My son owns an upstairs flat - with a garden behind the one owned by the downstairs flat. He rents it out.
The only way to access the garden is by walking down the road, then up the unmade/private road to the garages. Still, his latest tenants use the garden a lot for BBQs and socialising. They've planted it up and had an outside tap installed behind their kitchen.
From there (flat roof level) a hose runs down and along the fence to their second tap in their garden. They got permission from the downstairs chap and my son - although it's not a costly legal arrangement, just a casual, temporary one. They say they'll empty the hose for the winter to stop it splitting if it gets frozen.