Which documents a Class AA upward extension needs

Upward extension under Part 20 of the General Permitted Development Order is a prior approval route, so the class aa upward extension documents you submit are governed by a closed list of matters the council may consider. That list is shorter than a planning application's, and it is the reason these submissions get decided on evidence about daylight, structure, appearance and amenity rather than on development plan policy.

Read the right class first. Part 20 separates detached blocks of flats, detached and terrace buildings in commercial or mixed use, and buildings in use as dwellinghouses, and each has its own class, its own storey limit and its own list of matters.

What the right permits and where it stops

The Part 20 classes allow new dwellings to be created by adding storeys to an existing building, subject to limits that the Order sets out in detail. The limits that catch most schemes:

• the existing building must have been constructed within the date range the class specifies, which rules out both older and very recent buildings
• the building as extended must not exceed 18 metres in height
• the height of each additional storey is capped, and the floor to ceiling dimension is controlled
• the right does not apply on article 2(3) land such as conservation areas, National Parks and National Landscapes, nor to listed buildings or scheduled monuments
• it is excluded on certain other land, including sites in safety hazard areas and military explosives storage areas

Check the class text rather than a summary. The storey limits differ between a detached block and a terrace, and getting this wrong means an invalid application and a lost determination period.

Where the building has already been extended upwards once, the allowance is used up. That history needs establishing from the planning register before anything else.

The prior approval matters the council may consider

The classes list the matters for consideration, and the council cannot refuse on anything outside them. Across the Part 20 classes they include transport and highways impacts, air traffic and defence asset impacts, contamination risk, flooding risk, the external appearance of the building, the provision of adequate natural light in all habitable rooms of the new dwellings, the effect on the amenity of the existing building and of neighbouring premises including overlooking, privacy and loss of light, and whether the development would affect a protected view identified in the relevant direction. Some classes add the effect of noise from commercial premises and the effect of introducing residential use.

The Order also requires the developer to provide a report dealing with the construction management of the development, covering the hours of operation and how the developer will mitigate the effect on occupiers of the building and on neighbours.

So the document bundle maps onto that list, one document per matter, and nothing more.

Daylight evidence, which is the one that fails

Adequate natural light in all habitable rooms is an express matter, and it applies to the new dwellings being created rather than only to the neighbours. A top floor flat with a deep plan and a single window to a light well will struggle.

A Daylight and Sunlight Assessment following BRE 209 is the right document, assessing both the new units and the effect on existing windows in the building below and in neighbouring properties. Where the new storeys sit above an existing block, the effect on the top floor flats underneath needs modelling, because those windows lose sky view to the new parapet.

The honest position is that this matter has been the most common ground of refusal under Part 20, and an assessment produced after a refusal rarely rescues the scheme. Model it at concept stage, when the plan depth and the window sizes can still change.

Structural capacity of the existing building

Nobody grants prior approval on the strength of an assurance. Where two storeys are going on top of a 1960s frame, the council will want to know the proposal is buildable, and the leaseholders below will want it rather more urgently.

A Structural Survey by a chartered engineer, with intrusive investigation where the frame type is uncertain, is the proportionate document. Councils usually expect a suitably qualified author and professional review is strongly advised. The survey should address the existing foundations, the frame or load bearing walls, the lateral stability of the extended building and the practicality of transferring loads.

This is also where the commercial decision gets made. A scheme needing a new transfer structure and piled underpinning often costs more than the units are worth.

External appearance and the street

External appearance is a matter the council can consider, which gives it real control over the design even though development plan design policy is not directly in play.

Submit elevations in context, showing the extended building against its neighbours, with a materials schedule. Set back the new storeys where the roofline matters. Address the parapet, the plant enclosure and the lift overrun, because those are the elements that actually change the skyline and the ones applicants leave off the drawings.

Where the building sits in a view identified in a direction, deal with it directly. A Townscape and Visual Impact Assessment is proportionate for a tall building in a sensitive view and excessive for a two storey addition in a suburban street.

Construction management and access

The construction report required by the Order is not a formality. Building above an occupied block means craneage over people's homes, scaffold through their car park and noise for a year.

A Construction Management Plan covering hours, deliveries, crane operation, scaffold, dust and noise mitigation, and the arrangements for maintaining access and services to the existing occupiers, is what the council will condition. Write it with the contractor if one is appointed, because a plan the builder cannot work to becomes a variation request in month three.

Contamination and flooding are both listed matters, so where the site has an industrial history or sits in Flood Zone 2 or 3, a Flood Risk Assessment or a contamination appraisal is needed in support. For a rooftop addition the flood risk argument is usually straightforward, provided you address access and egress in a flood event rather than only the finished floor level.

When a full application is the better route

Three situations where the prior approval route is the wrong choice.

Where the scheme needs more height or more storeys than the class allows, applying under the class and losing is worse than applying properly. Where the building is listed or in a conservation area, the right does not exist. And where you want a mix of uses, a different unit layout at lower levels, or external alterations beyond what the class covers, a full application supported by a planning statement lets you argue the whole scheme on its merits.

The evidence base for a small residential scheme going the full application route is set out in our note on the documents behind a nine-dwelling application.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.

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