Class AA upward extension: what prior approval covers

Class AA of Part 1 of Schedule 2 to the General Permitted Development Order 2015 allows up to two additional storeys on a house, without a planning application, subject to prior approval. It sounds like a large right. In practice a series of eligibility tests removes most of the housing stock before the design begins, and the ones that survive are usually detached houses on generous plots.

Work through the exclusions first. They are quicker to apply than the height arithmetic and they are where the answer usually lies.

Which houses Class AA applies to

The house must have been built after 1 July 1948 and before 29 October 2018. Anything Victorian or Edwardian is out, and so is anything completed in the last few years.

It must be a dwellinghouse rather than a flat or a building containing flats. It must not be listed or within the curtilage of a listed building, and it must not sit on a scheduled monument or its curtilage, a site of special scientific interest, a safety hazard area or a military explosives storage area.

It must not be on article 2(3) land, which covers conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites. And the right is not available where the dwelling itself was created through one of the prior approval conversion routes, such as an office or agricultural building converted to residential use.

Run those tests before anything else. A single afternoon on the council's constraint mapping settles most sites.

Height limits and the storey count

Three separate caps apply, and the binding one differs by house type.

That fourth row is what defeats terraces and semis. In a run of identical two-storey houses, adding a full storey typically lifts the roof by more than 3.5 metres above the adjoining house, so the right delivers something closer to a raised roof than a new floor. Measure the neighbours before drawing anything.

The additional storeys have to be constructed on the principal part of the house, meaning the main body rather than a rear outrigger, a garage or a previous extension. Engineering operations are limited to works within the curtilage to strengthen existing walls and foundations, so a scheme needing a new structural frame is outside the class.

The matters prior approval can consider

Prior approval is not planning permission and the council's discretion is confined to a list. For Class AA that list covers:

1. Impact on the amenity of adjoining premises, including overlooking, privacy and loss of light.
2. External appearance, including the design and architectural features of the principal elevation and of any side elevation fronting a highway.
3. Air traffic and defence asset impacts.
4. Protected views, where the site falls within an area where those are specified.
5. Construction management, including hours of operation and how effects on occupiers of the building and neighbours will be handled.
6. The provision of adequate natural light in all habitable rooms of the dwellinghouse.

That last item is easy to underestimate. A new top storey with rooms lit only by a rear-facing window, or a reconfigured floor below that loses its light to the new structure, can fail on this ground alone.

Neighbours are notified and their representations are considered under headings one and two, which makes the process feel much like a planning application from the outside.

Construction management and the standing conditions

A construction management plan has to be submitted with the prior approval application, covering hours of operation and the steps taken to limit the effects of building work on people living in the building and nearby. Write it as a real document rather than a paragraph.

Standing conditions then apply to the works themselves. Materials must be of a similar appearance to the existing house. Windows are restricted in any side elevation wall or roof slope forming a side elevation, other than obscure-glazed non-opening units below a stated height. The development has to be completed within three years of the date prior approval is given, and the developer must notify the authority in writing when it is finished.

Miss the completion notification and you have an unlawful element in an otherwise lawful scheme, which surfaces on sale.

Two practical matters sit outside the Order but decide whether the project happens. The Party Wall etc. Act 1996 applies to work on a shared wall and to excavation near a neighbour's foundations, and notices have to be served in good time. And the additional storeys have to satisfy the Building Regulations in the usual way, with structural, fire and access requirements that on a two-storey addition can be the harder half of the job.

Where Class AA is excluded entirely

Beyond the designations, three practical situations end the conversation.

A house with an existing roof already close to 18 metres, which is rare for houses and common for the taller post-war terraces in some cities. A house whose neighbours are lower, since the 3.5-metre rule measures against the adjoining building rather than against your own. And a house where the principal part is small relative to later extensions, because the new storeys can only go on the principal part.

Illustrative example: a 1960s semi with a two-storey rear extension added in 2005. The owner wants two extra storeys across the whole footprint. Class AA permits them only over the original principal part, and the adjoining house caps the increase at 3.5 metres, which is less than the two storeys the internal height rules would otherwise allow. The scheme becomes a planning application.

Evidence to submit and common refusals

The application needs plans and elevations of the existing and proposed house, a construction management plan, and enough information for the council to assess each prior approval matter. Adding a short Planning Statement that walks through the eligibility tests, with the relevant dimensions stated, makes the officer's job straightforward and reduces the risk of a refusal on a technicality.

Refusals cluster around three things: external appearance where the addition reads as an alien storey on a uniform street, amenity where the new floor overlooks a neighbour's garden at close range, and inadequate natural light in habitable rooms. A Design and Access Statement that shows the proposal in the street scene answers the first far better than an elevation drawing alone.

Where the geometry is marginal, consider a lawful development certificate under section 192 of the Town and Country Planning Act 1990 for the eligibility question, since prior approval and lawfulness are separate points and a certificate is the only formal confirmation that works are lawful. Councils will not usually determine both together, so budget the time for two applications rather than one.

To see which supporting documents your scheme is likely to need, browse the report types or enter your postcode on PlanScope.

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.