This was a real puzzle to me, only resolved finally when we extended the back room and in digging the foundations found a sewer pipe below this window. It had clearly at some time been a toilet - perhaps the first one in the house when such things first came in from the cold? It must have been very difficult to use, since the door almost touched the outside wall when opened, making it almost impossible to reach the area behind it, where the toilet must have been.
Note also how low the ceiling was at this stage: the reason for this was that the only access to this back section of roof space was by means of a lead-covered trap-door set into the
north-facing roof over this area, itself safely accessed from the valley between the two roof sections. Having the floor/ceiling lowered made it (slightly) easier to crawl in through the trap-door.
This "floor" was constructed out of solid timbers of enormous dimensions, all around 3" thick, one was about 15" width and rather difficult to remove.
This set of images © John Woodhouse, 2005