What Makes a Building Higher-Risk? The Definition, Thresholds and What Applies to You

Before a project team can understand what the Building Safety Act 2022 regime requires of them, they must first answer a threshold question: is this building a higher-risk building? The answer determines whether the BSR has jurisdiction, whether the Gateway system applies, what duty holder appointments must be made, and what obligations will follow the building through design, construction and occupation.

The question sounds straightforward. In practice, it generates more uncertainty — and more risk of incorrect determinations — than almost any other aspect of the regime. This article sets out the statutory definition, how it is applied, what the exemptions are, and what the consequences of an incorrect determination look like.

The Statutory Definition

The definition has two limbs, both of which must be satisfied. A building that is 20 metres tall but contains only commercial space is not a higher-risk building under the Building Safety Act. A residential building of 6 storeys, regardless of its height in metres, is not a higher-risk building. Both conditions — height/storey threshold AND residential use — must be met.

The detailed provisions are set out in The Higher-Risk Buildings (Descriptions and Supplementary Provisions) Regulations 2023, which amplify the primary definition and deal with the edge cases that inevitably arise in practice.

How Height Is Measured

For the purposes of the 18-metre threshold, height is measured from ground level to the floor of the top storey of the building. Ground level is the level of the ground immediately adjacent to the building, or — where the ground is not level — the average level of the ground adjoining the outside walls of the building.

This measurement methodology has practical implications for complex sites. On a sloping site, the height of the building may differ significantly depending on where you measure. The regulations provide that where the ground level varies, the measurement is taken from the lowest point of ground adjoining the outside walls — which means a building on a significant slope may be in scope when measured from its lowest side even though it appears to be under 18 metres on the upper side.

The storey count — the alternative trigger of 7 or more storeys — is determined by counting every storey from the ground floor to the top floor of the building. Basement levels are generally not counted for storey purposes, but a storey that is at or above ground level is counted regardless of its use. A building with a ground floor, six upper floors and a plant room at the top is 8 storeys if the plant room constitutes a storey of the building.

The Residential Unit Requirement

The second limb of the definition — at least 2 residential units — is generally straightforward for conventional residential developments. A residential unit is any dwelling within the building: a flat, maisonette, studio apartment or similar. Purpose-built student accommodation and care homes require closer analysis.

Use Class C3 residential

Buildings in Use Class C3 (dwellinghouses) form the primary category of HRBs. A residential apartment building of sufficient height containing two or more flats is unambiguously within scope. Mixed-use buildings — commercial at ground floor, residential above — are within scope if the residential component satisfies the unit threshold and the building as a whole satisfies the height threshold.

Mixed-use buildings

A mixed-use building is assessed as a whole. The height of the building — including its commercial floors — is measured for the threshold test, even if the residential units are only in the upper portion. A 20-storey building with retail at ground floor and a hotel from floors 1 to 15 and flats on floors 16 to 20 is a higher-risk building (subject to the residential unit count), because the building as a whole satisfies both the height threshold and the residential use threshold.

Exemptions: What Is Not a Higher-Risk Building?

The Higher-Risk Buildings (Descriptions and Supplementary Provisions) Regulations 2023 set out a number of building types that are excluded from the definition, even where they might otherwise satisfy the height and use criteria:

• Hotels and temporary accommodation. Buildings providing hotel-type accommodation are excluded from the definition, even if they are of sufficient height. The rationale is that the residential unit definition requires a degree of permanency of occupation that hotels do not provide.
• Military premises. Buildings used solely for military purposes are exempt.
• Secure residential institutions. Prisons and similar secure facilities are excluded.
• Purpose-built student accommodation (PBSA) — partial caveat. PBSA is a complex area. The default position is that student accommodation buildings with individual self-contained units that are permanently occupied do fall within the residential unit definition. Projects should obtain specific legal and regulatory advice on PBSA schemes, particularly where there is mixed tenure or shared facilities.
• Hospitals and similar health premises. Buildings used primarily for the provision of health care that do not contain residential units as their primary function are generally outside scope, though care must be taken with mixed-use hospital developments that include permanent residential accommodation for key workers or patients.

What "In Scope" Means for Your Project

Once a building is determined to be a higher-risk building during the design and construction phase, the following obligations apply:

• The BSR (not local authority building control or an approved inspector) is the sole Building Control Authority. All building control functions are exercised by the BSR.
• All three Gateways apply: Gateway 1 at planning, Gateway 2 before construction and Gateway 3 before occupation.
• The duty holder regime — Principal Designer (Building Regulations), Principal Contractor and their respective obligations — applies in full.
• The Golden Thread must be maintained from the design phase.
• The Mandatory Occurrence Reporting regime applies throughout construction.
• On occupation, the Accountable Person regime applies, requiring registration with the BSR, maintenance of the safety case and regular safety case review.

The Design Phase vs the Occupation Phase

It is important to understand that the BSA 2022 creates two related but distinct regulatory regimes for HRBs. The design and construction phase regime — governed by the Gateway process — is administered by the BSR acting as building control authority. The occupation phase regime — governing occupied buildings — is administered by the BSR acting as regulator of the Accountable Person.

The definition of "higher-risk building" applies to both phases, but the threshold test is applied at the point in time relevant to the phase. During design and construction, the question is whether the building as designed will satisfy the definition. During occupation, the question is whether the building as occupied satisfies it — which may differ if, for example, the building is altered after occupation.

Why Getting the Determination Right at the Start Matters

An incorrect HRB determination at the start of a project has cascading consequences. A project that proceeds through planning, design development and into procurement without correctly identifying itself as an HRB may find itself at Gateway 2 stage without the required duty holder appointments, without a compliant Gateway 1 fire safety statement and without the design documentation that the BSR requires. Remedying these failures retrospectively is expensive, time-consuming and — in some cases — impossible without design revision.

Where there is genuine uncertainty about whether a building satisfies the definition — borderline height, complex mixed-use, unusual site topography — the correct approach is to obtain a formal determination at the earliest possible stage, not to proceed on the assumption that the building is out of scope.

Applicable Legislation and Guidance

Determining whether a building is a higher-risk building is the first — and most fundamental — question the project team must answer. The entire BSR regime, with all its obligations and complexity, flows from that determination. Getting it right, early, with appropriate professional advice, is the foundation of a compliant project.