I am thinking of knocking down my single garage and building a double, trying to find some software that will draw it on the pc.

 

AutoCAD
Photoshop

Theres another Auto(something?) i used at school that would be up to the job aswell.. cant for the life of me think what it was caled though

Sure there will be plenty more aswell, but that's a couple of options for ya

 

i use microstation at work