[andrewaylett]

The L comes at the back of the property -- most visible on the first floor of the floor plan linked. Someone's roofed over the side passage on the ground floor to make a bigger kitchen and knocked the two main rooms together to make a larger living room.

That design of terrace is exceptionally common in the UK, where each pair of houses is mirrored, with a kitchen out the back and a side passage letting light into the middle room on each level.