It's a bit like a loft conversion, only there were already habitable rooms. There is a 45 degree roof pitch, with rafters supported by purlins roughly half way up on each side of the roof. There will be low stud walls blocking off the eaves.
To the point - when the BCO was up there, he was umming and ahhing about my intention to put the new ceiling *above* the horizontal ceiling "joists" (bits of 3 x 2 about 3 feet down from the apex, which tie the rafters together to a certain extent), creating a beamed ceiling. Fire protection was the issue, as they are part of the roof structure - but he decided that it wasn't an issue, because by the time the fire had got that high, everyone would have escaped or be dead.
I'm concerned because I also intend to leave the purlins exposed to the front of the house (those on the rear are being largely replaced by dormers). It's a major part of my design, and I don't want the same issue being raised over that. I'm now puzzled, because I've since re- read Part B, which specifically excludes the roof from its definition of "structure" that needs to be fire-protected (Sect. 4.4, from memory). So I'm not sure why he raised the issue over the ceiling *joists* in the first place. I also had a loft conversion done at a previous (terraced) house, where we left purlins exposed, with no issue being raised (different BCO, different council).
Obviously I'll discover what he thinks when the time comes, but at the moment I'm puzzled - is there any reason why the roof structure would fall under Part B, because it has rooms in it? I can't see that mentioned in the regs, which seem specifically to exclude it from these requirements.