Class AA: adding storeys to a house under prior approval

Class AA lets you add up to two storeys to a house without a full planning application, but only through prior approval, and only if the house passes a list of tests that rules out a large share of the country's housing stock. Before you pay anyone to draw anything, check the construction date, the designation of the area and the height arithmetic. Those three checks take ten minutes and settle most cases.

Class AA sits in Part 1 of Schedule 2 to the GPDO 2015. It was added in 2020, and it is one of the more tightly drafted permitted development rights.

What the right allows

Where the existing house has two or more storeys, Class AA permits up to two additional storeys. Where it has one storey, it permits one additional storey. The new storeys must be built on the principal part of the house, which means the main roof, not over a rear outrigger or a side extension.

Engineering operations are limited to what is reasonably necessary to strengthen existing walls and foundations. So the right allows you to build up, and to make the existing structure capable of carrying it, and nothing else.

The right applies to a dwellinghouse. Part 1 does not apply to flats or to a building containing flats, so a maisonette owner cannot use it. A separate right in Part 20 deals with adding storeys to a detached block of flats.

The houses it excludes

Work through this list first, because a single answer here ends the enquiry.

• The house must have been constructed between 1 July 1948 and 28 October 2018. A Victorian terrace is out. A house completed in 2020 is out.
• The house must not be on article 2(3) land, which covers conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites.
• The house must not be a listed building or within the curtilage of one, and must not sit on land within a scheduled monument.
• Sites in safety hazard areas and military explosives storage areas are excluded.
• The right can be removed by a planning condition on the permission for the house or by an Article 4 direction, so check both.

New estates are the common trap. Houses built after October 2018 fall outside the date window entirely, and many estates built before it carry conditions removing permitted development rights.

The limits that shape the drawings

Three sets of numbers govern the design.

The overall height of the extended house must not exceed 18 m. That is rarely the binding constraint on a two storey semi, but it matters on anything already tall.

The increase in height is capped by reference to the existing roof, with a lower cap for one additional storey than for two. Where the house is attached to another, the drafting also ties the acceptable height to the neighbouring houses, which is what stops a single terrace house rising two storeys above the rest of the row. Read AA.1 in full before fixing a ridge level, because this is the provision that most often defeats a scheme on a terrace.

Floor to ceiling height of each new storey is limited, and the limit is the lower of a stated maximum and the floor to ceiling height of the existing storeys. Generous ceilings are not available.

Materials must be of a similar appearance to the existing house. And no windows are permitted in any wall or roof slope forming a side elevation. That single restriction shapes the internal layout more than anything else, because a new top floor lit only from front and rear needs careful planning to avoid dark central rooms.

Making the prior approval application

Prior approval is an application, with drawings, a fee and a statutory timetable. Submit it properly and it behaves predictably.

1. Prepare existing and proposed plans and elevations, drawn to scale, showing the site, the house, and the neighbouring houses on both sides with their ridge and eaves heights.
2. Obtain a report from a suitably qualified engineer on the structural strength of the existing house and its ability to take the additional storeys.
3. Submit the application to the local planning authority with the fee.
4. The authority notifies adjoining occupiers and displays a site notice, then has a set period, normally 56 days, to determine it.
5. Do not start work before the authority gives prior approval, or before the determination period expires without a decision.

The structural report is the item most often missing, and an incomplete submission does not start the clock. Where the roof structure is unusual, a structural survey of the existing building prepared early is worth having in any case, since it also tells you what the build will cost.

What the council can refuse on

Prior approval is narrower than planning permission, but it is not a formality. The matters the authority considers are set out in the Class, and they include the impact on the amenity of adjoining premises, covering overlooking, privacy and loss of light, and the external appearance of the house, including the design and architectural features of the principal elevation and of any side elevation fronting a highway. Air traffic and defence asset impacts are included, and in London so is the effect on protected vistas.

Amenity is where most refusals come from. Two additional storeys on a semi-detached house cast a longer shadow across the neighbour's garden and put new windows at a level that overlooks it. Anticipate that. A shadow study prepared at the equinox and midsummer, and a window layout that avoids direct overlooking, answers the objection before the neighbour makes it. On a constrained plot, a daylight and sunlight assessment prepared to the recognised methodology carries far more weight than an assertion that the effect is minor.

External appearance is the second ground. A top storey that reads as a separate box, with different proportions and window rhythms from the floors below, invites refusal even where materials match.

Illustrative example: a 1960s semi in a suburban street applies for two additional storeys. The neighbour objects on privacy and light. The revised drawings drop the rear windows to high level obscure glazing at the flank end, set the top storey back behind a parapet, and include a shadow study. Approval followed at the second attempt, six weeks later than it needed to.

After approval, and what it does not cover

Prior approval under Class AA carries a completion deadline, so read the condition on the notice and diarise it.

Three regimes still apply.

The Building Regulations, in full, and adding two storeys to an existing house is a structural project with fire, means of escape, thermal and sound requirements.

The Party Wall etc. Act 1996, where the works affect a party wall, which on a semi or terrace they almost certainly will. That is a separate process with its own notice periods.

The Community Infrastructure Levy, where the authority charges it and the new floorspace exceeds the threshold, though a self-build or residential extension exemption may be available if it is claimed correctly and before works start.

A planning statement is not required for a prior approval application, but a short covering note that walks the officer through each limit in AA.1 and each prior approval matter shortens determination noticeably.

This is general guidance rather than legal advice. A lawful development certificate is the only formal way to confirm that works are lawful, so check the position with your local planning authority or a qualified professional before relying on any rule of thumb about permitted development.

Enter your postcode on PlanScope to see which supporting documents a prior approval application on your house is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.