There are a number of factors to consider- I attach a section from page 66 of the DCLG guidance document which may be downloaded free here:
http://www.communities.gov.uk/documents/fire/pdf/1508652.2 Automatic fire detection
Automatic fire detection may be needed for a
number of reasons. These can include:
• if you have areas where people are isolated
or remote and could become trapped by
a fire because they are unaware of its
development, such as students working
in unsupervised areas or where the premises
are only partially occupied, outside of
core hours;
• if you have areas where a fire can develop
unobserved (e.g. storerooms);
• as a compensating feature, e.g. for
inadequate structural fire protection, in
dead ends or where there are extended
travel distances;
• where smoke control and ventilation
systems are controlled by the automatic
fire-detection system; and
• to reduce the effects of arson.