Full height side of house extension

We have a 3 bed semi in OX5, we are thinking we would like to extend out the side of our house demolishing the existing garage and converting the loft all the way across into this too. We don't have a clue what this sort of thing would cost and don't want to waste builders time coming out to us if it's never going to be feasible for us to afford. Can anyone give us a rough guide as to what this might cost? Thank you