When you add an extension, modern building regulations are going
to require cavity walls and double glazing and the heat losses
in this part are going to be a lot less (could be only a quarter)
of the equivalent size of existing house.

Indeed, I've heard of cases where adding a two storey extension has actually
resulted in reduced calculated heat losses from the house. This is because
an uninsulated solid wall was appended by the heavily insulated 2 storey
extension, which had considerably lower heat loss than the original wall,
despite have many times the surface area.

Christian.