Now you can design your own extension.
SketchUp is 3D modeling software that’s easy to learn and incredibly fun to use. Download SketchUp today for free and get started drawing in 3D.
Download link: http://www.sketchup.com/download
There are literally hundreds of videos on ‘youtube’ that can help you to use Sketchup.
I drew this image using sketchup and then rendered it using ‘Kerkythea’ another free bit of software.
This was the finished job.