
When we were planning an extension we contacted a reputable building company who had it's own planner. He came around to do the quote and discussed what we required before drawing up a rough plan for us. That way it doesn't cost anything and you get to talk to an expert who can sometimes put a different light on things.
We didn't go for the extension in the end (cost and big problem with drains), but went with a large conservatory, a lot cheaper and I have it as half utility and dog room and half as a dining room for us. Again with the conservatory we saw several different companies and got different ideas from each of them. In the end we put the bits together we liked which worked well.
The only regret I have is that we should have done it years ago :)
Sandra