Good builders need to see your drawings before they price the job, so get an architect to make them up, get them passed by your council if necessary or get your building regulations (phone the council and ask them what you need, with measurements) then get three local builders (recommended if possible) and go for the middle price or the one you have a good feeling about. Look around your area where work is being carried out, when the builders have gone home knock on the door of the home owner and see if they are pleased with the work, take it from there. Good luck ! Don't do anything without permission from the local council.