Eighteen metres or seven storeys: the higher risk test

A building is higher risk if it is at least 18 metres tall or has at least seven storeys, and contains at least two residential units. Both limbs of the height test are alternatives, and the residential unit count is what brings a building into the regime at all. Cross that line and you are not adding a document to the planning submission. You are changing who approves the building, when work can start, who can be appointed to design and build it, and what has to be handed over before anyone moves in.

For a developer appraising a site, the point is that the threshold is a cost and programme cliff rather than a gradient. The difference between six storeys and seven is not one floor of value.

Where the test comes from and what it attaches to

The definition sits in the Building Safety Act 2022 framework and the regulations made under it, which also describe how the height and storey count are worked out and how independent sections of a building are treated. The measurement conventions matter enough to be resolved on a section drawing before the design is fixed, because the datum on a sloping site can decide the answer.

Hospitals and care homes meeting the height condition are treated as higher risk during design and construction, without carrying the occupation duties that apply to residential buildings. That distinction catches out mixed development with a care element.

The regulations also deal with what counts as one building. Two blocks joined by a podium may be one building or two independent sections depending on how they are structurally and functionally separated, and the answer changes the regime for both. This is a question to settle with the Building Safety Regulator early rather than to assume from the architectural drawings.

Mixed use buildings and the residential element

The residential test is a floor, not a proportion. Two residential units in a predominantly commercial building satisfy it, so a nine storey office with two apartments at the top is caught, while a nine storey office with none is not.

That asymmetry drives real design decisions. On a scheme where the residential element is marginal, converting the top floor flats to serviced office or to a commercial use can take the whole building out of the regime, and converting a plant level into two flats can bring it in. Neither decision should be made without pricing the consequence across the whole construction programme.

Build-to-rent, co-living and student accommodation all need the unit definition checked against the regulations rather than against commercial terminology, because what the market calls a bedspace and what the regulations call a residential unit are not the same thing.

Consequences across the three gateways

Gateway Two is the one that reshapes a programme. Building control for higher risk buildings sits with the Regulator rather than with a local authority or an approved inspector, and the approval has to be in place before work starts rather than being resolved alongside construction. Changes during construction then run through a change control process, with some changes notifiable and some requiring approval before they are built.

Alongside the gateways sits the dutyholder regime: a client, principal designer and principal contractor with defined duties, competence requirements for those appointed, and an obligation to create and maintain a golden thread of accurate, accessible building information. In occupation, a principal accountable person has to register the building, prepare a safety case report, obtain a building assessment certificate and operate a resident engagement strategy.

None of that is a planning matter. All of it is priced into the scheme at the point the planning application is made, as the discussion of fire and regulator consultation as building height rises sets out.

The cliff edges below 18 metres

Treating 18 metres as the only threshold is a mistake. Several requirements change at lower heights, and a developer who designs to clear one line can land on another.

Sprinkler provision in blocks of flats changes at 11 metres under Approved Document B. Restrictions on combustible materials in external walls apply to relevant buildings above 18 metres. Approved Document B has also been amended in relation to second staircases in new residential buildings above 18 metres, and the plan efficiency consequence of a second core is substantial on a small floorplate. Check the current editions and their transitional provisions on gov.uk before committing to a core strategy.

So the honest appraisal position is that there are several thresholds, each with its own cost step, and the scheme should be tested at each rather than optimised against one.

Design decisions that sit just under the line

Illustrative example, not a real scheme: a six storey block of 46 flats at 17.4 metres, on a site where a seventh storey would add eight flats. Those eight flats carry the gateway regime for the whole building, a second staircase, the Regulator as building control authority, a hard stop before work starts, registration and a safety case in occupation, and a different insurance and warranty position. Whether the eight flats pay for all of that is an arithmetic question, and on small floorplates the answer is often no.

The reverse decision is also real. And it is the one developers under-use. A scheme already comfortably over the threshold has no reason to shave storeys, and pushing height where policy allows it spreads the fixed compliance cost across more units.

What a developer should not do is design at 17.9 metres and hope. A measurement that tight will be rechecked by the Regulator, by the warranty provider and by a purchaser's technical adviser, and a screed or substructure variation during construction is enough to move it. Build in margin or accept the regime.

Evidencing the position

1. Section drawing with existing and proposed levels to Ordnance Datum, the datum point marked, and the measured height stated in metres.
2. Storey count annotated, with any excluded level labelled and the reason given.
3. Residential unit schedule with the count stated, including anything arguably a residential unit.
4. A statement of whether the development is one building or independent sections, with the reasoning.
5. A note confirming which Approved Document editions the design has been prepared against.
6. Early written engagement with the Building Safety Regulator where any of the above is marginal.

Keep that pack together. It answers the planning officer, the Regulator, the warranty provider and eventually the purchaser's solicitor, and assembling it twice costs more than assembling it once.

How the threshold shows up elsewhere in the application

Crossing it tends to pull the application towards committee rather than delegated determination, partly because of scale and partly because of local sensitivity to tall buildings, a pattern covered in scale and the shift from delegated to committee decision.

It also shows up in viability. The compliance cost of the gateway regime, the second staircase and the external wall specification all land on the appraisal, and they interact with the affordable housing position in ways that change as the scheme grows, as the analysis of affordable housing tenure mix as scheme size changes describes.

In planning terms the submission needs a fire statement, and the design team will separately need a fire safety strategy for building control. A competent fire engineering professional is legally required for this work, and an AI-assisted draft is a starting point for that professional rather than a substitute.

This is general guidance, not legal advice. Check the position with your local planning authority 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.