Hi David,
Thanks so much for your help, I really appreciate it!
I wasn't aware of the "trick" to first have a short floor, build the roof and then increase to height of the room again.
And I'm glad I didn't do everything wrong Maybe some background information just in case you're interested: I live in that house which has at the moment only two floors. Now we're thinking of lifting up the roof to be able to have this third floor. And now I need to visualize that. It's located in Switzerland and yes, the exterior walls are indeed as thick as they are drawn as there's a pretty thick layer of insulation to save energy in winter (when heating).