In earlier times people did not live with parts of their family in separate houses. All of the family lived together in one house. This is how it had been since the Stone Age for most people in Britain. It was only in the Roman times that a few wealthy people had houses with more than one room.
So, by the Middle Ages, when this house was built, people were still building single-roomed houses. By this time they were building with a frame of timbers and filling in the spaces with daub and wattle (the timber has been painted black and the daub white only recently). It is easier to build a long house for many people than one which is square, for the roof span can be smaller.
When this house was built there was one room and one floor. It has been modified since. An extra floor has been put in for bedrooms, windows with glass added, and a chimney put in at the far end.