Some houses have alleyways running behind them. When the houses are joined together to form a terrace this forms the best way to get access to the back of the house. It seems the way to tag these in OSM is as highway=service, service=alley. In Hull and the surrounding areas these are known as tenfoots - they are usually about ten feet wide. Some streets have much narrower access to the back of the house that is really only as wide as a footpath and these are not tenfoots.
We went out to look at an area of Hull that we know have tenfoots, partly to see how easy it is to get access and partly to see what the extra detail might look like. Access was very easy. The tenfoots were all paved and only a couple of places had potholes. We didn't see many people, a few dog walkers, a few cyclists and a few children playing. I think I'll take tenfoots much more seriously, they add extra detail with little problem and they are not well mapped elsewhere. My only comment is that they might need to be rendered a little less prominently on some Mapnik zoom levels. Take a look and see what you think here.