The best 2D building design program is AutoCad but it is expensive.
Newegg.com - Autodesk AUTOCAD LT 2008 - Digital Media Editing
You might be able to get by using Sketchup. It is a 3D modeling program but can be used to do 2D and is free.
Google SketchUp
Autodesk also has a program called Architectural Desktop that might work but I have never used it so I am not sure.