Storey height definitions and how thresholds are measured

Two buildings of identical appearance can fall on opposite sides of the 18 metre line depending on where the measurer stood. That is not a loophole. It is the consequence of a measurement convention that fixes a datum, defines the top of the measured zone, and then tells you which levels to ignore, and a developer who has not resolved all three on a drawing is carrying an unpriced risk into the design.

The number itself is familiar enough. The regime a building enters, the consultees it attracts and the construction duties that attach to it all turn on a figure that is rarely stated anywhere in the submission.

Why the measurement decides the regime

Several different sets of rules use height and storey count as their trigger, and they do not all measure the same way or ask for the same thing.

Mixing the conventions produces the worst outcome: a design that was measured against one rule and assessed against another. Settle on a single measured figure, state it, and then test it against each regime separately.

Where height is measured from and to

The convention that matters most measures upwards from ground level and stops at the top of the floor surface of the top storey. It does not stop at the parapet, the roof finish or the top of a lift overrun, which is why a building with a generous plant enclosure can be visually much taller than its measured height.

Ground level is the harder half. The regulations defining a higher risk building set out how to deal with ground that is not level, and Approved Document B works from the lowest level of the ground adjacent to the building. Read the definition that applies to the regime you are testing rather than assuming they agree, because on a site falling two metres across the footprint the choice of datum can be the whole answer.

In practice you need a topographic survey tied to Ordnance Datum, a datum point marked on the section, and the measured height written on the drawing in metres to one decimal place. A figure derived from a scaled PDF is not evidence. And the survey should predate the design, not chase it.

Counting storeys and the exclusions

The storey count is an alternative test, not a check on the height test. A seven storey building below 18 metres is caught just as surely as a five storey building above it.

The conventions in the higher risk building regulations exclude a storey that is below ground level from the count, and exclude a roof-top level that is exclusively plant or machinery from both the count and the height measurement. A mezzanine is treated as a storey where its internal floor area reaches a stated proportion of the largest storey in the building, so a small gallery is not a storey and a near full-floor mezzanine is. Check the current wording of the regulations, since these definitions have been amended and the proportion is the kind of detail that gets misremembered.

Three cases cause most of the arguments.

Roof gardens and terraces at the top of a building are not plant areas, so an accessible roof level needs thinking about rather than assuming it falls away.

Basements and lower ground floors turn on what counts as below ground level, which on a sloping site is not obvious. A lower ground floor fully exposed on the downhill elevation is a different proposition from a true basement, and the geotechnical and flood consequences of that level deserve their own evidence, as the discussion of basement light wells, escape and flood risk shows.

Plant levels only fall out where they are exclusively plant. A level containing a plant room and a small store is not exclusively plant, and the whole regime can turn on whether a cleaner's cupboard was drawn into the corner of the roof level.

Sloping sites and the datum problem

On a site with a cross-fall, the measurement from the lowest adjacent ground will almost always be the one that puts you over, and the measurement from an average or uphill level will be the one your architect used. Resolve that early.

Illustrative example, not a real scheme: a seven storey block on ground falling 1.9 metres from the rear to the front measures 17.6 metres from the uphill side and 19.4 metres from the lowest adjacent ground. On the storey count it is caught in either case, so the dispute about datum is academic for the higher risk test. Reduce the design to six storeys and the datum becomes decisive, and the same drawing produces two different regimes depending on which convention is applied. That is why the storey count is worth checking first. It is the test that is hardest to argue about.

Where the design is being trimmed to sit under a line, be honest about the margin. A scheme measured at 17.9 metres will be remeasured by the Building Safety Regulator, by a warranty provider and probably by a purchaser's technical adviser, and a 100 millimetre screed variation during construction is enough to move it.

Evidencing the measurement on drawings

Put the evidence where it can be found.

1. A long section through the steepest part of the site showing existing and proposed ground levels to Ordnance Datum.
2. A marked datum and a dimension line to the top of the floor surface of the top storey, with the figure stated.
3. The storey count annotated on the section, with excluded levels labelled and the reason given.
4. A mezzanine area calculation, where any level is arguable, comparing its internal floor area against the largest storey.
5. The residential unit count, since the higher risk definition needs at least two.
6. A note stating which regime each figure has been tested against.

Keeping that material together also makes the handover to construction cleaner. The planning file is where a measurement dispute is resolved three years later, and the broader argument for assembling that record properly is set out in structural warranties and what the planning file should hold.

Consequences of being just over

Crossing the threshold is not a single administrative step. It changes who grants building control approval, it introduces Gateway Two before construction and Gateway Three before occupation, it imposes dutyholder and information management duties through the design and build, and in planning terms it brings a fire statement and consultation with the Building Safety Regulator.

Approved Document B has also been amended in relation to second staircases in new residential buildings above 18 metres. Check the current edition and its transitional provisions on gov.uk before relying on a core layout designed under an earlier version, because the plan consequences of adding a second stair are not marginal.

Material choice interacts with all of this. Engineered timber structures above certain heights attract particular scrutiny at planning and insurance stage, and the questions that follow are set out in mass timber schemes and the questions planners ask.

But the first thing to do is simpler than any of that. Measure it, write the number on the drawing, and say which rule you measured it against.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. Fire engineering and building safety compliance require a competent professional, and an AI-assisted draft is a starting point for that professional rather than a substitute.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.