Hi there,
For a standard house build E90/E100 for sustainable build expect to pay up to E142 per sq foot which would be more or less turnkey. A lot depends on the specification. You have to decide whether it be block, timberframe, poroton, ICF - then down to windows, heating systmes etc. My husband and I hope to build sometime next year, we got alot of different quotes mainly for sustainable build, some were outlandish and some were normal. One of the normal quotes we got was for E127 per sq foot and that was for poroton block, aluclad windows double glazed with argon, concrete floors, concrete stairs, external entrance wall, geothermal and solar panels and so on. Hopefully this helps.