1.5 A house may be regarded as large if any of its storeys exceed 200SqM.

1.6 A large house of more than 3 storeys (including basement storeys) should be fitted with an L2 system as described in BS 5839: Part 1: 1988,
except that the provisions in clause 16.5 regarding duration of the standby supply need not be followed.
However with unsupervised systems, the standby supply should be capable of automatically maintaining the system in normal operation (though with audible and visible indication of failure of the mains) for 72 hours, at the end of which sufficient capacity remains to supply the maximum alarm load for at least 15 minutes.

1.7 A large house of no more than 3 storeys
(including basement storeys) may be fitted with an automatic fire detection and alarm system of Grade B type LD3 as described in BS 5839: Part 6 instead of an L2 system.

System classifications