Does Building Height Really Matter in a FRAEW?
July 2026
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“When a building is close to an important height threshold, a small difference in measured height may lead to a disproportionate effect on the assessment process — even though the physical difference in height may be relatively small.”
Building height is an important consideration in an external wall fire risk assessment. The 11m and 18m thresholds can affect the fire-safety provisions and the way certain aspects of an external wall are considered.
However, building height is only one factor in a FRAEW. The overall assessment should consider the building as a whole, including the external wall construction, fire spread mechanisms, compartmentation, means of escape, building configuration and other relevant factors.
How should building height be measured?
For the purposes of the relevant guidance, building height is measured from the lowest adjacent ground level to the finished floor level of the highest occupied storey, rather than to the roof.
For example, for a typical five-storey residential building, the relevant measurement will generally be to the floor level of the fourth floor rather than the roof.
The problem with estimating height
A common industry rule of thumb is to assume approximately 3 metres per storey.
In many circumstances, this may provide a reasonable initial indication. However, we should take the actual height measurement on site and avoid to estimate a buidling height in the FRAEW assessment when a building is close to an important regulatory threshold.
For example, a typical five-storey residential building might be estimated at around 12 metres using a 3m-per-storey assumption. However, if the actual floor-to-floor dimensions are closer to 2.6–2.7 metres, the measured height to the relevant floor level could potentially be below the 11m threshold - a building height subject to a lesser control on fire safety provision under the relevant guidance.
Why do the 11m and 18m thresholds matter?
The distinction can be significant because the fire-safety provisions applicable to residential buildings above and below the 11 metres and 18 metres thresholds are not identical.
A difference of around one metre due to height estimation, by itself, does not automatically mean that the physical fire risk presented by a building has materially changed.
However, it can change the regulatory context and the factors that need to be considered when the actual height is close to a threshold. An assessment based on an assumed height above or below a height threshold could potentially result in certain risk factors being considered differently from a building in the FRAEW assessment.
What about combustible materials on buildings below 11m?
This is an area where careful assessment is particularly important.
Under the current Approved Document B for residential buildings 11m or less in height, where the external wall is 1,000mm or more from the relevant boundary, there is no minimum reaction-to-fire classification specified for the external facing and underlaying insulation. In other words, it is permissible for a combustible cladding or insulation to be present in a building under 11m, provided that other fire safety requirements are fullfilled.
This does not mean that combustible materials are automatically acceptable in every circumstance. Other provisions and the overall fire performance of hte external wall still need to be considered.
Rather, it illustrates why the actual building height and the full building context matter when assessing the fire risk being posed by the external wall.
“How was the building height established — was it actually measured, or simply estimated?”
"How if it was actually below or above the regulatory threshold, potentially affecting the FRAEW assessment and EWS1 rating?"
Why this matters to an EWS1 rating?
An EWS1 is not itself a fire risk assessment. However, the findings and conclusions of the underlying external wall assessment can impact the EWS1 rating and potentially the other aspects, for example, a property transaction or a mortgage application.
Where an existing FRAEW has relied on an estimated building height, which is close to an important regulatory threshold, it may therefore be reasonable to ask: “How was the building height established — was it actually measured, or simply estimated?” "How if it was actually below or above the threshold, impacting the FRAEW and EWS1 rating?"
A Proportionate Approach
Building height should not be considered in isolation, and neither should the presence of combustible materials automatically lead to a conclusion that extensive remediation is required.
The appropriate outcome depends on the overall risk profile of the building and the interaction of the relevant factors.
This is why a properly evidenced, risk-based assessment is important.
If you have an existing FRAEW or EWS1 assessment and are unsure how the building height was established, an independent review of the available information may be worthwhile.
Editorial note:
This article is based on our professional observations and is intended as general information. It should not be taken as a substitute for a project-specific FRAEW, EWS1 assessment or professional advice.
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