Creating floor plans with SweetHome3D how do I create an attic?

I was too stupid to plan an entire house, I simply planned both floors separately, in separate files. The thing with the walls works as Steffi described. Depending on the scenario, you sometimes have to split a wall in the middle. You can then create angled walls. I managed the knee wall in our old house quite well with that.

: I just tried it out because I also have to/want to make a plan for the attic.

As others have already said, you divide the walls and let them slope towards each other so that they meet at the ridge.
If you need a roof surface for illustration, you can generate it over a box with thickness 1cm, room width, and inclination in roof pitch.
A roof window (not found in the standard) is done via the frame.

With a bit of experimenting, you can really do a lot with the program...

I have now done it the way recommended as well, always splitting the walls and then defining the starting height and ending height. It fits, great. Not quite intuitive and in some places where walls stand in the middle of the slope, it is either Pythagoras or a bit of trial and error and approximation.