How can I make a mirror (reverse) image of a 3d floor plan
Have a look at this trick and the GIF:
Scaling is one way of doing it but the mirror extension is way better in my opinion.
The interface of the extension is very similar to that of AutoCAD and very easy to use. Its also free.
Or you can use Flip Along from the Context menu. I have keyboard shortcuts established so mirroring a selection along an axis is a simple single button press.