we had a much bigger extension than the one you describe, so can't be sure my advice is spot on. Are you sure you don't want a two storey one? It would be cost effective.
I think you are probably looking somewhere around 15 thousand - you will obviously need new kitchen units etc. Are the outbuildings definitely structurally sound, with the right foundations? If not, it will obviously cost you more.
You may also need a damp proofing course for the outbuildings. This could be £1k. You will need architects drawings, planning permission and building control approval. All of this costs money, obviously
Have you checked the local plannning regulations to make sure your plans would be approved?
Flooring is very expensive, as is plumbing work, electrics and gas fittings. Make sure the builder qoutes for absolutely everything, right down to the sort of lights you want in the ceiling, the sort of windows you want, radiators, light switches. Think this through very carefully, or it will get out of hand.
Our extension was a garage, bedroom, ensuite, kitchen, conservatory and downstairs loo. The builder also replaced the existing windows to UPVC, reskimmed the plaster in three rooms, laid a floor in the hall, sanded and varnished floorboards and laid two patios in the garden. We spent a total of £90k, which increased the value of the house by just about the same amount.
The work took eight months to complete, the cost got out of hand because we hadn't had the finer details on the original quote. We also had to store most of our furniture in a unit, which cost £100 per month
Good luck!