Thanks newhome and Onoff!
Im looking to make open timber frame panels out of 6x2, Lintels with 8x2 stood on edge, insulation inside along with services then batten off the panels exterior for render boards to go up. Thats the rough idea... still undecided with heating options, initially thought standard radiators but having floated the idea of underfloor heating (thanks Onoff) she seems quite keen, guess its price dependant as are the doors to the garden bifold vs double sliding doors?!