Designing houses is very closely related to CAD families in the past, but that about to change in the future as more and more open source applications are available as an alternatives for the designers.
One example is Sweet Home 3D. It's a free interior design application that helps you place your furniture on a house 2D plan, with a 3D preview. It's also nominated as one of the candidates for SourceForge's Project of The Month for this month. It's Java-based so it can run on many platforms, including Windows, Linux, Mac, and Solaris.
Since it's built with MVC as the base, it's easy to extend using plugins. You can also import your own textures and 3D models.
Could you really believe that these images are created using an open source application? Well, you have to because it's a real deal.
Still unconvinced yet? Try the online demo (requires JRE installed and plugins are linked to your browser).