I was going to ask how to add more space, horizontally or vertically, to the middle (or, not at the edge) of a house, without having to completely redraw the present design to the left or right of the line where new space is to be added. However, just as I was about to do that, I had an idea, which seemed to work! If I do a multi-select across all the rooms and walls I want to keep, including all of the outside walls, then move the selection (e.g. using the arrow keys), I found that the outside walls and most of the inside walls, just stretched at the connection to the unselected portion. This is pretty much what I was after, so this is more a note than a question but if anyone has any thoughts on this, feel free to add them.
This is quite a common action for my designing, so it has been a real frustration up to now. I thought I might have to buy a really expensive architectural program to be able to do that. So my TurboFloorPlan 3D V16 lives on!