We encourage lessees in our large block to leave a spare key at Reception in case we have to enter their flats in an emergency. This is especially important in the case of leaks in the 80+ year old plumbing, or because many of the flats are unoccupied some of the time as they are used as pied a terres. Our question is what might constitute an emergency.
In short we feel the logical answer is Fire or Flood, and in this context flood means MAINS PRESSURE leak and to support this, these are generally the 2 instances that the fire brigade would kick a door down to gain access. For a non mains pressure leak, only if the owner was living overseas and the flat unoccupied and one could demonstrate sufficient attempt to contact the owner then such situation could become an emergency.