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Probably not. An accessible raised platform like that generally requires planning permission as you might overlook neighbours. Add in requirements for balustrades etc and it is likely to be over height for permitted development anyway. You would have to really want one for it to be worth the bother.
Someone round the corner from me had to reduce a kids climbing frame after the neighbours complained because it was too tall.
Are garden rooms/sheds with stairs up the side and walkable roofs a thing?
I feel like I saw this on a gardening show once but can't remember.