I’m 20sqm 5x4
I might try keep it at 2.5 but I’m 6ft4 so need to see what head room I’ve got.
So your 76" tall (1.93M) only leaves you 57cm to play with, looks like you do indeed need that 3M.
Planning application required sadly unless your happy with a very thin roof to keep it under 2.5M, could work out.
Regarding it being over 15 SQM and within 1M of boundary, just email your council BCO and ask which fire proof cladding
would be acceptable, then use exactly what they state and keep the email (printed) for evidence, just to cover yourself.
When you buy the cladding also print the spec of it and keep it with the BCO's email, you can then relax knowing you've done what was asked.
Ron