Thank you Hans and OK!

The term "cross section" helped me out to search for the right thing and i could find the workaround that OK suggested now as well.
This should be enough for my needs. I can remove all the walls and elements except the 1 wall including windows etc., export and import it while rotating to put it on the ground. Then i can add all the measures needed.

Excellent, thanks again.

Maybe one last question just out of interest: why is it, that the imported .obj file is as unsharp as in the example of OK?

Thanks,
Siddie