Adding storeys under Class AA: structural feasibility first

The upward extension rights in the General Permitted Development Order let some buildings gain storeys without a full planning application, and the prior approval that replaces it says almost nothing about whether the existing structure can carry the load. That question is answered by Building Regulations later, and by then the deal is usually done. So the order of work on a Class AA upward extension should be structural appraisal, then feasibility, then prior approval, because a scheme that needs the existing frame strengthening from the ground up rarely stacks up at the price the right appeared to justify.

What the rights cover and exclude

Two parts of Schedule 2 matter. Part 1 Class AA allows a dwellinghouse to be enlarged by additional storeys, subject to prior approval. Part 20 contains the classes that create new self-contained homes by building upwards, on detached blocks of flats, on buildings in commercial or mixed use and on terraces and houses, each with its own conditions.

The limits differ between classes and have been amended since the classes were introduced in 2020, so read the current text of the Order for the class you are using rather than a summary. What is common across them is a set of gateway conditions worth checking before any design work.

• The building must have been constructed within the stated date range, which for the Part 20 classes runs from 1 July 1948 to 5 March 2018.
• The existing building must be of the type the class describes, and detached means detached.
• The rights do not apply on article 2(3) land, which includes conservation areas, National Parks, the Broads, National Landscapes and World Heritage Sites.
• They do not apply to a listed building or within its curtilage, to a scheduled monument, or on a site of special scientific interest.
• Height limits apply both to the number of new storeys and to the resulting building height.
• Floor-to-ceiling height of the new storeys is restricted, which affects whether the additional homes work at all.

That last condition catches people out on commercial conversions, where a generous existing floor-to-floor height sets an expectation the new storeys cannot match.

Prior approval matters the authority assesses

Prior approval is not a formality, and the list of matters the authority may consider is set out in the class itself. Across the upward extension classes it typically includes transport and highways impacts, air traffic and defence asset safeguarding, contamination and flood risk, the external appearance of the building, the provision of adequate natural light in all habitable rooms of the new dwellings, and the impact on the amenity of the existing building and neighbouring premises, including overlooking, privacy and loss of light.

Natural light is the ground that refuses most schemes. It is a statutory consideration, not a matter of judgement about quality, and a top-floor flat with a single small window serving a deep plan will fail it. Design the light in before submitting, and show the analysis.

External appearance is the second most common ground. A roof extension in a different material, with different window proportions and a flat roof where the building has pitches, is refused on appearance even where every dimensional limit is met.

Structural capacity of the existing frame

This is the work to do first, and it needs a structural engineer with the existing building's drawings, or a survey where the drawings are lost.

Adding storeys changes more than the vertical load. It raises the centre of mass, increases wind loading and lengthens the load path, so lateral stability and overturning need checking as well as compression. And where the additional storeys take the building past the thresholds in Approved Document A for consequence class, the requirements for structural strength and resistance to disproportionate collapse increase, which can mean strengthening work throughout the existing building rather than only at the top.

Commission a desktop appraisal before exchange, with a sum allowed for opening up. An engineer who says the frame is probably adequate subject to intrusive investigation has given you a risk, not an answer.

Foundations and load path checks

Foundations are the item most often skipped and the one that most often kills the scheme. You need the existing foundation depth, width and formation level, the bearing stratum and its capacity, and an assessment of the settlement the additional load will cause.

Trial pits against the existing footings are the only reliable way to establish the first three. Where the building sits on shallow strip footings in shrinkable clay, or on made ground, the additional load may need underpinning or piling through the existing footprint, which is disruptive, expensive and sometimes impossible with the building occupied.

Differential settlement is the subtler risk. Loading part of a building and not the rest produces movement at the junction, and that shows up as cracking in the flats below, which brings its own liability with existing leaseholders.

Access, stairs and fire escape effects

Additional storeys change the means of escape for the whole building as well as for the new flats. Travel distances, stair capacity, smoke control and firefighting access all have to be reassessed against Approved Document B for the building as extended.

Two thresholds matter. Where the resulting building reaches 18 metres or seven storeys it falls within the higher-risk regime under the Building Safety Act 2022, which brings the building safety gateways and, for a planning application, a Fire Statement under the development management procedure rules prepared by someone with fire engineering competence. How height and storeys are counted for those purposes is set out in counting storeys and measuring height for higher-risk buildings. Requirements for a second staircase in new residential buildings above 18 metres have been introduced with transitional arrangements, so check the current Approved Document B position rather than relying on an older scheme's precedent.

External wall materials are constrained above certain heights too, which affects the elevations you draw, as covered in combustible materials rules and planning elevations.

Evidence to submit with a prior approval application

Submit more than the minimum. A prior approval application that arrives with plans, elevations, a daylight analysis for the new habitable rooms, a construction method statement covering access, cranage and protection of the occupied building below, and a structural statement confirming feasibility, gets determined on its merits. One that arrives with two drawings invites refusal on appearance and natural light.

The structural position belongs in a Structural Survey, and where the building is or becomes a relevant building, in a Fire Statement prepared by a competent fire engineer, which is a legal requirement rather than a recommendation.

When a full application is the better route

Three situations point that way. Where the design that works needs more height or a different floor-to-ceiling arrangement than the class allows. Where the building is on excluded land, in which case the rights are unavailable anyway. And where the scheme would benefit from being considered as a whole, including changes at ground level, reconfiguration of the existing flats or a change of use, because prior approval cannot deliver any of that.

A full application also gives room to negotiate on appearance, which is where prior approval applications for roof extensions most often founder.

PlanScope drafts the supporting documents an upward extension needs, structured to the sections authorities expect, and flags the site-specific figures it cannot verify rather than filling them in. For specialist technical reports, qualified professional input may be required, and for a Fire Statement or Fire Safety Strategy a competent fire engineer is legally required to take responsibility.

Enter the postcode on PlanScope to see which documents an upward extension is likely to need.

This is general guidance, not legal advice. Permitted development rights are conditional and a Lawful Development Certificate is the only formal way to confirm that works are lawful, so 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.