my Engineer hat is on now.....build it in block work, did mine a few years ago by direct labour, 300mm block work wall ( two 4" inch walls) with a 100mm cavity with you can fill with insulation beads after, stick a prefabricated truss ( dormer truss & you can use the storage space upstairs) on it & cover it with concrete tiles, 9m x 6m internally & napp render finish outside with two 2.7m metal roller doors & you'll get it up for 15 grand