My client is converting an old stables block to provide event space on ground floor and two short term rental flats on the first floor. There is a single stairway that will have 60 minute fire protection from the flats, however the stair enclosure and final exit is shared by those using the event space. Although 2 purpose groups, during an event, the flats will either be unused or rented to people attending the event. BC are still not happy to approve as there are two purpose groups. Would an escape window in each of the flats be acceptable provided the opening was a minimum of 450mm, height from floor was between 800 and 1100mm, and external height from sill was no more than 4.4 metres above ground? 

It sounds like the flats are ancillary to the use of the event space and if the stair is lobbied does have a situation that is detailed as acceptable if using Approved Document B. Building control are getting more strict on designs that don't fully reflect Approved Document B and increasingly will only consider these if justified in a full fire strategy by a Fire Engineer