While you're drawing the floor plan, the Ortho mode is active to draw the walls horizontally or vertically. To turn it off, click once on the O keyboard button, so you can draw out square walls indicating size (a) and angle (B).
Press the O button again or select "Ortho" in the status bar Application (C) to re-enable Orthal mode.
This article is valid for DomuS3D® 2017 and later versions