The 18 metre and seven storey higher-risk threshold
Height and storeys are alternative limbs, not cumulative. Measurement runs to the top storey floor surface from the lowest adjacent ground level, and mezzanines can count.
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. That test comes from the Building Safety Act 2022 and the Higher-Risk Buildings (Descriptions and Supplementary Provisions) Regulations 2023, and crossing it changes who your building control body is, when you can start on site, and what you have to hand over before anyone moves in.
Height and storeys are alternatives, not cumulative. A seven-storey building of 17 metres is caught. So is a five-storey building of 19 metres.
How height is measured
From ground level to the top of the floor surface of the top storey. Not to the parapet, not to the roof, not to the top of a plant screen.
Two supplementary rules do most of the work in practice. A storey consisting exclusively of roof-top plant or machinery, or of areas serving only plant or machinery, is disregarded. And where the ground is not level around the building, the measurement is taken from the lowest part of the surface of the ground adjacent to it, which is the rule that catches sloping sites.
Get this settled on a dimensioned section early. On schemes hovering near the threshold, arguments about the datum can cost more than the design change would have.
Basements and lower ground floors deserve a separate look. A storey that is below ground on one elevation and at grade on another is exactly the situation the lowest-adjacent-ground rule was written for, and getting it wrong in either direction is expensive.
How storeys are counted
Storeys below ground level are excluded. A roof-top storey used exclusively for plant or machinery is excluded. Everything else counts.
Mezzanines and galleries are the awkward case. A gallery floor counts as a storey where its internal floor area reaches a defined proportion of the internal floor area of the largest storey above ground, so a substantial mezzanine in a double-height ground floor can take a building over the line. Check the regulations rather than eyeballing it.
Two design consequences follow. A generous double-height entrance with a mezzanine gallery may be more expensive than it looks. And a podium level that reads architecturally as landscape may still be a storey.
What crosses the line commercially
The threshold is not a paperwork change. Five things shift.
The Building Safety Regulator becomes the building control authority for the works. You cannot appoint a registered building control approver instead, and the choice that exists for ordinary projects disappears.
Building work cannot start until Gateway 2 approval is granted. This is a full plans style approval with a statutory determination period, and the period differs between new buildings and work to existing ones. Treat it as a hard programme constraint with float, not as a formality that runs alongside mobilisation.
Change control applies during construction. Major changes need approval before they are made, and notifiable changes have to be recorded. A design and build contract that assumes free-flowing design development after contract signature is mispriced.
The golden thread of information has to be created and maintained, in a structured and accessible form, and handed on.
Occupation cannot begin until a completion certificate is issued at Gateway 3, and the building must then be registered before residents move in. An accountable person and a principal accountable person have duties for the occupied building, including preparing a safety case report and applying for a building assessment certificate.
The commercial point is simple. Two approval gates with statutory periods, sitting either side of construction, change the cash flow model for a residential scheme materially. Programme them as milestones, not as administration.
The definitions that do not match
There are three overlapping definitions in circulation and confusing them causes real problems.
| Regime | Trigger | Residential test |
|---|---|---|
| Planning Gateway One | 18 m or 7 storeys | Two or more dwellings, or educational accommodation |
| Design and construction, Gateways Two and Three | 18 m or 7 storeys | Two or more residential units, plus care homes and hospitals |
| Occupation regime | 18 m or 7 storeys | Two or more residential units, with exclusions |
Educational accommodation sits in the planning definition, so a student scheme needs a Fire Statement at planning even where the later regime treats it differently. Care homes and hospitals are within scope for design and construction but are handled differently for occupation. Hotels, secure residential institutions and military accommodation are outside the occupation regime.
The result is that a building can be caught by one regime and not another, and a project that assumes one answer covers all three will discover the difference at the worst moment.
Designing around the threshold
Deliberately designing to sit under 18 metres is a legitimate commercial decision, and plenty of schemes do it. Three points about doing it well.
Do it early. Losing a storey after the massing is fixed costs far more than designing to six storeys from the start.
Do it with margin. A top storey floor level at 17.9 metres is a scheme that will breach the threshold the first time a level changes on site or a screed depth is revised. Half a metre of headroom is worth having.
Do not confuse the planning threshold with the construction one. A building designed under 18 metres may still have seven storeys, and the storey limb catches it just as effectively.
And be clear about what the threshold is and is not. It is a regulatory trigger, not a statement about whether a building is safe. Buildings below it still need a proper fire strategy, and a Fire Safety Strategy to BS 9991 or BS 9999 is a fire engineering document whichever side of the line the building falls. These are report types where a professional with fire engineering competence is required, and an AI-assisted draft is a starting point for that person rather than a substitute for them.
Related requirements that track the same figure
Several requirements have converged on 18 metres, which is convenient but not identical in every case. A second staircase requirement for new residential buildings above 18 metres has been introduced through the Approved Document B route with transitional arrangements, and the transitional dates matter to anyone with a consented scheme in the pipeline. Check the current position on gov.uk rather than relying on a summary, because the dates have moved.
Structural verification also tends to arrive with the threshold. Where an existing building is being extended upwards towards it, a Structural Survey establishing what the existing frame and foundations can carry belongs at feasibility stage rather than after consent.
Illustrative example: a six-storey block on a site that falls 1.2 metres from front to back. Measured from the highest ground level the top storey floor sits at 17.6 metres. Measured from the lowest, it sits at 18.8 metres. The lower figure is the one that governs the measurement, so the building is caught and the whole consenting route changes.
This is general guidance, not legal advice; check the position with your local planning authority, the Building Safety Regulator or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
How is the 18 metre height measured?
From ground level to the top of the floor surface of the top storey, disregarding any storey consisting exclusively of roof-top plant or machinery. Where the ground is not level, the measurement is taken from the lowest part of the surface of the ground adjacent to the building, which frequently catches schemes on sloping sites.
Do mezzanines count as storeys?
They can. A gallery or mezzanine counts where its internal floor area reaches a defined proportion of the internal floor area of the largest above-ground storey. A substantial mezzanine in a double-height ground floor can therefore take a building over the seven storey limb, so check the regulations rather than assuming it is excluded.
What changes once a building is higher-risk?
The Building Safety Regulator becomes the building control authority, work cannot start until Gateway 2 approval is granted, change control applies during construction, a golden thread of information must be maintained, and occupation cannot begin until a Gateway 3 completion certificate is issued and the building is registered.
Is the Gateway One planning definition the same?
No. Planning Gateway One uses a relevant building test that covers buildings containing two or more dwellings or educational accommodation. The design and construction regime covers two or more residential units plus care homes and hospitals, and the occupation regime carries its own exclusions. A building can fall inside one and outside another.
Is it sensible to design deliberately under 18 metres?
It is a legitimate commercial decision, provided it is taken early and with margin. A top storey floor at 17.9 metres will breach the threshold as soon as a level or screed depth changes on site. Remember that the storey limb applies independently, so a low building with seven storeys is still caught.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.