Hi all, I've been reading a lot but this is my first post. I'm buying a 4 bed terraced with a hipped roof. It used to be semi-detached but effectively became terraced when it got extended to the side. I'd like to get another bedroom+ensuite instead of the current loft room.

The existing loft build is poorly done in terms of design and exterior build, but internally the staircase is in the right place and it's a nice space with a velux window. Unfortunately, the roof looks like Tetris as a result. The staircase protrudes out of the roof and does not join nicely with the ridge. The roof tiles are also quite old and need changing soon.
I've exchanged and awaiting completion, but I've made this drawing based on photos, floorplans, neighbours' planning permission, Google Maps, Bing bird's view, etc.

Taking into account permitted development volumes, this is the maximum I think I can get out of it. This is also what the neighbour next door has done. The window to the side is in the staircase.

I'll get some quotes from local conversion builders, but I just had a couple of question for you guys:
Is the fact of having an existing loft room make the hip to gable + dormer conversion cheaper or faster? I'm thinking since the staircase is done and trusses removed, but I don't know how significant.
Would it be preferred to modify the current roof structure or would it be best to just strip out the roof structure completely and start from scratch with the desired shape? Also bearing in mind I'd take the chance to put new tiles throughout.
Any obvious issues I've missed?
Thanks very much in advance.
PS: I'm by no means DIYing this. Probably some of the internal stuff after the structure is in place.

The existing loft build is poorly done in terms of design and exterior build, but internally the staircase is in the right place and it's a nice space with a velux window. Unfortunately, the roof looks like Tetris as a result. The staircase protrudes out of the roof and does not join nicely with the ridge. The roof tiles are also quite old and need changing soon.
I've exchanged and awaiting completion, but I've made this drawing based on photos, floorplans, neighbours' planning permission, Google Maps, Bing bird's view, etc.

Taking into account permitted development volumes, this is the maximum I think I can get out of it. This is also what the neighbour next door has done. The window to the side is in the staircase.

I'll get some quotes from local conversion builders, but I just had a couple of question for you guys:
Thanks very much in advance.
PS: I'm by no means DIYing this. Probably some of the internal stuff after the structure is in place.
