Guidance needed.

Purpose built flats (2004) 3 floors, 2 flats per floor. lobbied at all levels.

Single common stair with lift serving all floors. Risk assessor wants fire alarm/detection in common areas. Is this correct and where would guidance be found.

Depends what the fire action is. 

No, not required unless as a temporary measure due to combustible cladding being involved or some other critical defect in structure.

Only requires a smoke control system (unless using manually openable windows/vents as part of the small development exemption) which would have detection to open the vents (no alarm sounders, etc).

Not required in current Building Regulations (or those at the time of build): https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/937931/ADB_Vol1_Dwellings_2019_edition_inc_2020_amendments.pdf

Not required in existing flats guidance (currently being revised so no on line copy available) either - in the interim the above document is referenced.