Plans for extension

Have they considered having a crack at the design themselves first?
A lots of draftsmen are just that, they can drive CAD. Don't expect them to come up with anything inspirational in terms of design. They want to get the permits with the minimum amount of fuss and time so that they can make a profit, and they are not necessarily considering what is best for you or your site.
have a look at this article: http://www.architectyourself.com/design-your-own-home/

If they are going to use a draft, it is often much better to go to them with a clear idea (or better still a set of basic drawings) of the design you want to build.