Had a long look at it this year. Any change in the exteria look of a roof on street facing side is most likely going to be a no-no (velux windows are allowed but not dormers). Unless you are detacted I don't think they can raise the roof.
I think you need something like 2m minimum height at highest point (its really not very high, a tall person would struggle) after taking but I think its measured to bottom of rafters, not top so I have just about OK by 5cm. Instead of lifting the roof, if you have enough space they can drop the ceiling of upstairs.
Like most modern houses, I had a truss construction, so RSJ's would be required to support roof, then truss can be removed. There are loads of other items to consider like water tank if you have one, etc
I was thinking about master bedroom with en-suite + an office, full length dormier at rear of house, extending existing stairway, velux windows to front, firedoor, etc. I got a quote inc stairs, plumbing, electrics, etc, to plaster. From national company specialising in truss roof (£45k) and local builder who had his own architect (£37k). But knew local regs and what could and could not be done. Didn't cost anything so worth getting one in to at least have a look.
