Agree with vinnyeh above re costs. m2 cost would not include external works either. Depends where you are though obviously. I would always say use a good Architect (I would say that!). Tell them what you want, how you want to use your house, your budget and timescales. Give them some images of the sort of thing you like (in keeping/modern etc). Get a couple of Architects to give you fee proposals and base your decision on this and how you feel about them. Costs include the build, VAT, contingency, statutory fees, professional fees including Architect and structural engineer – it adds up.