In the guidance notes https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/830643/190910_Tech_Guide_for_publishing.pdf the last point (any statement) says that, "If any part of the building, container or enclosure is within 2 metres of the boundary of the
curtilage of the house, then the height limit for the total development is restricted to 2.5
metres if it is to be permitted development." This means that regardless of slope, if you build within 2 meters of the boundary there is a restrictive height limitation otherwise you could end up with this scenario.
The limit on height of structures is height from ground level where it is constructed, not from the ground level of surrounding properties. If ground has been levellled to form a terrace there is nothing that I am aware of which changes that rule.
I'm still working on the shed. I have taken a chunk out the hillside to inset it, so it isn't a monster on the hill. And it has been more work than I ever imagined, basically to placate neighbours. Then my other neighbour said, it would be easier for them to just throw a shed at the top of the slope near to the house, which will pretty much undo any savings I bothered with. Imagine a very steep rise with a shed on the top close to the house.
Regarding decking, the immediate neighbour had enough of being overlooked and moved out they had no privacy in their garden at all with decking and loft extension. Then another neighbour erected some huge deck probably to counter the effect of the other one.
I'm glad to say one of the decks toppled in a storm.
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“Raised” - In relation to a platform means a platform with a height greater than 0.3 metres.
https://planning.merton.gov.uk/MVM.DMS/Planning Application/1000111000/1000111875/20P3043_Decision Notice.pdf
https://planning.merton.gov.uk/MVM.DMS/Planning Application/1000111000/1000111875/20P3043 Site Photo.pdf
Regarding decking, the immediate neighbour had enough of being overlooked and moved out they had no privacy in their garden at all with decking and loft extension. Then another neighbour erected some huge deck probably to counter the effect of the other one.
I'm glad to say one of the decks toppled in a storm.