If the area is open plan, and assuming there are sufficient exits why need a detection system in the first place? Is it for property protection? Is it not reasonably likely that, being open plan with high ceiling, any fire would be identified fairly quickly to allow quick evacuation? Is the risk such that a fire would develop so quickly that it could adversly effect an evacuation?
Multi occupancy with vastly differing hours of operation - if a fire develops 'out of hours' in one bay there will be people from another firm several bays down who are still at work will not know about a fire until it's developed significantly.
There is lots of noise & smoke and dust from the work processes in several bas, plus the use of RPE etc that would all reduce the efficacy of human detection.
I agree that normally it would need only a cat M, or possibly an L5 for any specific risks, but the place is under a Section 64 for not just the alarm, but separation and the owner wants to try for AFD in lieu of walls which the cost to put in is not economically viable for the site. A protected tunnel already retrofitted makes the travel distances OK, put in under the terms of a temporary warrant from the BCO a few years ago, but the FRS still doesn't like it and wants it to fully comply with Building Regs and the compartment/separation requirements in the benchmark guide.
Personally an M or L5 is all that is viable, the different hours thing is not itself enough to justify the extremely difficult task of putting in an L1 (that would go off all the time & be a bugger to maintain) and that if the walls don't go in then as an alternative smoke control or supression - although with the addition of a building wide alarm travel distances and times with existing and recently added routes would suffice to get everyone out the FRS don't want to do it - it seems they don't like it being a sacrificial structure even if everyone gets out.
