Garage / Outbuilding roofline - how much wriggle room?

I'm looking to re-roof or stand alone garage,dry line it and put it to better use (play room for the kids)
i've been to the council my proposal of a lean to roof at a height of 3m incurrs planning app.
if i can get the eaves height down to 2.5m then i don't need palling app.
so i'm now thinking flat roof. my question is, just how much wriggle room is there on the 2.5m to the eaves when it's a flat roof?
can i build up to 2.5m of brickwork then sit the roof structure on top and let the barge board be at the top end of 2.5m? i don't want o push it if they'll make me take it down of apply retrospectively.
to be fair it's currently gable'd in concrete asbestos sheeting and max height is about 2.8m
so it's actually goign to be lower than it is currently.

thanks

simon