Agree with Marko.
Pay an architect to do some basic drawings to get you through planning. The guy my Mum used was cheap (a few hundred quid) and prepared hand drawn plans and helped out with the planning process. He then used a structural engineer friend to design and specify the 3 main steels used where load bearing walls were affected and we had a sound basis upon which to choose a builder. All of this meant that planning permission was painless and Building control were happy on site (apart from the builder closing works in before inspection!
After this is where it went a bit wrong.
I’m ok with the process of appointing a contractor (it’s part of my job). The work was on behalf of my Mum though and despite best efforts to bring in 3 quotes she was taken with the first company (after being let down by two other builders who didn’t keep their appointments).
They were a main contractor in their own right and whilst they did domestic work this was at the smaller end of their scale. We agreed the price and scope of works and put in place a contract for domestic works.
When works started on site things went awry, labour was sporadic, quality average and in fairness there were a small number of changes. In the end it took a lot of effort and withholding of significant money before we got the job finished and it really took the shine off what should have been a straightforward build.
A few things I’d suggest looking at in addition to the architect involvement would be;
1. Use a contract. For 20 quid you can buy a contract form here http://www.jctltd.co.uk/category/home-owner-contracts
I would strongly recommend using someone who has experience in this to help you complete it. On first glance it is a fill in the blanks format but to get the best out of it it’s better that you understand what it is you’re filling in.There’s bound to be someone you or a friend knows who can offer some face to face advice.
2. Decide how much you want the builder to do. Everything up to final decorations or basic structure, electrics and plumbing? Referring back to my experience if we had chosen our own plasterer, electrician and plumber we’d have been rid of a shonky builder much sooner and managing these trades isn’t the hardest thing in the world to do.
3. Be clear on your scope of works. Make sure you compare like for like as people say above. Specification of major parts such as boilers, cabinets, high value fittings etc
4. Don’t change things. Get your scope clear from the start and don’t ask the builder to change things after he has started unless you absolutely must.
There will be all sorts of other things to consider but it’s important to consider them. Reading my comments back it all seems daunting but from personal experience I know how a seemingly straightforward sub £30,000 job can end up being far from straightforward.