If, as a converted house, its classed as an HMO, then the BS5839 rules are rather steep on that now.
There should be a fire panel on the communal hallway, witha zone for each flat, plus one for the communal area. All circulation spaces, living areas and kitchens should have detectors. PLUS, the detectors within each flat (zone) should be hushable from within the zone. Sounders AND strobes within each zone, and the comunal areas.
If its NOT classed as HMO; then the guy why comes to assess it will be making his determination of the system which, in his opinion, should be installed. That entails quite a bit of work; as well as liason with local fire service & council; and his liability if he gets it wrong.
Hope that helps mate.
KME