Measuring height for planning and for the fire threshold
A building can be under 18 metres for the fire regime and over a tall building threshold in the local plan at the same time. Both numbers belong in the submission.
A building can be 17.8 metres tall for the fire safety threshold and 22 metres tall in the local plan's tall building policy, and both numbers are correct. The two regimes measure different things from different points. The fire threshold measures to the floor surface of the top storey and discounts rooftop plant. Planning policy usually measures to the highest point of the structure, plant enclosures included. Getting the distinction wrong at design stage produces either an invalid application or an unlawful start on site.
Why two measurements exist
They answer different questions.
The building safety measurement exists to identify buildings where evacuation and firefighting become materially harder with height, which is a function of how far up the highest occupied floor is, not of how tall the parapet looks. Plant at roof level does not change the evacuation problem, so it is excluded.
Planning policy measures visual and townscape effect. For that purpose, a 4 metre plant enclosure on the roof is entirely relevant, because it is visible from the street and from long views. Policy also tends to care about the relationship with neighbouring eaves and ridge lines, which is again a matter of the whole envelope.
Neither is wrong. What is wrong is quoting one number in both places and assuming a single figure serves the whole application.
How height is measured for the fire threshold
The relevant buildings for a fire statement at gateway one, and the higher-risk buildings caught by the building control regime, are identified by a height of at least 18 metres or by having at least seven storeys, together with a residential content test.
The height is measured from ground level to the floor surface of the top storey, not to the roof, and not to the ceiling. Where the ground falls across the site, the measurement is taken from the level of the lowest part of the surface of the ground adjacent to the building, which means a sloping site can push a building over the threshold that would sit below it on flat ground.
A rooftop storey used only for machinery or plant is left out of the measurement. So is a storey that exists only to house lift motor rooms, tanks or similar.
Check the current definitions in the regulations before relying on any of this, because the descriptions have been amended and the detail carries the outcome. A building within 300 millimetres of the threshold deserves the regulation text in front of you, not a summary.
Storey counting and what is excluded
The seven storey limb catches buildings with generous floor to floor heights that fall below 18 metres, and it has its own rules.
- Storeys below ground level are generally not counted towards the total
- A rooftop storey consisting only of plant or machinery rooms is excluded
- A mezzanine counts where it has a floor area above a stated proportion of the storey it sits within, so a small gallery is ignored and a substantial mezzanine floor is not
- The ground storey counts as a storey
Mezzanines are where teams most often disagree, because a commercial ground floor with a large mezzanine can tip a building from six storeys to seven without any change to its external appearance. Resolve that before the application, not at gateway two.
Height in local plan tall building policy
Local policy is a patchwork and it has to be read individually.
Some authorities define tall buildings by absolute height in metres above ground level, measured to the highest point including plant. Some define them relative to the prevailing context, for example a building substantially taller than its surroundings. Some use storeys. London boroughs must define what constitutes a tall building for their area in their local plan, and the thresholds vary widely between inner and outer boroughs.
The datum also varies. Height above ordnance datum is used where the policy is concerned with a skyline or a protected view, and height above adjacent ground level is used where the concern is local townscape. A scheme on a hill can comply with one and breach the other.
Read the policy wording and its supporting text before fixing the massing. A building designed to sit one metre below a threshold measured to the parapet will breach a threshold measured to the top of the lift overrun.
Roof plant, parapets and mezzanines
Roof level is where the two measurements diverge most sharply, and where drawings are least often explicit.
Plant enclosures, lift overruns, photovoltaic arrays, telecommunications equipment, green roof build-ups, parapet upstands and safety balustrades all add height that planning policy counts and the fire threshold does not. On a residential scheme with a communal roof terrace, the balustrade and the stair access enclosure can add a metre and a half to the planning height.
Show them. A section that stops at the parapet, with plant indicated as a dashed outline and no dimension, invites a query and sometimes a refusal on the basis that the height cannot be assessed.
The same applies below ground. A basement car park changes the storey count analysis and the ground level datum, and a section that cuts through only the upper floors leaves the case officer guessing.
An illustrative example
A scheme has six residential storeys over a double height commercial ground floor with a mezzanine, topped by a plant enclosure and a lift overrun. Top storey floor level is 17.6 metres above the lowest adjacent ground. Overall height to the top of the plant enclosure is 23.1 metres. The mezzanine exceeds the area proportion, making eight storeys in the count. The building is caught by the fire regime on storeys despite being under 18 metres, and by the borough's tall building policy on overall height. This example is illustrative.
Consequences of getting it wrong
Understating the height or the storey count has two distinct failure modes.
At validation, an application for a relevant building submitted without a fire statement is liable to be rejected as invalid, costing weeks. The requirement comes from Article 9A of the Development Management Procedure Order 2015, and the Health and Safety Executive is consulted where it applies. A fire statement prepared by someone with the necessary fire engineering competence is required, and it is not a document to produce in a hurry.
At construction, a building that turns out to be higher-risk after work has started is in a far worse position, because construction must not begin before building control approval is granted. The detail of what gateway two requires before construction starts shows how much design development that approval needs, and discovering the requirement late means stopping.
There is a third, quieter consequence. Where a building crosses 18 metres, second staircase provision arises, and what changed for buildings above 18 metres has direct effects on core area, net to gross ratio and unit mix. A scheme that creeps over the threshold during design development loses saleable area as well as programme.
Recording the measurement in the submission
Make the numbers explicit and make them findable.
- A dimensioned section showing the lowest adjacent ground level, the floor surface of the top storey, and the highest point of the structure, each with a level to Ordnance Datum.
- A short note stating which measurement convention each figure uses and the source of the convention.
- A storey schedule listing every storey, whether it is counted, and why any is excluded.
- A roof plan with plant, overruns and balustrades dimensioned rather than indicated.
Put the same figures in the planning statement, the design and access statement and the fire safety strategy documentation. Where they differ between documents, someone will find it, and the time for that to happen is before submission rather than after.
Enter your postcode on PlanScope to see which reports a residential scheme of this height is likely to need.
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. For specialist technical reports, qualified professional input may be required.
Frequently asked questions
How is the 18 metre threshold measured?
From ground level to the floor surface of the top storey, not to the roof or the parapet. Where the ground falls, the measurement is taken from the lowest part of the surface of the ground adjacent to the building, so a sloping site can push a building over the threshold. A rooftop storey used only for plant or machinery is excluded.
Which storeys count towards the seven storey test?
Storeys below ground level are generally excluded, as is a rooftop storey consisting only of plant or machinery rooms. The ground storey counts. A mezzanine counts where its floor area exceeds a stated proportion of the storey it sits within, which is where design teams most often disagree, so resolve it before the application rather than at gateway two.
Does rooftop plant count for planning height?
Usually yes. Local plan tall building policies generally measure to the highest point of the structure, which includes plant enclosures, lift overruns, balustrades and stair access enclosures. On a scheme with a communal roof terrace these can add well over a metre, so they should be dimensioned on the section rather than shown as a dashed outline.
What happens if the height is understated?
Two things. At validation, an application for a relevant building submitted without a fire statement is liable to be rejected as invalid under Article 9A of the Development Management Procedure Order 2015. On site, a building that turns out to be higher-risk after work has begun is in breach, because construction must not start before building control approval is granted.
How should the measurements be recorded?
With a dimensioned section showing the lowest adjacent ground level, the top storey floor surface and the highest point of the structure, each tied to Ordnance Datum, plus a note stating which convention each figure uses. Add a storey schedule explaining any exclusions, and keep the same figures consistent across every submitted document.
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.