They may be questioned, it depends on the insulation of the
rest of the place, AFAIK - couldn't your local pen-pusher
be consulted? Slim chance of a sensible answer, I know...
Extension is to an old house with solid walls, some single-glazing etc.
I am putting in my plans to building control soon.
I will probably have to make a calculated risk at this point about how
much it's worth paying / if they get refused etc.
Simon.