ᐅ Creating Floor Plans with SweetHome3D: How Do I Design an Attic or Loft?

I wasn’t able to plan an entire house at once, so I simply planned both floors separately, in different files. The walls can be done as Steffi described. Depending on the situation, you sometimes need to split a wall in the middle. This way, you can create angled walls. I managed to get the knee wall right on our old house using this method.

@hausBauMTK: I just gave it a try because I need/want to create a plan for the attic as well.

As others have already mentioned, you divide the walls and let them slope towards each other so they meet at the ridge.
If you need a roof surface for illustration, you can create it using a box with a thickness of 1cm (0.4 inches), room width, and the pitch set to the roof slope.
A roof window (not found in the standard options) is created via the frame.

With a bit of experimentation, you can do a lot with this program...

I followed the recommended approach as well, dividing the walls and then defining the starting and ending heights. It works perfectly. Not entirely intuitive, and in some cases where walls stand in the middle of a slope, you either have to use the Pythagorean theorem or experiment a bit to get close.