Higher risk buildings at 18 metres or seven storeys

A higher-risk building is one that is at least 18 metres in height or has at least seven storeys, and contains at least two residential units. Both limbs of the height test are alternatives, so a seven storey building of 17 metres qualifies, and an 18 metre building of five tall storeys qualifies too. Get either measurement wrong and the scheme enters or escapes the Building Safety Act 2022 gateway regime on a false footing.

The definition that matters for planning is not quite the same one, which is where most of the confusion starts. At Gateway One the trigger is a relevant building under Article 9A of the Development Management Procedure Order 2015, and that definition reaches buildings the higher-risk definition does not.

Where the threshold comes from

The Building Safety Act 2022 created the gateway regime, and the detail of what counts as a higher-risk building for the design and construction stage sits in the Higher-Risk Buildings (Descriptions and Supplementary Provisions) Regulations 2023. The height condition is met where the building is at least 18 metres tall or has at least seven storeys. The use condition requires at least two residential units, with care homes and hospitals meeting the height condition also caught at the design and construction stage.

For the occupation phase under Part 4 of the Act, the definition narrows: at least 18 metres or seven storeys, and at least two residential units, with care homes and hospitals outside it.

So one building can be higher-risk for gateways two and three and treated differently once occupied. Say which definition you mean whenever you use the phrase.

How height is measured

Height is measured from ground level to the floor surface of the top storey. Not to the ridge, not to the parapet, not to the top of the plant enclosure.

Two refinements matter. Ground level is taken from the lowest part of the surface of the ground next to the building, which on a sloping site produces a greater height than a designer working from a finished floor level datum expects. And a storey consisting exclusively of plant or machinery rooms at roof level is disregarded for the measurement.

The practical consequence is that a scheme designed at 17.6 metres to the top floor on the high side of a site can be over 18 metres measured from the low side. Check it from the lowest adjacent ground before you rely on being below the threshold, and put the measured dimension and its datum on the section drawing.

The seven storey alternative

Storeys are counted excluding any storey below ground level, and excluding a roof-top storey used exclusively for plant. A mezzanine counts as a storey where its floor area is at least half the area of the largest storey that is not below ground level.

A six storey building over a large mezzanine is a seven storey building. That is the calculation most often got wrong, and it is worth doing before the design is fixed rather than after a Gateway Two submission is rejected.

Consequences at each gateway

Gateway One is the planning stage. An application for a relevant building must include a fire statement on the prescribed form, and the Health and Safety Executive is a statutory consultee. The fire statement is a planning document about the fire safety implications of the design, not a fire engineering strategy for construction, and the content checklist for a Gateway One fire statement sets out what the form actually asks for.

Gateway Two comes before construction, as a building control approval application to the Building Safety Regulator, which has replaced the local authority building control route for these buildings. Gateway Three is the completion certificate, needed before occupation.

The programme point is that Gateway Two is a hard stop. There is no equivalent of starting on site under a building notice, and a design that has not resolved its fire strategy will not pass. Work done at planning stage that anticipates Gateway Two therefore has real value, which is why the fire strategy for a mixed use building at planning stage matters even though planning is not the place it is finally approved.

The regulator as statutory consultee

At Gateway One the Health and Safety Executive comments on the fire safety implications of the proposal as submitted. Its comments are a material consideration and, in practice, are given substantial weight because no case officer wants to grant permission for a building the regulator has said raises fire safety concerns.

What the regulator looks at includes access and water supply for firefighting, the principles of the means of escape, the external wall construction approach, and whether the site layout allows appliances to reach the building. None of that is detailed design. All of it constrains the layout, which is why it belongs in the design process before the application rather than in a response to a consultation.

Planning stage implications for the design

Crossing 18 metres changes the building, not just the paperwork. Expect the second staircase requirement for new residential buildings above the height threshold to shape the core layout, and check the current position in Approved Document B and its transitional arrangements before assuming which version applies to your scheme. The practical effect on plan efficiency is set out in the note on second staircase provision in planning layouts.

Beyond the stair, the consequences run through firefighting shafts, lift provision, corridor arrangements, external wall material choices, and the space needed at ground level for appliance access and hard standing. Those are all things a planning officer sees as massing, elevation and site layout, so they have to be settled at planning stage or the permission will not be buildable.

A fire statement is a report type where a professional with the necessary fire engineering competence is legally required under the Building Safety Act 2022 framework and Article 9A of the Development Management Procedure Order 2015, and an AI-assisted draft is a starting point for that professional rather than a substitute.

Where the threshold is misapplied

The recurring errors:

1. Treating the Gateway One relevant building definition and the higher-risk building definition as the same. Gateway One also reaches buildings containing educational accommodation, which the higher-risk definition does not.
2. Measuring height to the ridge or parapet instead of to the top storey floor surface.
3. Measuring from the highest adjacent ground rather than the lowest.
4. Counting basement storeys, or failing to count a large mezzanine.
5. Forgetting the two residential unit condition, and applying the regime to a single very tall dwelling.
6. Assuming a roof-top storey containing a plant room and a lift overrun is disregarded when it also contains habitable or ancillary accommodation.

Where fire safety matters are described in the design narrative rather than the statutory form, the approach in fire safety in a design and access statement keeps the two documents consistent.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. For specialist technical reports, qualified professional input may be required.

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