Ok so the wind is already blocked by the house from the NW if it were to face that way. It would still have to be an exceptionally large extension to block the wind from the SW? And as mentioned by Bozza, I've never seen it working either whenever I've gone past it.
I'm reall ynot trying to...
I assume the house is the one I've boxed in this picture?
Not being funny but it would have to be a bloody huge extension to reduce windflow wouldn't it? As in the size of another house?