The way it was described to me by a planner is that none of the extension should be visible to the sides of the dwelling. Therefore you cant simply go off the back and then shoot out the side. If thats what you want to do it will require planning.
I got onto my local council about the ruling & I now have it in writting that their policy is that once the 40 sqm is behind "the rear line of the house" then it's fine. I had submitted a sketch of the proposed extension (with a bit jutting out to the side) & they said it would be exempt.