It generally comes down to exit routes (and prevention of fire spread) and distance from a safe point of exit.
Any residental property with a floor over 4.5m above ground level is classed differently to one not more than 4.5m above ground level.
In general unless the floor over 4.5m from gorund level is compltly separated from the others with a fire comparment then the stairwell will need to be fire rated and the doors off the stiarwell would need to be fire doors.
Technical Guidance Document B is a good read and is freely downloaded from the dept of environment website. Any Architect or fire cert consultant will assist you with the finer details.
We got our attic converted a couple of years ago - making it a three storey house. From my knowledge of what was said at time, doors around stairwells need be fire rated, except for bathrooms as they are not habitable rooms. For a "regular" 2 storey house, this means doors off the hallway at ground floor level plus all bedrooms on upper floors.