Increasing the period of fire resistance of the compartment walls between flats beyond that specified in the Approved Document is unlikely to have any significant impact on the safety of occupants of the building and would have no discernable benefit to persons in the flat where the fire has started.
It is estimated that the provision of a BS 9251 sprinkler system within a dwelling will reduce fire-related casualties by around 70%. Whilst it would be desirable to install such systems in all dwellings it was decided that it would only be reasonable to impose this on larger buildings.
This was following analysis of the costs and benefits in the research report: The effectiveness of sprinklers in residential premises and consideration of the increased hazards for fire-fighters and other persons associated with fires in tall buildings, as discussed in the Regulatory Impact Assessment: Changes to Part B (Fire Safety) of the Building Regulations 2000 (as amended) and Approved Document B.
The 30m trigger height is considered to be a logical provision which aligns with the provisions for sprinkler protection for other building uses.
There may be alternative fire suppression systems that could be used where it can be demonstrated that a similar level of performance as would be provided by a BS 9251 sprinkler system can be achieved.
Where water mist systems are proposed the guidance contained in the BRE publication An Independent Guide on Water Mist Systems for Residential Buildings may assist Building Control Bodies in assessing such systems.