Well the prize goes to freddymercurystwin, the area is 18.54m²!!!
Thanks for the help, I'll be keeping the external doors and currntly I have 47.8% of the wall as glass which the buildings officer said it would be OK and will be exempt.
I used AutoCAD Lt 2010, I run a CAD design company so the software is a must for me, but costs around £1000 so not cheap. I hear google's sketch up is pretty handy........and free!!!
I am at the sketch stage of an extension (and my first project), at the moment I have drawn it as a lean to with 50% glass on the outside, and it meets all the other cryteria to be exempt from building regs, however my fair lady doesn't want so much glass, so I was wondering if, what I have...