Class AA: adding a storey to a house

Class AA of Part 1 allows the enlargement of a house by adding up to two additional storeys where the house already has two or more storeys, or one additional storey where it has only one. It sounds generous. In practice the construction date condition, the height limits measured against neighbouring roofs, and the excluded land categories remove most houses from the class before a designer starts work.

The right first question is not what can be built. It is whether the house qualifies at all.

Which houses qualify and which are excluded

Class AA sits in Schedule 2, Part 1 of the Town and Country Planning (General Permitted Development) (England) Order 2015, inserted in 2020. The qualifying conditions turn on the building itself and on the land it sits on.

The house is excluded where it was built before 1 July 1948 or after 28 October 2018. That is the condition that disposes of most enquiries. Victorian and Edwardian terraces, interwar semis built before mid-1948, and anything completed in the last few years all fall outside the class. The window is post-war and pre-2018 construction.

The house is also excluded where it sits on article 2(3) land, which covers conservation areas, National Landscapes, National Parks, the Broads and World Heritage Sites. It is excluded where it is a listed building or within the curtilage of one, where it is or is within the curtilage of a scheduled monument, and where it sits on a site of special scientific interest, in a safety hazard area or in a military explosives storage area.

Establishing the construction date is an evidential exercise. Historic mapping, the council's building control records, title documents and rating records are the usual sources, and a house "about 1950" needs pinning down rather than assumed.

Height limits and the relationship to neighbouring roofs

Three height controls apply together, and all three have to be satisfied.

The height of the highest part of the roof of the extended house must not exceed 18 metres. The extended house must not exceed the height of the existing house by more than 3.5 metres where one storey is added, or 7 metres where two are added. And where the house is attached to another building, the height of the highest part of its roof must not exceed the height of the highest part of the roof of that adjoining building by more than 3.5 metres.

That third limit is the one that governs on a terrace or a semi. Adding two storeys to a two-storey semi will almost always breach it, because the result stands far more than 3.5 metres above the attached neighbour. In practice Class AA delivers a single additional storey on attached houses and its two-storey potential is realistic only on detached properties.

There is also a floor to ceiling limit for each new storey: no more than the lower of 3 metres or the floor to ceiling height of any existing storey. A bungalow with 2.4 metre ceilings therefore gets a new storey at 2.4 metres rather than 3.

Engineering operations are restricted to works within the curtilage to strengthen existing walls and foundations, which matters where the existing structure cannot carry the additional load.

Prior approval matters the council will assess

Class AA requires an application to the local planning authority for prior approval before development begins, and the authority considers a defined list of matters rather than the full range of planning considerations. Those matters include the impact on the amenity of adjoining premises, covering overlooking, privacy and loss of light. They also include the external appearance of the house, taking in the design and architectural features of the principal elevation and of any side elevation fronting a highway, plus air traffic and defence asset impacts and, in London, protected views.

The practical consequence is that the assessment concentrates on the two things neighbours object to: light and overlooking. A daylight and sunlight note addressing the effect on the attached neighbour's rear windows, and a window arrangement that avoids direct views into adjoining gardens, do most of the work. Roof form is also assessed against the principal elevation, so a flat-roofed box on a street of pitched roofs invites refusal of prior approval on external appearance grounds.

Conditions attached to the class include that materials must be of similar appearance to the existing house, that no windows may be installed in any wall or roof slope forming a side elevation, that the works must be completed within three years of prior approval, and that the developer must provide the authority with a report for the management of the construction.

Structural and party wall implications

Adding a storey loads the existing structure and foundations in a way they were probably not designed for. A post-war semi with shallow strip foundations may need strengthening works before the new storey goes on, and Class AA's restriction on engineering operations means that work has to be within the curtilage.

Party wall procedure applies. Raising a party wall, cutting into it to insert a beam or a flashing, and underpinning near a neighbour's foundation all require notice under the Party Wall etc. Act 1996. Serve notices early, because the award process runs on its own timetable and a neighbour who has objected to the prior approval application is unlikely to agree quickly.

Expect a structural appraisal to be needed at building control stage regardless of the planning route, and get it done before committing to the prior approval application. Finding out that the foundations cannot take the load after approval is granted wastes the fee and the programme.

Where a full application is the better route

Four situations where the householder application route is preferable even though Class AA appears available.

1. The house is attached and two storeys are wanted, since the 3.5 metre limit relative to the neighbour's roof will not allow it.
2. The design wants a different roof form, more glazing on the side elevation, or a window where the class prohibits one.
3. The house sits on excluded land, or its construction date cannot be evidenced.
4. The scheme combines the upward extension with a rear extension beyond permitted development limits, in which case one application covering both is simpler than a prior approval plus a householder application.

The fee for a householder application is lower than many people expect, and the assessment is broader rather than harder. A prior approval refused on external appearance leaves you applying anyway, with a refusal on the record.

The exclusion of flats explains why the class is of no use to a maisonette owner, and the reasoning behind that is set out in why flats and maisonettes have no Part 1 permitted development. Where the property is in a conservation area, the restrictions run wider than Class AA, as set out in Article 2(3) land permitted development in protected areas.

A Lawful Development Certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that works are lawful, and a short Planning Statement setting out the measurements against each limit in Class AA is what supports either a prior approval application or a certificate.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.