I have a flat roof extension (now refurbished) about the size of a single garage which has a ceiling height of only 2250mm & the BI never batted an eyelid when he inspected the renovation works. It’s a dinning room & looks fine with light oak floor, white walls & a pair of very large French doors in one side but I had to be careful about the choice of ceiling light! I think you could push it another 50mm but any more would really start to close it all in so I would say that would be the practical minimum unless you really have no choice. I would add that I inherited the extension & had to make the best of it or rip it down & start again!