
Being as my OH is a builder may I suggest that you get some idea of what you want before you approach a builder. They cannot price a project without plans. You need planning permission and building regulations before you start any building. Go to a builder whose work you can see, your homework to ask about and look at as much as you can that they have done, go and enquire at houses where you can see which builder is working there and ask the occupiers if they are happy!
Don't pay any money up front, if a builder said he needs materials then you order them and pay for them and have them delivered to your site. A good builder should be able to make a start without you paying out up front. You will probably be expected to make staged payments at the end of each month or so, don't pay up front! you may find your builder dissapears into the night with your money.
Try to make sure they have served an apprentiship and know what they are doing, and watch the big firms, they often send out apprentises to do the work, I know they have to learn but you may end up paying for there mistakes, they should be supervised at all times. Check your builder has good insurance cover, third party, indemnity and all risks, it may cost more to get it right but in the long run it is better for you.
Good Luck, oh and by the way the good ones are worth waiting for and the best price is not always the cheepest but should be the most detailed estimate so you know whats included, less of the dreaded "EXTRAS" that way