Does sweethome allow you to make plans in 2D?
I think what you're after isn't technically 2D vs 3D but 'Perspective View' vs 'Orthographic View' (but tl;dr: no).
Perspective view emulates your vision, but orthographic is what you would use for engineering / architecture drawings. Looking top down like this, for instance, in perspective view you can see both the top of the wall and the bottom of the wall (what's shown), but in orthographic view those would appear as a single line like in architecture drawings.
All that being said, I don't think Sweet Home 3D can do orthographic views, though don't have it in front of me to check.