Sometimes things turn out differently.
I still haven’t received a response from the building authority, but I have received an initial hearing from the monument office. Apparently, the floor plan with room layout and floors is also supposed to be protected. Unfortunately, this means the small house will no longer be habitable (with a ceiling height of 2.05 meters, I unfortunately cannot rent it out either). So I will renovate it with minimal effort, maybe add house connections, a laundry room, a small workshop or something like that. For this, I can now freely place the new building on the property, preferably in a closed construction style on the north side, so that more light comes in from the south.
It’s really a shame, I really liked the combination of old and new building fabric.