Jonny was here http://rockettube.fun/ roket tube
  In really confined spaces, it may be easier to go up a level, as it can be less noticeable. For a tiny London garden, we made a den on top of a garden shed. We sunk the ground level in the shed so it was about a foot below ground. That way we managed to get a flat-roofed den on the top without having to apply to the planning authorities. We disguised the flat roof by putting castellation around the top and set the structure within a pergola.