Counting storeys and measuring height for higher-risk buildings

Whether a building is a higher-risk building comes down to two numbers and one occupancy test: is it at least 18m in height or at least seven storeys, and does it contain at least two residential units. Get the measurement wrong and you either submit a fire statement you did not need or, far worse, fail to engage Planning Gateway One on a scheme that required it, which means an invalid application or a permission granted without a statutory consultee having been consulted.

The measurement rules are not intuitive. Height is taken to the floor surface of the top storey rather than to the roof, some storeys are left out of the count entirely, and uneven ground changes the answer.

Why the measurement decides everything

Several duties cluster around the same dimension, and they do not share a single definition. The higher-risk building definition used for the Building Safety Act 2022 regime is set out in that Act and in the Higher-Risk Buildings (Descriptions and Supplementary Provisions) Regulations 2023. The relevant building definition that triggers a fire statement at Planning Gateway One sits in article 9A of the Development Management Procedure Order 2015. The definition governing combustible materials in external walls sits in the Building Regulations 2010.

Those three overlap heavily and are not identical, so check each against its own instrument for the scheme in front of you. A building can be caught by one and not another.

What they do share is the measuring convention, and that is the part worth learning properly.

Measuring from ground level to the top storey

Height is measured from ground level to the floor surface of the top storey of the building. Not to the ridge, not to the parapet, not to the top of the lift overrun.

The practical effect is that a building can have a parapet and plant well above 18m while measuring below it, and the reverse is not possible. On a scheme sitting close to the threshold this single point is often the difference between engaging the regime and not.

Take the measurement from the general arrangement section, dimension it on the drawing, and label both ends of the dimension: the ground level datum used and the finished floor level of the top storey. Do not rely on a levels plan and a separate section that were drawn at different times.

Storeys that are left out

A storey consisting exclusively of plant or machinery rooms at the top of the building is left out of account both for the height measurement and for the storey count. That is why a building with a rooftop plant level can measure lower than the visible mass suggests.

The exclusion is narrow. It applies where the storey consists exclusively of plant, so a plant level that also contains a small office, a store used for something other than plant, or accessible amenity space does not qualify. Roof terraces, in particular, are not plant.

Storeys below ground level are not counted towards the storey total. That does not make a basement irrelevant for other purposes, and it does not remove a basement from other fire safety duties. It simply does not add to the seven.

Stepped sites and uneven ground

Where the ground around the building is not level, the measurement is taken from the lowest part of the surface of the ground adjacent to the building. On a sloping site that pushes the measured height up, sometimes by a full storey.

Two situations worth flagging early on a site plan.

• A building set into a slope, where the rear elevation shows one storey fewer than the front. The measurement follows the lowest adjacent ground, so the taller elevation governs
• A sunken courtyard, lightwell or service yard immediately adjacent to the building, which can constitute the lowest adjacent ground and add its depth to the measured height

That second point catches basement car park ramps and refuse yards. Where the geometry is arguable, resolve it with the building control body in writing before the planning application is submitted, not afterwards.

Illustrative example: a six storey block on a site falling 3.2m across its footprint, with a sunken service yard at the low corner. Measured from the high side it sits comfortably under 18m. Measured from the lowest adjacent ground at the service yard it exceeds it, which brings the scheme into the regime and requires a fire statement with the application. The measurement, not the design, changed the submission.

Mezzanines, basements and split levels

A mezzanine counts as a storey where its internal floor area is at least half the internal floor area of the largest storey in the building. Below that proportion it does not add to the count. Double height spaces with a small gallery are usually outside the rule, and a substantial inserted floor is usually within it.

Split level arrangements need a decision rather than an assumption. Where two half-levels sit at different heights within what reads as a single storey, record which you have treated as a storey and why, and keep that reasoning with the drawings.

The residential unit test

Height and storeys are only half of it. The regime is aimed at buildings containing at least two residential units, and the scope of relevant buildings for the planning gateway is framed around buildings containing two or more dwellings or educational accommodation.

Three configurations that need thought. A single large dwelling over seven storeys is not caught by the residential unit test. A building with commercial floors below and two flats above is caught, and the whole building height is measured rather than the residential part. Student accommodation and other communal residential forms need the definition checked rather than assumed, because the treatment of rooms for residential purposes differs between regimes.

Where a scheme sits above commercial units the fire strategy is entangled with escape and compartmentation as well as with the classification, and fire strategy for a residential block above commercial units covers that ground.

Recording the measurement on the drawings

This is the part that saves a validation query, and it takes one afternoon.

1. A dimensioned section showing the ground level datum, the finished floor level of the top storey and the measured height between them
2. A note on the section stating which storeys have been disregarded and why
3. A site section showing ground levels around the whole footprint, with the lowest adjacent ground identified
4. A storey schedule listing every level, its use, and whether it is counted
5. A short statement of the conclusion, with the instrument relied on named

Put all five in the Fire Statement rather than scattering them. A fire statement is required at Planning Gateway One for relevant buildings under the Building Safety Act 2022 framework and article 9A of the Development Management Procedure Order 2015, and it must be prepared by someone with the necessary fire engineering competence. That is a legal requirement, not a recommendation, and an AI-assisted draft is a starting point for that professional rather than a replacement for them. The same applies to a Fire Safety Strategy prepared to BS 9991 or BS 9999.

Other duties trigger at nearby heights, including sprinkler provision and fire main type, and dry risers, wet risers and the height that separates them sets out where those thresholds fall. Check them against the same dimensioned section rather than against a different drawing, since inconsistent height figures across a submission are a common source of consultee questions.

This is general guidance, not legal advice; check the current text of the relevant regulations and take advice from a competent fire engineer and your building control body before relying on any classification.

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. Browse the PlanScope report types to see what a scheme at this scale usually needs.