Permitted development and the 18 metre height threshold
The Part 20 height caps and the building safety height condition use the same number for different purposes. Where schemes get caught between them.
The 18 metre threshold permitted development problem is that the same number appears in two unconnected places and means different things in each. In Part 20 of Schedule 2 to the General Permitted Development Order 2015, 18 metres is a height cap on most of the upward extension classes: build above it and the class simply does not apply. Under the Building Safety Act 2022 framework, 18 metres, or seven storeys, is the point at which a building containing residential units becomes a relevant or higher-risk building, bringing fire safety requirements and a statutory consultee into the process. A scheme can clear one and be caught by the other.
Where the 18 metre figure comes from
The height condition in the building safety regime is 18 metres measured from ground level to the floor surface of the top storey, or seven storeys, whichever is reached first, for buildings containing at least two residential units. Both limbs matter, because a building of seven storeys with generous floor-to-floor heights crosses the storey test well before the metric one, and a building with shallow floor zones can reach 18 metres at six storeys.
The storey count also has rules of its own about what counts, including the treatment of roof-top plant areas and basements, so a count taken from a sketch section is not reliable. Get it measured and recorded against the definition in the regulations rather than estimated.
The caps in Part 20 are a different thing entirely: they are limits on what the permitted development right allows, with no fire safety purpose at all. They happen to use the same figure.
The upward extension classes and their caps
Part 20 contains several routes for adding storeys, and they differ in what they apply to and how high they let you go.
| Class | Applies to | Additional storeys | Height cap |
|---|---|---|---|
| A | Detached blocks of flats | Up to two | 30 m |
| AA | Detached buildings in commercial or mixed use | Up to two | 18 m |
| AB | Terrace buildings in commercial or mixed use | Up to two | 18 m |
| AC | Terrace buildings in use as a dwellinghouse | Up to one | 18 m |
| AD | Detached dwellinghouses | Up to two | 18 m |
Each class carries conditions and limitations beyond the height cap: the age of the existing building, whether it sits on article 2(3) land or within a scheduled monument or site of special scientific interest, floor to ceiling heights for the new storeys, and the requirement that the external appearance be considered. Verify the current wording of the class you intend to use, because Part 20 has been amended repeatedly since it was introduced.
The caps are absolute. A building at 17 metres cannot gain two storeys under Class AA if the result would stand at 19 metres, and the right is not available in part.
Prior approval and the fire safety overlay
Part 20 operates by prior approval, which means an application to the authority for approval of specified matters rather than a full planning application. The matters typically include transport and highways impacts, air traffic and defence asset safeguarding, contamination and flood risk, external appearance, the provision of adequate natural light to all habitable rooms, impact on the amenity of the existing building and neighbours, and, for the taller cases, fire safety.
The Fire Statement requirement in Article 9A of the Development Management Procedure Order 2015 attaches to applications for planning permission for development involving a relevant building, with the Building Safety Regulator within the Health and Safety Executive as the statutory consultee at Planning Gateway One. How that interacts with a prior approval application under Part 20 is a question to put to the authority in writing before design freeze, since the application route is not the same and the classes carry their own fire safety matters. Do not assume either that Gateway One is avoided by using prior approval, or that it automatically applies.
Where a Fire Statement is required, it must be prepared by someone with the necessary fire engineering competence under the Building Safety Act 2022 framework and Article 9A, and a PlanScope draft is a starting point for that professional rather than a substitute.
Where the thresholds interact badly
Three cases come up repeatedly.
A six storey block at 17.2 metres adding two storeys under Class A. The 30 metre cap is not the problem; crossing the building safety height condition is, because the finished building will contain residential units above 18 metres and will be a higher-risk building for the purposes of the building control gateways as well.
A commercial building converted to flats and then extended. The conversion and the upward extension may each be permitted development under different parts of Schedule 2, and the combined result can be a relevant building even though neither step looked like one on its own.
A terrace house gaining a storey under Class AC. Nowhere near 18 metres, so the building safety regime is irrelevant, and the real constraints are the one storey limit, the floor to ceiling heights, and the amenity and natural light matters the authority will assess.
The pattern is the same each time: the right is tested against its own cap, and the building is tested against the regulatory definition. Two separate exercises on the same drawing.
Checking the position before design freeze
Do four things, in this order, before the massing is fixed.
- Measure the existing building properly: height to the floor of the top storey, total height, and a storey count against the statutory definition rather than a count of floor plates.
- Confirm the Part 20 class you intend to rely on and read its current conditions and limitations in full, including the designated land exclusions.
- Model the finished building against both the class cap and the 18 metre or seven storey condition, and note how much headroom you have. A scheme with 300 mm of margin is a scheme that will change.
- Ask the authority in writing which application route it expects and whether it considers Gateway One engaged, and keep the answer.
Where the headroom is tight, a full planning application is sometimes the better route. Prior approval is faster on paper and inflexible in practice: the right is either available or not, the matters for approval are fixed, and there is no scope to negotiate a scheme that falls a little outside. A supporting planning statement for a full application can at least argue the merits.
Structural and other consequences
Adding storeys to an existing frame changes the loading, and the structural question usually determines whether a scheme is deliverable long before the planning one does. A Structural Survey establishing the capacity of the existing foundations and frame is worth commissioning early, and councils expect a suitably qualified author for it.
Two other items follow from height. Means of escape and the stair strategy change at the threshold, which affects the net to gross ratio and therefore the viability. And overheating assessment under the relevant standard becomes harder on upper storeys with large glazed areas, which an Overheating Risk Assessment has to address alongside the energy strategy.
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 structural assessment legally require competent professionals, and for specialist technical reports qualified professional input may be required. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
What is the 18 metre height cap in Part 20?
It is the maximum height of the finished building for most of the upward extension classes, including Classes AA, AB, AC and AD. Class A, for detached blocks of flats, allows up to 30 metres. The caps are absolute: if the result would exceed the figure, the class does not apply and the right is not available in part.
How is the building safety height condition measured?
From ground level to the floor surface of the top storey, with an alternative test of seven storeys, for buildings containing at least two residential units. Whichever is reached first applies. The statutory definition includes rules about which storeys count, so measure and record against the regulations rather than counting floor plates.
Does Planning Gateway One apply to a prior approval application?
The Fire Statement requirement in Article 9A of the Development Management Procedure Order 2015 attaches to applications for planning permission involving a relevant building. Prior approval under Part 20 is a different application route and the classes carry their own fire safety matters, so put the question to the authority in writing before design freeze.
Can a building cross the threshold through two separate steps?
Yes. A commercial building converted to flats under one part of Schedule 2 and then extended upwards under another can produce a relevant building even though neither step looked like one alone. Assess the finished building against the regulatory definition rather than assessing each permitted development step in isolation.
Is prior approval better than a full application?
Not always. Prior approval is faster on paper but inflexible: the right is either available or it is not, the matters for approval are fixed, and a scheme falling slightly outside a limitation cannot be negotiated. Where headroom against a cap is tight, a full application lets you argue the merits instead.
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.