No sure whether it helps, but I drew my own extension plans. I only have a very basic schoolboys technical drawing at "O level". My local planning dept were most helpful, once I explained what I wanted to do with my very limited skills. It showed up how little I knew, but wasn't that complicated.
I'll ask around, but if you do decide to instruct another architect, I would suggest a local one might be best.
Regards,
Mark.