Class AA of Part 1: adding storeys to a detached house

Upward extension of a detached house under permitted development is possible, but it is a prior approval right rather than a free pass. Class AA of Schedule 2 Part 1 of the General Permitted Development Order 2015 allows up to two additional storeys on a house that already has two or more storeys, and one additional storey on a single storey house. Before any of that applies you have to clear a set of exclusions that rule out a large share of the housing stock, and then get prior approval from the council on matters including the external appearance of the house and the amenity of your neighbours.

The right exists. It is narrower than the headlines suggested when it was introduced in 2020, and the prior approval stage is a real hurdle rather than a formality.

Which houses the class covers

Start with the exclusions, because they are where most schemes stop.

Class AA does not apply where the house was built before 1 July 1948 or after 28 October 2018. That single condition removes Victorian and Edwardian terraces, interwar semis built before the cut-off, and anything recently completed. The house also has to be a dwellinghouse, so flats and maisonettes are outside Part 1 entirely.

The right is withdrawn on article 2(3) land, which means conservation areas, National Parks, the Broads, National Landscapes and World Heritage Sites. It is also withdrawn where the building is listed, within the curtilage of a listed building, on the site of a scheduled monument, or on a site of special scientific interest. Safeguarded land around aerodromes and certain defence assets brings further restrictions.

An Article 4 direction can remove the right locally even where none of the above applies. Check the council's own mapping before spending money on drawings, and note that our guide to article 2(3) land and permitted development in protected areas covers those designations in more detail.

Houses created through a change of use under Part 3, such as an office converted under Class MA or a barn under Class Q, are excluded as well.

How many storeys the Order allows

The allowance depends on what is there now.

The new storeys must be built on the principal part of the house. That means the main body of the building, not on top of a rear extension, a side addition or a garage, however tempting that looks on a drawing.

Each new storey is limited in height: no more than 3 metres internally floor to ceiling, or the floor to ceiling height of any storey of the principal part of the existing house, whichever is the lower. A bungalow with a 2.4 metre ceiling height therefore gets a 2.4 metre storey, not a 3 metre one.

Height limits

Three limits operate together, and the binding one differs from house to house.

The overall height of the extended house must not exceed 18 metres, which is academic for most detached houses but relevant on anything already tall. The increase over the existing roof height is capped, and the cap is more generous for a detached house than for one attached to a neighbour. And the finished roof must not be higher, relative to the existing, than the Order permits for the number of storeys added.

Measure the existing height properly before designing. The reference point is the highest part of the existing roof excluding any chimney, aerial or similar, and on a hipped roof with a ridge running the wrong way the number is often lower than the owner assumes.

The prior approval matters

This is where the application is won or lost. The council can consider a defined list, and it can refuse prior approval on any of them:

• impact on the amenity of any adjoining premises, including overlooking, privacy and loss of light
• the external appearance of the house, including the design and architectural features of the principal elevation and any side elevation fronting a highway
• air traffic and defence asset impacts
• whether the development would affect a protected view identified in the relevant national direction

Loss of light to a neighbour is the most common ground for refusal on an upward extension, followed by external appearance. An officer assessing a two storey addition to a bungalow in a street of bungalows is entitled to conclude that the appearance is unacceptable, and plenty have.

Design for the street, not for the maximum envelope. A proposal that reads as a house rather than as a box dropped on a bungalow survives the appearance test far more often.

Neighbour consultation and procedure

The council notifies the owners and occupiers of adjoining premises and displays a site notice. There is a period for representations, and the council must take any it receives into account. Your neighbours will be told, and their objections go into the file.

The application itself needs a plan showing the site, a written description of the development, drawings of the existing and proposed elevations, and a report from the developer dealing with the structural position.

The council has a set period to determine the application. Check the Order and the council's acknowledgement letter for the exact deadline and what happens if it is missed, because the rules on deemed approval differ between permitted development classes and this one is not the same as a householder application.

Expect conditions if approval is granted: materials of a similar appearance to the existing house, obscure glazing and restricted opening on side elevation windows, and a requirement to complete within a set period from the date of approval.

Engineering evidence

The structural report is not a formality. Adding two storeys loads existing foundations that were designed for a two storey house, and the report has to address whether the existing structure can take it, what strengthening is proposed and how the works will be carried out.

Get a structural engineer involved before the prior approval application rather than after. A scheme that turns out to need underpinning and a new frame may be less attractive than a rear extension under Class A and a loft conversion achieving similar space, and that is better discovered on paper.

Building Regulations approval is entirely separate and covers fire safety, structure, insulation and means of escape. The Party Wall etc. Act 1996 will usually apply where the house adjoins another. Community Infrastructure Levy may be payable where the council charges it, with a self-build or residential extension exemption available in some circumstances if you claim it before work starts.

Where the right is excluded entirely

To summarise the position rather than repeat it: the right is unavailable on pre-1948 and post-October 2018 houses, on flats, in conservation areas and other article 2(3) land, on listed buildings and in their curtilage, on scheduled monuments, on sites of special scientific interest, where an Article 4 direction applies, and where the house was created under a Part 3 change of use.

If any of those apply, a full householder or full planning application is the route, with a Planning Statement and design material to support it. That is not a worse outcome. A full application is judged on its planning merits and can accommodate a design that the rigid limits of Class AA would not allow.

One thing worth being clear about: nothing in a prior approval decision confirms that your works are lawful in every other respect. A Lawful Development Certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that proposed works are lawful, and it is worth obtaining if you intend to sell within a few years.

Enter your postcode on PlanScope to see which documents an upward extension is likely to need, whether you go down the prior approval route or make a full application.

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.