Ours is 3.8 metres by 9 metres. We did it 5 years ago so costs may be different now,also you don't have to have planning permission if your only extending by so much.
I think we paid £45k for the actual build plus extra for the company to project manage.
We got numerous quotes from friends recommendations. If you have a house nearby that's had an extension , knock their door and ask if they've been happy with the build... We were scrupulous about this as we'd already lost £90.000 from an off plan apartment we bought in Spain (grrrrrrrrrrrr!).
If your going to project manage you really need to be on the ball with things, ours went relatively smooth because the boss was using his regular guys that pretty much turned up when they should have. It's a nightmare if say the electrician doesn't come because then that puts hold to the plasterer and so on.
You'll need to get an architect to draw your plans... Can't remember how much that was but its expensive! Also costs rise if you've drains to be redirected... We had 3!!
That's the price of the build, everything else was on top and its sooo easy to get carried away with buying! Our kitchen budget was £6k.... We actually spent £12.5k, the en suite we we doing on the cheap but putting in a decent showere enclosure... Budgeted £1.500.... Just the sink alone was £1000!!!
It sounds cheesy but its really changed our lives... The way we socialise ect! I love entertaining but was always stuck in the kitchen on my own, now we have a huge open plan space with an island that everyone sits around nibbling on food!
The number one thing though is get a few quotes from people who can recommend on their builds... We had loads of people who had 'mates' that were builders... There's no way we were risking our money again!
Let me know if there's anything else!