New construction: Attic expandable or not

: Do I understand you correctly – in our situation you would go from 35 degrees roof pitch to 38 degrees? (We have already discarded the additional effort of a knee wall)

No. I had two thoughts. First, I calculated for you what the planned 38 instead of the minimum 35 degrees roof pitch means in terms of "height."

1) then I warned you that legally a third full floor might possibly arise from that and

2) derived from that the suggestion to deliberately do exactly that, namely: to make the attic the second full floor. That is, with even 40° roof pitch and the knee wall to make the roof the upper living floor, thus using it sensibly and not adding another floor between the ground floor and attic.

A knee wall is not additional effort. You generally want to avoid an attic part with "usable height zero" if you want to use the attic space. Therefore, you would need a knee wall if you did not make one. I would always prefer the knee wall.

Only if you do not need the attic space at all and only build to satisfy a roof pitch-loving development plan, would you rather build without a knee wall (so as not to have a then useless exterior wall stub included in the insulation calculation).

- due to edit time exceeding as "P.S." -

I summarize:

Don’t build two stories with straight walls plus a useless pitched attic floor with a roof slope that is already steep for purely storing Christmas decorations even without a knee wall;

but only one story with straight walls plus a pitched attic floor with a roof slope that together with the knee wall really makes it a full floor also in terms of usability.

You compensate for the lack of a basement by more ground area of the house or the garage. That is always preferable to drowning in the illusion of a usable blind-appendix attic.

Make a concrete ceiling, a proper roof (no truss/studio truss or similar), put in a few skylights and rather a knee wall instead of increasing the roof pitch. It might also look rather mediocre otherwise. And definitely a proper staircase.

If it is to be used as a recreational room, then possibly underfloor heating. Otherwise as an unheated ancillary room for technology, storage, and hobbies. Either way, you can still integrate it into the ventilation system ([Kontrollierte-Wohnraumlüftung]).