An occupied building presumably. The temporary structure has to be considered as part of the premises emergency evacuation arrangements. Those on the scaffold need to know what to do in the event of alarm just the same as everyone in the building. Some local consideration may be needed as to the means of 'alarm' - sometimes the internal sounders aren't discernable, and you may have to get a message through to a guy wearing hearing protection and weilding a Kangol hammer. Similar issues apply with roof work.