Why building height changes everything: the 11m and 18m thresholds explained
Of all the numbers that come out of a fire risk assessment, building height is the one that quietly does the most work. Cross a threshold, and a set of separate duties switches on — not because anything about the building's use has changed, but because of a single measurement.
11 metres
At 11m, several things change at once under Approved Document B, as amended:
- Sprinklers become a trigger consideration — the threshold was reduced from 30m to 11m in the 2020 amendment.
- Wayfinding signage recommendations apply.
- Combustible materials are banned in or on external walls, following the 2022 amendment.
- A secure information box becomes a recommendation.
18 metres
At 18m (broadly, seven storeys), the picture changes again:
- Evacuation alert systems become a recommendation under the 2022 Approved Document B amendment.
- The building falls within scope of the Higher-Risk Building regime under the Building Safety Act 2022, bringing Principal Accountable Person duties, secure information box requirements, and enhanced record-keeping obligations.
Why one number does so much
Height correlates with things that matter directly to life safety in a fire — how long it takes to evacuate, how far smoke has to travel, how quickly a fire service can intervene at upper floors. Rather than writing bespoke rules for every building, the regulatory framework uses height as a proxy and attaches duties to it. That's efficient for legislation, but it means a small error in establishing a building's height — or overlooking which measurement method applies — can mean a landlord is unknowingly short of a legal duty.
What this means for a fire risk assessment
Establishing the building height accurately, and tracing through which thresholds it sits above, is one of the first things a proper assessment does. It's not a formality — it determines which parts of the assessment even apply.