Rooftop flats: Class AA compared with a full application
Airspace schemes look simple on a section drawing. The permitted development route brings a long list of prior approval matters, and on many buildings a full application ends up being the quicker of the two.
Adding storeys to an existing building to create flats can be done through the permitted development classes in Part 20 of Schedule 2 to the General Permitted Development Order 2015, or through a full planning application. The Class AA upward extension route, and the other classes alongside it in Part 20, offer a defined procedure with a defined list of matters the council may consider. A full application offers no such limits, and no such constraints either.
Which is better depends on the building, and the answer is less often the permitted development route than airspace promoters suggest.
What the classes permit and on which buildings
Part 20 contains several upward extension classes rather than one. They are distinguished by whether the building is detached or part of a terrace, and by what it is currently used for: a block of flats, a commercial or mixed use building, or a building in use as dwellinghouses.
Each class carries its own conditions. The building has to have been constructed within a defined date range. The number of additional storeys is limited, with more allowed on buildings that already have several storeys. And the overall height of the extended building is capped, with a higher cap for blocks of flats than for the classes applying to commercial buildings and houses.
Read the class rather than a summary of it, including this one. The figures have been amended since the classes were introduced and they differ between classes, which is exactly the kind of detail a lender's solicitor will check.
Several categories of land and building are excluded outright: article 2(3) land, which covers conservation areas, National Landscapes, National Parks and the Broads, together with listed buildings, scheduled monuments, sites of special scientific interest and various safety and defence areas.
The prior approval matters the council can consider
The list is long, and it is the reason these applications are less straightforward than they look.
| Matter | What it covers in practice |
|---|---|
| Natural light | Adequate daylight to all habitable rooms in the new dwellings |
| Amenity of the existing building and neighbours | Overlooking, privacy and loss of light to others |
| External appearance | Materials, fenestration and the relationship to the building below |
| Transport and highways | Access, servicing, parking and construction traffic |
| Air traffic and defence assets | Where the building sits near safeguarded areas |
| Contamination and flooding | Site conditions affecting the new residential use |
| Construction management | How the works are carried out over an occupied building |
| Fire safety | Where the extended building meets the relevant thresholds |
Two of these do most of the damage to schemes. Natural light, because roofscapes are cluttered and the new units often have constrained aspects. And amenity for the existing occupiers, because the people living in the building below are the ones who will object, and their objections are squarely within a matter the council may consider.
Structural, fire and access constraints
The existing building has to carry the load, and a report from a suitably qualified professional on structural strength is part of the submission. Commission a Structural Survey early, because a building that cannot take two storeys without strengthening the frame or the foundations is a building where the scheme economics change completely.
Access is the second constraint. New storeys need a stair and usually a lift, and retrofitting a lift into an existing core takes floor area from somewhere. Where the extended building crosses the relevant height or storey thresholds, a Fire Statement is required with the application and fire engineering competence is legally required to prepare it.
Plant is the third. Existing rooftop plant has to go somewhere, and relocating it interacts with noise, with the rooftop plant enclosures question and with the residual roof area available for the new units.
Daylight and amenity for existing occupiers
This is where most schemes are won or lost, and it deserves proper assessment rather than an assurance.
For the new units, the test is adequate natural light in all habitable rooms. For the existing occupiers, the question is loss of light, overlooking and privacy. Both are assessed by the same techniques, and a Daylight and Sunlight Assessment carried out to the recognised methodology is the evidence that answers them.
Do it before you apply. A scheme that goes in without the assessment, receives objections from every flat on the top floor, and then commissions the work, has already lost the argument in the officer's mind.
PlanScope can produce an AI-assisted draft of that assessment structured to the standard sections, with every modelled input marked as to be confirmed until a specialist supplies it. Councils normally expect a suitably qualified author for daylight work, and professional review before submission is strongly advised.
Where a full application gives more flexibility
Four situations point towards a full application.
The building is excluded from the classes, by designation, listing or date. The design needs something the classes do not allow, such as a set-back that changes the footprint or a reconfiguration of the floor below. The scheme includes other works, such as new entrances, cycle stores or refuse provision, which cannot be done under the class and would otherwise need a second application. Or the local plan is positive about the principle, in which case the policy assessment is not the risk it might be elsewhere.
The trade is a longer determination period and an open assessment against the whole development plan, including affordable housing policy on schemes above the local threshold. That last point is worth modelling before choosing the route.
Construction management on an occupied building
Whichever route is used, building over occupied flats is the part that generates complaints and delays.
Set out the crane or hoist position, the working hours, the access route, how residents reach their homes throughout, how refuse and deliveries continue, and what happens to the roof covering while the works are in progress. Weatherproofing during the works is the issue residents care about most and the one least often addressed in a submission.
Offer a resident liaison arrangement rather than waiting to be asked for one. It costs little and it changes the tone of the consultation responses.
Choosing the route and the documents each needs
Work through it in order.
- Identify the class that would apply and check the building against its exclusions and date conditions.
- Test the height and storey allowance against the scheme you actually want.
- Get an early structural view on whether the building can carry it.
- Run a daylight assessment for the new units and for the existing occupiers.
- Price both routes, including the cost of a failed prior approval.
- Check the affordable housing position under a full application before dismissing it.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. A lawful development certificate is the only formal way to confirm that works are lawful. Enter your postcode on PlanScope to see which reports your site needs.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Which class applies to my building?
It depends on whether the building is detached or forms part of a terrace, and on its existing use. Part 20 contains separate classes for detached blocks of flats, for commercial and mixed use buildings, and for buildings in use as dwellinghouses. The conditions differ between them, including the height caps, so identify the class first and read its own text rather than working from a summary.
How many storeys can be added?
The classes limit the number of additional storeys and cap the overall height of the extended building, with the number depending on how many storeys the building already has. Buildings of three or more storeys generally get a larger allowance than shorter ones. Take both figures from the relevant class, because they have been amended and they differ between classes.
What does the natural light condition mean in practice?
The prior approval covers the provision of adequate natural light in all habitable rooms of the new dwellings. That is a substantive test, not a formality, and on a constrained roofscape with plant, lift overruns and neighbouring buildings it can be the matter that defeats a scheme. Assess it properly before applying rather than after a refusal.
Can the council refuse on design grounds?
The prior approval includes the external appearance of the building, which gives the authority a real say over materials, fenestration and the relationship of the new storeys to what is below. It is narrower than a full design assessment, but it is not nothing, and a scheme that ignores the existing building's proportions will struggle.
Is a full application ever faster?
Often, once the whole route is counted. A prior approval refusal followed by a full application is slower than starting with the full application. Where the building is excluded from the classes, where the design needs flexibility the classes do not allow, or where you also need to alter the building below, a single full application is usually the cleaner route.
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.