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I am - I dug 30cm+ below ground level to keep the height under 2.5m
That being said, I have looked through the local planing portal to see what they do / do not allow - There are a lot of outbuildings on the neighbouring and adjacent properties, many in excess of what is allowed under PD.
There have been investigations, but no enforcement - the planning offices have been fairly generous in their judgements.
It's also exempt from most building regs (other than part P) as it's *ahem* below 15 sqm
For the most part, it does comply with building regs though. And if it didn't, an extra 100mm of PIR would take 1.5sqm off the floor space.
Are you doing this under permitted development? I'm assuming not. Sorry if mentioned already.