I might have taken the risk if I was simply installing wood and glass walls (not too much bother or expense to take them down again if someone complained) but I wanted to enclose the space by brick walls, rendered on the outside, tiled on the inside and install plumbing, drains, handbasin, toilet and shower. Much too risky!
My rear terrace juts out from the main building line by about 20cms but the Planning guy didn't even ask about that. His refusal was immediate and as far as I could tell, non-negotiable. Balconies and Terraces are deemed as 'empty spaces' and must remain so.