There is so much more to “designing a home” than figuring out square footage and arrangement of rooms, that I doubt you could get anything (for free) that would do much more than that. After you get a layout, then it has to be plumbed, wired, insulated, and most important of all “supported” (and covered), which is unlikely to be found in any kind of simple app. Architects and engineers can spend hundreds of hours on full-featured programs such as AutoCad and still make crucial mistakes.