Many thanks Hans.

Where it says "95.32 m2" there is a double wall (one wall on top of the other one).

Sorry, I couldn't either find something like "95.32 m2" nor a double wall.

After doing my things (eg. making the three yellow boxes invisble), I noticed that the red box is almost completely visible, but it isn't square.

I deleted these boxes / made them invisible, but without success. And I deleted all walls of the big room, but it kept the same with the boxes staying partially invisible.

If that doesn't fix the problem, I have another suggestion: The large room at right has quite a few "loose" (read: not-connected) walls. That makes it difficult to make a good room/ceiling. So... delete ALL walls of the large room on the right and redraw them as ONE consecutive wall.

I did and the boxes stayed as they were...

When I import the hedge-model it looks 100% OK.

Hmmm, very strange...do not know, what I do in another way...

Thank you Emmanuel,

Okay, there are different heights, but good to know.

Many thanks again, nice greetings, Dirk