doesn’t quite fit. 26 cm is a full tread of foot length in shoe size 39; with larger feet, when going up the heel is partially unsupported, and when going down the toes; barefoot both come more into play than when shod. If the step edge is in front of the ankle going upwards, the limit of comfort or the feeling of safe walking is probably reached at about that point, possibly earlier with "overweight."

That sounds like a "only what no longer fit below comes up" planning principle. Besides, I would plan a staircase starting from the exit; knowledge of the upper floor would be helpful to the fellow discussants.

We actually just tried to realize a wooden house within our budget. A glass front in the living area and an open design were important to us.

It is difficult for me to find another position for a proper staircase. If I want to have a 26cm tread, the staircase protrudes too much into the living area.

It must be nice to be able to step fully with the sole of your foot quite relaxed in old age.