Midland, I think you just made his point for him. Simply not informing building control does not mean it stays in its original purpose group.
However, I still say you cannot call it a private domestic dwelling until you compartment it off from the other purpose group as would be required under building regs. Until you do that it remains part of the hotel, thus NOT private.
This might take someone looking through the Housing Act in more detail than I am willing to bother with......
If I found this happening on inspection, I would look at it from the point of these rooms being no different to having a member of the public in them. If the hotel owner wanted to say that I couldn't ask for the detectors in those rooms maintained etc then I would point him towards building regs and the housing act, and until he has changed the purpose group I would deal with it under the RRO. I would let anything he appealed go to determination unless some information was forthcoming that proved it was private and domestic. (Or a ship) Once he has changed the purpose group and got it through building regs then we should have no problem anyway.