What are the Class AA permitted development rules to get an additional storey?

The new Class AA Permitted Development right offers property owners and developers in the UK the opportunity to add up to two extra storeys to residential properties.

Instead of having to apply for planning permission, you can use the Class AA Prior Approval application process instead. 

If your application is approved, you’ll save a lot of time and money and free yourself from the normal paperwork and bureaucracy.

As with all permitted development rights, you have to get the go-ahead from your local planning authority to use these new permitted development rights. 

That means that you’ll almost certainly need to work with architects like Windsor Patania through what amounts to an airspace development project.

In this article, we’ll explain the new Class AA rights and how you can take advantage of them.

Jump to:

• What Class AA and the types of developments it covers
• What Class AA developments do and do not permit
• Aspects to consider before going for prior approval for Class AA project
• The “other” class AA – “new flats on detached buildings in commercial or mixed use properties”
• Book a discovery call with an architect

What Class AA is

The UK Government introduced Class AA permitted development rights to offer greater flexibility to owners of residential properties who want to expand their existing building by building upwards.

What type of development does Class AA permit?

Expressed in the simplest terms, a Class AA development allows the following type of work to be carried out on qualifying property:

• You can add one storey if the existing property is one storey.
• You can add up to two storeys if the existing property has two or more storeys.

What type of properties are Class AA?

To qualify as a Class AA property under the Permitted Development Rules 2023, your property must not:

• Have been built before July 1st 1948 or after 28th October 2018
• Be on Article 2(3) land like a conservation area, a World Heritage site, a National Park, a site of special scientific interest or an area of outstanding natural beauty.
• Be a listed building
• Have had extra storeys added to it previously (in other words, it’s not been extended upwards with an additional storey or more before this point) 
• Have been converted under Class G, Class M, Class MA, Class N, Class O, Class P, Class PA or Class Q of the Town and Country Planning (General Permitted Development) Order 2015.

What can and can’t you do under Class AA?

When you apply for prior approval from your local planning authority, follow these rules when planning:

• If you live in a terrace house or semi-detached (in other words, not a detached house), your new roof height must not exceed the height of the adjoining premises’ roof by more than 3.5 metres after project completion.
• The height of your home’s roof must not exceed 18 metres or exceed the current roof’s height by more than 3.5 metres (for a one-storey house) or 7 metres (for a multi-storey house).
• The floor-to-ceiling height of the new storey must not exceed 3 metres or the height of any existing floor in the main part of the house.
• The new storey must be added to the main part of the property (meaning you can’t start building a second floor on top of an existing extension)
• The project can not include visible exterior support structures.
• You do not require any engineering operations beyond those needed to strengthen existing walls or foundations. Engineering operations to, for example, strengthen walls or existing foundations must be within the curtilage of the property.

Getting permission granted relies on other considerations relating to design and architectural features too, notably:

• The materials you use to build up must match those of the existing house.
• The roof pitch must be the same as with the original house after project completion (in other words, no roof slope forming a side elevation to the property).
• You can’t add windows to the side of the house.
• The property must still be used as residential property (a “dwelling house”) when finished.

Applying for prior approval via your local planning authority

Before you approach local government, you should take your project through a standard retrofit development appraisal to assess its viability. You’ll also get an idea of how likely your project will proceed through the prior approval process of or whether it’ll have to go through the standard planning application process instead. 

Prior approval application success for your additional storeys depends on the tests your local authority applies which are based on the National Planning Policy Framework.

You’ll need to include information in your application on:

• The extent to which the proposed development will “impact on the amenity” of neighbouring premises and existing buildings by, for example, blocking their light or overlooking them.
• Changes to the external appearance of the property, especially if the front of a building faces a road.
• Any air traffic or defence asset impacts 
• If the new building might block any important views that are protected by law.
• Diagrams of existing and proposed elevations
• How the construction of additional storeys will be managed 

Although the prior approval process is much faster than standard planning permission, there are still a number of hoops that you have to go through.

You must complete the project within three years of the date prior approval was granted.

Working with a modern classic design architect

So far, this article has covered the General Permitted Development Order named “Class AA – enlargement of a dwellinghouse by construction of additional storeys”. 

There is another Class AA – “new flats on detached buildings in commercial or mixed use properties” – which is not a permitted development right but a use of building classification.

This allows for the construction of new flats on top of detached buildings that are currently commercial or mixed use, such as shops, professional services, restaurants, cafes, offices, betting shops, payday loan shops, or launderettes. 

These buildings could also have mixed-use with residential properties, such as shops with flats above.

Adding new floors onto commercial properties under the other Class AA

The guidelines differ for any proposed development to add additional storeys to an existing building in commercial or mixed-use.

The main point of difference is that, whereas with the first class AA, the new storeys form part of an existing house, under this Class AA, you must build apartments that are separate from the rest of the property.

In summary, the rules are:

• The building must be detached and have at least three storeys
• Any upward extensions must be added to the principal part of the building.
• The highest part of the roof of the building, after an upwards extension is added but not including plant mustn’t be higher than 30 metres.
• The floor-to-ceiling height of any storey in upwards extensions, measured internally, cannot be more than either 3 metres or the floor-to-ceiling height, measured internally, of any storey of the principal part of the existing building (whichever is the smaller).
• The height of the roof of the extended building can’t be more than the height of the existing building’s roof, not including plant, by more than 7 metres.
• You can only strengthen existing walls or foundations or install or replace services within the existing curtilage of the building. Any other engineering operations are not permitted.
• You can install additional plant onto the roof of the extended building only if there is existing plant on the building. The height of any additional plan mustn’t exceed the height of any existing plant.
• Entry to and exit from both the new flats and existing premises can’t be beyond the curtilage of the existing building.
• All storage, waste and other ancillary features must be kept within the curtilage of the existing building. It can’t be placed on land out the front of the building
• Any new facilities related to storage, waste management, or other services needed to support the building must be within the boundary of the current building. They should not be located in front of the main front wall or the side wall that faces the street.

Work with WindsorPatania

If you want to add one or more storeys to your residential, commercial or mixed-use property, click on the “Speak to an architect” button below. We can also advise you on the likely cost of adding a second floor to a house.

We look forward to hearing about your project.