Sweet Home 3D is a free interior design application
I used this software to design my tiny house that I built with great success. It really helped me visualize the space. I didn't create an engineering ready floor plan with this so to speak. In other words, I intentionally left the measurements slightly relative which wouldn't work for most construction projects. In other words, if my bedroom ended up being 14ft by 10ft and 2 and 1/2 inches instead of 14x10' I was fine with that, it just meant other rooms slightly smaller. But give or take the width of dry wall and other factors it was worked fine. I found getting super precise measurements with this app difficult and so I didn't even try. I just let the guy building walls have a tad bit of freedom. Like I said, for most projects that would not work. Having said that, this app was a huge life savor and allowed me to really visualize the space and make key design decisions.