Can someone explain why OFSTED require a fire risk assessment on a premises before they will allow it to be occupied, but a fire risk assessor won't come until it's occupied?

The regulator are correct in asking that the premises is not occupied without and FRA, albeit this would need to be a 'pre-occupation FRA' and a caveat attached to that effect. This would ensure that gaps in policy and procedure are identified. Also, it would seek confirmation is provided that building regulations had been complied with and all safety systems have been commissioned and signed off. A full FRA would then be carried out after occupation.

Because they don't understand the law. There is no such thing in law as a pre-occupancy FRA, this matter is controlled by Building Regulations separate fire safety requirements. However it is true that Building Regulations only cover a small part of the fire safety regime required for an occupied building and if the Responsible Person does not have the knowledge or competency as to the management of a building (training, maintenance, policy & procedures etc) and the aspects not covered by Building Regulations (signage, extinguishers, etc) then they need guidance and steering. From this the lucrative pre-occupancy FRA is born. You can't do a proper FRA on an empty building & it becomes worthless the moment the first people, furniture & equipment are moved in, so you need to quickly do it again. General consultancy & a pre-occupation checklist achieves the same thing (& would cost less) but instead an FRA is being requested as a quasi 'certificate of occupancy' which it isn't intended to be.

Although a Fire Risk Assessment (FRA) can be undertaken for an unoccupied building, the assessment may no longer remain valid once the premises are occupied, as occupancy can materially alter the building’s risk profile and operational conditions.

The Regulation 38 fire safety information should be reviewed prior to occupation, and a pre-occupation inspection of the premises should be carried out to identify any obvious omissions, defects, or non-compliances. Any identified issues should be referred back to the contractor for rectification before the building is brought into use.

A full FRA should then be undertaken after a short period of occupation, once the use of the premises, occupant behaviours, and management arrangements can be properly observed and assessed.