Messy,
Extract from Glyn Evans' paper published April 2006 that may help.
"The idea of an alteration notice first came about during the Fire Safety Orders passage through Parliament, when it was scrutinised by a Select Committee. Concern was expressed that the Order might not be seen by the public as offering the same level of safety as the fire certification procedure under the FPA ... This concern was compounded by the fact that, under the new regime there will be no requirement for a responsible person to advise an enforcing authority of any changes they intend to make to their premises which might increase the the risk for persons working or resorting there.
It was decided that what is needed is an enforcement tool whereby an enforcing authority can ensure that a responsible person whose premisses they believe constitute an above average risk to relevant persons, or might constitute such a risk if they are changed, can be required to notify the enforcing authority of proposed changes before they occur."
Simplistically - they were worried about losing the old Section 8(2) of the FPA, so Alterations Notices were conjured up as the replacement.