how much extra per m2 should an extra storey be?

[td2304]

I'm weighing up whether or not to add another storey to my project, just wondering if anyone can give ballpark figures for this. Considering foundations and drainage are already taken care of how much should the extra storey be in theory?

 

[noseall]

I'm weighing up whether or not to add another storey to my project, just wondering if anyone can give ballpark figures for this. Considering foundations and drainage are already taken care of how much should the extra storey be in theory?

Depends how complex the roof is and say the span of the floor joists and vagaries of knock-through etc.
I'd work on about £1500p/sqm for building work.