Naming of parts?

What would you call this? In a terraced house configuration (typically c1850 to about 1910), a rear projection, generally slightly subordinate and at right angles to the main dwelling. Usually has a kitchen at gf level and a bedroom above. It gives the house an L-shaped plan. The projection is shared by a pair of houses and has a division/compartment/party wall running down its centre. Its purpose is obviously to make the best possible use of narrow urban sites etc while retaining as much daylighting as possible. These are very common in London and other UK cities (and there are probably US examples too). It's sometimes refered to as a 'back addition' but I'm wondering if there's another less vague technical term for it??