The BS5839 pt6 is meant primarily for new build residential properties but some fire officers do accept this standard for smaller HMO (this is fairly rare) This Standard basically states that smoke alarms or heat alarms are required in escapes routes i.e. corrider and stairways but also they are required in main habital rooms such as living rooms and dinning rooms. As well as this all the smoke alarms need to be linked either wired or wirelessly and all detectors need to have mains supply with a battery backup.
There is a lot of information on the British standards on this website
http://www.sdfirealarms.co.uk/shop/DomesticFireAlarms-a-22.html It contains information on all BS5839 standards from the very basic to the top end domestic smoke alarm.
In Some cases a fire officer may require some HMO to comply to BS5849 pt 1 which is a commercial system. The general installation standard is L2 which bascially requires smoke alarm sounders to be above 65db and all rooms and fire escapes to have a smoke alarm or heat alarm to cover them
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