I forget where this was, but it's been reported only this week that one couple have discovered their house in in Tier Two, but the far part of their back garden is in Tier Three.
Why?
Whoever scribbled their areas' Tiers map set the arbitrary boundary along a water-course that happens to be in a culvert under the garden.
Probably the same would-be cartographer who caused a friend problems with his insurer refusing flood insurance by the house being within some remotely-set, meaningless distance from a river. His home is close to the river, horizontally – but he said it would be a handy-sized flood to reach some 200 feet up the valley side. Still, it does seem law that where possible, homes must be built on flood-plains, not up the bank as in centuries past when local knowledge and common-sense were common!
(I assume the householders with the multi-tier garden simply say "Rhubarb" and ignore it. After all, they are not going off their property – even if the rhubarb patch is beyond the culvert.)