A ginnel is a narrow passageway between or through buildings, an "alleyway". Possibly early 17th century: perhaps from French chenel ‘channel’.
In traditional industrial towns and cities across Britain, hundreds of thousands of these homes were built in the 19th century and before. Often called two up-two downs , or back to backs. They were generally what are called terraced houses, or long rows of any length of houses all attached, with alleys at various points for access to the back yards for rubbish collection etc. I have lived in one many years ago.
All were built with washing and toilet facilities in separate outbuildings at the back sometimes one per house but in poorer dwellings communal facilities for several homes. Sometimes a washhouse was also included where a fire could be lit under a "copper" to boil water to clean clothes. A two up-two down usually consisted of as you would expect four rooms, two front and two back with fireplaces in all rooms. Back to backs were split further into only two rooms, what you might call one up-one downs. Families of more than ten used to occupy these homes, with sometimes all sleeping in one room.
Most of these houses would have originally been rented to families but latterly many were privately purchased and modernised. Today it is not uncommon for them to fetch half a million pounds or more in some desirable areas. The most common modernisation was an extension out the back of two stories with kitchen below and bathroom upstairs. Depending on the size of the house, access to the bathroom upstairs may have been by a corridor or through the rear bedroom.
There arose as a result a legal anomaly termed a "flying freehold". Freehold is a term whereby the property owner owns both the building and the land it is on. Leaseholders only own the property and pay "ground rent" to the land owner. But there is a problem when your home is built over a ginnel. A ginnel is an access for maybe four or six houses so that land is communal, not a problem when a developer owns the whole row. But what happens when all become privately owned? You have to invent the "flying freehold" a sort of "air rights" of the area above the ginnel. Also it was common to equalise the size of the houses by having the house, say, on the left have a room that extended over the ginnel at the front, while the house on the right had a room at the back that extended over the ginnel. You are by now either fascinated or really bored.