In reply to Gwilym H:
We have a similar issue with a track that is ours but that has shared access rights for other houses.
You’ve still got 2.5 meters height above ground to work with. I wanted over 3 meters height under the tall side of the single pitched roof so I dug down half a meter, and waterproofed the buried part. Membrane under the concrete base and up the sides of the burried blockwork walls and a bonus bitumen coat or 3 on the inside of the blockwork. I also took care to take all roof water far from the building to be safe.
If 3 meters isn’t enough height, you’ll just have to overhang the wall and get strong! I wasn’t going to dig further because we hit clay. Digging clay is not rewarding.
There’s no harm in asking the council in writing to clarify their understanding of your curtilage with regards permitted development, unless hypothetically you were going to build a non compliant outbuilding and keep it under the radar until you can trigger the “4 year rule” and get a certificate of lawfulness for it.
It’s amazing when I walk around our village how many blatant planning violations I see (admittedly a lot relate to tree works in a conservation area and section 211 notification) - there seems to be some ill defined line below which people can’t be bothered to complain unless you piss them off.
For the avoidance of doubt, my next comment is absolutely not related to the OP who acted in good faith and is being eminently reasonable in my view and also not aimed at you, it’s just a General Rant. Taking your chances with planning violations and then complaining bitterly through the press is one of my pet peeves. There’s some punter’s mansion with egregious violations that’s been in the news recently because the owners are claiming they can’t afford to do the works required by the enforcement action and would be made homeless. It made a BBC News too 10 storey. If it wasn’t so petty and mean spirited I’d write them a letter telling them how little I care. Planning law is one of the pillars of modern society - without it things would fall apart hellishly quickly.