[locofocos]

I used this when modeling a shed-to-house I built. Like most open source software, it's not SUPER polished but it's useful. Making basic walls, doors, windows, and placing furniture is fairly easy. It gets very complicated using their workaround for things like ceilings, roofs, and such but it's doable.

One great feature is being able to export a 1:12 scale reference for my shed builder. It took some very careful toying with background layers, but I was eventually able to get a large-scale Kinko's printout from a PDF where 1 inch on the printout = 1 foot in the real world.