How to Convert a Wall Corner Into an Interior Wall
Let's say you have a wall joined at both corners, with an interior length of 20 ft. You can extend this wall by simply clicking on it, and then choosing one of the end control points.
Essentially, you end up changing the vertical corner into an interior wall.
Notice that when you do so, the interior length of the wall is preserved. This is a powerful way to extend a single room floor plan, into a floor plan for an entire house.
Watch this video to see how to convert a wall corner into an interior wall.
Note: This video has no sound; it has captions pointing out the relevant steps (program version 2.1).