Eighteen metres and seven storeys, measured properly

Two separate tests decide whether a residential building sits inside the higher-risk regime, and either one is enough: a height of 18 metres or more, or seven or more storeys. A six-storey block with tall floor-to-floor heights can be caught on height alone. A seven-storey building of 17 metres is caught on storeys alone. The measurement conventions are set out in regulations, not left to judgement, and an architect who measures to the wrong datum discovers the error when the Building Safety Regulator is consulted at Planning Gateway One.

Height is measured to the floor surface of the top storey, not to the ridge, the parapet or the top of the lift overrun. That single point corrects most of the mistakes.

Why two tests exist and either can apply

The Higher-Risk Buildings (Descriptions and Supplementary Provisions) Regulations 2023 define the higher-risk building for the purposes of the Building Safety Act 2022: a building of at least 18 metres in height, or with at least seven storeys, that contains at least two residential units. In planning, the parallel trigger is Article 9A of the Town and Country Planning (Development Management Procedure) (England) Order 2015, which requires a fire statement with applications for relevant buildings meeting the same height or storey condition.

Having two tests is deliberate. Height alone would let a building with generous floor-to-floor dimensions escape at six storeys, and storey count alone would let a shallow-floored building escape at 17.9 metres. Officers check both, and so should your submission.

Note that the thresholds in the London Plan referral arrangements work on their own basis and are not the same numbers, so do not assume one measurement answers both questions. The referral position is covered separately in our note on the Greater London Authority referable thresholds.

Where height is measured from and to

The convention has two ends and both are commonly got wrong.

The bottom datum is ground level, defined as the level of the surface of the ground immediately adjacent to the building. Where that surface is not level, the relevant point is the lowest part of it. So on a site that falls across the footprint, you measure from the low side, which makes the building taller for the purposes of the test than a measurement from the entrance would suggest.

The top is the floor surface of the top storey of the building. Not the ceiling of that storey. Not the roof deck above it. Not the parapet, the balustrade, the lift motor room, the plant screen or the mast. If the top occupied storey has a finished floor level 17.6 metres above the lowest adjacent ground, the building is below 18 metres on the height test even if the parapet stands at 20.4 metres.

One carve-out matters: a storey that is a roof-top machinery or plant area, or that consists exclusively of machinery or plant rooms, is excluded when identifying the top storey. A building with occupied flats to level six and a plant enclosure above measures to the level six floor.

Record the ground level you used and where you took it. A section drawing with the datum annotated settles the question in one page.

What counts as a storey and what does not

A storey counts where any part of it is above ground level. A wholly below-ground basement does not count, and nor does a car park at a level entirely below the surrounding ground.

That gives the common sequence on a sloping urban site: a lower ground floor that is below ground at the back and above ground at the front counts, because part of it is above ground level. Designers who label it basement on the drawings and then omit it from the count are not describing the building the regulations describe.

A roof-top machinery or plant area is not counted as a storey, on the same logic that excludes it from the top-storey datum. A gallery or mezzanine is counted where its internal floor area is at least half the internal floor area of the largest storey in the building, and is not counted where it falls below that. So a small void-edge mezzanine in a duplex does not add a storey, while a near-full-floor mezzanine does.

Count the storeys on the proposed section drawing, label each one, and state the total. It takes a line of text and removes an argument.

Roof plant, parapets and mezzanines

Three features produce most of the disagreements.

Plant at roof level is excluded from both the height datum and the storey count where the area is a machinery or plant area or consists exclusively of plant rooms. Where that enclosure also contains a residents' room, a store let to flats, or a lift lobby serving a roof terrace, it is no longer exclusively plant, and the exclusion is lost. Design it as plant or design it as accommodation, and label it accordingly.

Parapets, upstands, balustrades and screens sit above the top floor surface and are irrelevant to the height test. They matter to townscape and daylight assessments, which is why drawings carry the parapet dimension prominently and the floor level dimension in small type. Reverse that for a submission dealing with the threshold.

Mezzanines turn on the 50 per cent floor area comparison, and the comparison is against the largest storey, not against the storey the mezzanine sits within. Calculate it, show the two areas, and state the result.

Split level and sloping sites

A building on a slope has no single ground level, and the regulations resolve that by taking the lowest adjacent surface. The practical consequence is that the downhill elevation governs.

Where a scheme is designed as two linked blocks with a shared core, the question of whether it is one building or two becomes live, because the test applies per building. There is no neat formula. Structural separation, separate entrances, independent cores and whether the blocks are physically attached all feed into it, and the honest position is that borderline cases need a reasoned note rather than an assertion. Put the reasoning in the submission and invite the authority to agree it early, because discovering at Gateway One that the Regulator treats a linked pair as one building is an expensive late surprise.

Stepped roofs raise the same point in reverse. A block that steps down a hill may have its tallest section measured from its own lowest adjacent ground, so the section through the tall end is the one that decides it.

Consequences of crossing the threshold

Crossing either threshold changes the planning submission and the whole downstream process.

At application stage, a fire statement is required with the application for a relevant building, and the Health and Safety Executive acting as the Building Safety Regulator is consulted at Planning Gateway One. The statement must be prepared by someone with the fire-engineering competence to do it, and that is not a document to draft in-house. The procedure, the consultee's role and the timescales are set out in our note on eighteen metres, seven storeys and Planning Gateway One.

Beyond planning, the building falls within the Building Safety Regulator's gateway regime for design and construction approval and for occupation, and the dutyholder obligations follow. Design decisions taken to stay below the threshold, such as reducing a floor-to-floor height or omitting a storey, are legitimate and common, but they need to be taken on a correct measurement rather than an optimistic one.

A fire statement and a fire safety strategy both require professional fire-engineering competence under the Building Safety Act 2022 framework, and an AI-assisted draft is a starting point for that professional rather than a substitute for them.

Recording the measurement in the submission

Put the measurement where an officer will find it, in the design and access statement and on the drawings, and make it checkable.

A short, dimensioned section through the tallest part of the building, taken at the lowest adjacent ground level, with the top storey floor surface annotated and the resulting height stated. A storey schedule listing every storey, whether it is above ground level in part, and whether any is excluded as plant. The mezzanine area calculation where one exists. A single sentence stating the conclusion against both tests.

Where the building is marginal, say so and show the margin. An officer who can see that the top floor level is 17.82 metres above the lowest adjacent ground, and how that was derived, will accept the conclusion. An officer given a number with no derivation will ask for the derivation, and that request arrives after validation. Scoping the rest of the pack at the same time is quicker from the 66 report types by category than from a validation list alone.

Enter your postcode on PlanScope to see which reports a scheme at this scale is likely to need, then send the drafts for professional review.

This is general guidance, not legal advice. Check the current regulations and 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. For specialist technical reports, qualified professional input may be required.