I'm looking for some offline (not web-based) downloadable floor planning software that's free for designing a 2d floor plan (I don't really care whether it can be visualized in 3d). It should be something like the old offline version of Home By Me (their new online-only version is not acceptable) - except that their version required you to save to Facebook and I'd like to be able to save these models to hard drive. I'd also like it to auto-calculate the area of rooms. Another piece of software (non-free though) that produces the kind of diagrams I want is EDrawSoft Floor Plan Maker.
I came across some software called Sweet Home 3D and tried it out, and it seems to do pretty much exactly what I want! Despite the name, the floor plans are created in 2D and then rendered in 3D, but the 2D plan looks fine and room areas are auto-calculated!
Live Home 3D has Free version for Windows that can be installed from the Microsoft Store. The app is pretty powerful, though very easy to start and get needed result. When a room is complete the app automatically indicates the room area on the floor plan.
The app doesn't stop on floor plans, if you want to play with it a bit more you can automatically create 3D model of your house from the floor plan you created. And then walk around it in 3D.
As others have mentioned, Sweet Home 3D is built for exactly this purpose. It is written in Java and thus available on the major desktop platforms; it is also on the repositories of some Linux distributions. This tool is built for the very purpose of creating floor plans and offers advanced features such as 3D rendering of your floor plan.
If you choose that route, you may also want to get the Advanced Editing plug-in for it (see the instructions for a download link and installation), as the wall drawing functionality in the base version is sufficient only for a visual impression. If your plan needs to be to scale, then the plugin is pretty much required.
Dia has some functionality for floor planning, including measurement lines. It is OK for drawing a 2D floor plan but lacks the advanced features of a purpose-built tool. Also, it hasn’t been updated in some time (the last preview release is some six years old).
Otherwise, your best bet is probably Inkscape (or any vector drawing tool of your choice): Create a drawing as large as the building (or the rooms you are interested in), and draw rooms and walls at a 1:1 scale. When you need to print it or embed it in another document, you can still scale it down as needed. Main drawback: no automated measurements, you will have to add those by hand.