To make a 2d plan of the walls, including doors and windows is not yet possible in sweet home 3d.
Either I didn't understand what you're trying to draw, or you missed some Sweet Home 3D features! 
Please, have a look at the plan images in the gallery (all the images in the first column) and tell me what is missing in them for you.
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Emmanuel Puybaret, Sweet Home 3D creator
