Would love to do this when we build our little conservatory at the back of our house, but we live in a Victorian end of terrace with solid walls and we have already done what we reasonably can to minimise our heat efficiency, such as replacement double glazed sashes, energy efficient boiler etc. Can't afford any more.
 

It is easy enough to have an originsl building constructed with cavity walls insulated to current standards but how do you insulate to current standards a building that consists of nine inch brickwork generally reffered to as solid wall construction (no cavity).A layer of insulating material would be required to be applied to the external walls also existing windows and doors would require changing to conform to thermal efficiency criteria.
 

cost of extension - £30k
cost of improving rest of house - 50k
total cost of new extension - £80k

errr. no thanks.