My understanding about selecting the appropriate guidance, is if it is a new build, you should use the building regulations guidance ADB, BS 9999 or BS 7974 and decide which one to use, you shouldn?t cherry pick. If it is an existing building you should use the appropriate DCLG guidance or other appropriate guidance, considering any previous guidance that may have been used.
Is this more or less correct or am I well out of touch with modern day thinking.
My understanding is that both the Regulatory Reform (Fire Safety) Order 2005 and Building Regulations 2010 are based on function requirements, Article 50 guidance and the suite of Approved Documents (respectively) are provided as guidance in support of these pieces of legislation.
Prescriptive application of these guidance documents SHOULD help achieve compliance with the legislation, but not in all cases. This "guidance" merely sets benchmark standards, although other guidance is also available, and there are many "determinations" which have shown that it is not necessary to adhere prescriptively to the guidance to achieve compliance.
Most of the guides do suggest that you should use them exclusively and not "cherry pick" between guides although, in reality, sometimes there may be no alternative.
The bottom line is that you have to convince the appropriate enforcing authority that you have complied with the functional requirements of the appropriate legislation; also, AI's, Building Control and Fire & Rescue Authorities don't always agree on what constitutes compliance.