What is a Class AA upward extension?
Class AA is the householder permitted development right, in Schedule 2, Part 1 of the Town and Country Planning (General Permitted Development) (England) Order 2015, that allows a house to be enlarged by building new storeys directly on top of it.
Most people know Class A (rear and side extensions) and Class B (loft dormers). Class AA sits between them and goes further than either: instead of adding volume inside or beside the roof, it lets you lift the whole roof and build full-height rooms underneath the new one. The right is aimed at houses where the ground is used up but the air above is not, which describes a good share of post-war London suburbs.
It is not a free pass. Class AA always carries a prior approval step, which means the council looks at a defined list of impacts before you can build. It is closer to a light-touch planning application than to a loft conversion under Class B, where no council involvement is needed at all.
Which houses can use Class AA?
The house has to pass every test in paragraph AA.1. Fail one and the right is not available, so the only route left is a full householder planning application.
| Test in paragraph AA.1 | Limit |
|---|
| When the house was built | Not before 1 July 1948 and not after 28 October 2018 |
| Storeys you can add | Up to two on a house of two or more storeys; one on a single-storey house |
| Rise in overall height | No more than 3.5 m on a one-storey house; no more than 7 m on a house of more than one storey |
| Highest part of the finished roof | No more than 18 m above ground |
| Semi-detached and terraced houses | The new roof may not sit more than 3.5 m above the highest part of the roof of the attached house (or houses) |
| Floor-to-ceiling height of each new storey | The lower of 3 m or the floor-to-ceiling height of any existing storey of the main house |
| Where the storeys go | On the principal part of the house only, not on a rear wing or earlier extension |
| Previous upward enlargement | Not available if storeys have already been added |
| Land | Not on Article 2(3) land (which includes conservation areas) or a Site of Special Scientific Interest |
Source: GPDO 2015, Schedule 2, Part 1, Class AA, retrieved 12 September 2026.
The build-date window does most of the filtering in London. A Victorian or Edwardian terrace was built before 1 July 1948, so it cannot use Class AA whatever its height, and a house finished after 28 October 2018 is excluded too. What is left is the post-war stock: council-built estates, 1960s and 1970s town houses, and later speculative housing on the outer boroughs.
The conservation area exclusion matters just as much. A conservation area is Article 2(3) land, so any house inside one is out, even if it was built in 1975. An Article 4 direction can also remove permitted development rights in a street that is not otherwise designated, so check your council's register before you spend anything on drawings.
What conditions apply once you build?
Paragraph AA.2 attaches conditions that last for the life of the work. They are about appearance and use, and they are where many upward schemes get redesigned.
- Materials on the exterior must be of similar appearance to those on the existing house.
- No window may be put in any wall or roof slope that forms a side elevation of the house.
- The roof pitch of the principal part must stay the same as it was before.
- The house must remain a single dwelling in Use Class C3; the new storeys cannot become a separate flat under this class.
- The work must be finished within 3 years of the date prior approval is granted.
- You must tell the council in writing, as soon as reasonably practicable after completion, giving the developer's name and address and the completion date.
The side-window ban is the one that catches detached and end-of-terrace houses. A deep house with rooms along the flank relies on side windows for daylight. If the only way to light the new floor is from the side, Class AA does not work and the scheme has to go in as a planning application instead.
The pitch rule means you cannot swap a pitched roof for a flat one as part of the works. The new top storey has to carry a roof of the same pitch as the old one, which pushes up the overall height and brings the 7 m and 18 m caps into play sooner than people expect.
What does the council check at prior approval?
The council cannot reopen the principle of adding a storey. It can only assess the matters listed in the order, and it notifies adjoining owners and occupiers so they can comment on them.
| Prior approval matter | What a London council tends to look at |
|---|
| Amenity of adjoining premises | Overlooking, loss of privacy and loss of light to the houses either side and behind |
| External appearance | Design and architectural features of the principal elevation, and of any side elevation that fronts a highway |
| Air traffic and defence | Relevant near aerodromes and technical sites; rarely an issue in residential streets |
| Protected views | Whether the enlarged house would affect a protected vista |
External appearance is the matter that decides most cases. On a uniform street of 1950s semis, one house two storeys taller than its neighbours reads very differently from a single lift across a varied frontage. Drawings that show the street elevation, with the houses either side accurately surveyed, answer that question far better than a plan of the new floor.
How much does a Class AA application cost?
The council fee for prior approval under Part 1, Class AA is £249 from 1 April 2026, the same as a larger home extension under Class A.
| Route | Council fee in England from 1 April 2026 |
|---|
| Prior approval: additional storeys on a home (Part 1, Class AA) | £249 |
| Prior approval: larger home extension (Part 1, Class A) | £249 |
| Householder planning application, single house | £548 |
Sources: Planning Portal, Application Fees England, 1 April 2026; MHCLG, Planning fees: annual indexation from 1 April 2026. Both retrieved 12 September 2026.
The fee is the smallest cost in the project. The drawings for an upward extension need a measured survey of the house and its neighbours, existing and proposed elevations, and a structural engineer's view on whether the walls and foundations can take the extra load before anyone commits to a design.
Do you still need building regulations approval?
Yes. Class AA deals with planning only. The new storeys are building work, so they need approval from building control in the same way as any extension.
Two parts of the regulations shape the design early. The first is structure: existing foundations on a two-storey house were sized for two storeys, and the engineer has to prove they can carry more, or show how they will be strengthened. The second is fire. Approved Document B Volume 1 treats a house with a storey more than 4.5 m above ground differently from a lower one, and it steps up again above 7.5 m.
| Highest storey above ground | Means of escape in Approved Document B Volume 1 |
|---|
| One storey more than 4.5 m | A protected stairway, separated by fire resisting construction (minimum REI 30) at every storey, running to a final exit (paragraph 2.5) |
| Storeys more than 7.5 m | As above, plus an alternative escape route from each storey over 7.5 m, or a sprinkler system to BS 9251 throughout (paragraph 2.6) |
Source: Approved Document B Volume 1, 2019 edition incorporating 2020, 2022 and 2025 amendments, retrieved 12 September 2026.
A two-storey house raised by two storeys will almost always have floors above 7.5 m, so the escape strategy has to be settled before the stair is drawn. Our guide to means of escape in a loft conversion explains the protected stair in more detail. If the house is attached, the Party Wall etc. Act 1996 is likely to apply to work on the shared wall as well.
Is Class AA better than a mansard or a planning application?
Only where the house qualifies and the design fits inside the conditions. Where it does not, a full application is usually the more honest route.
| Question | Class AA prior approval | Householder planning application |
|---|
| Victorian or Edwardian terrace | Not available (built before 1948) | Available; a mansard roof extension is the usual form |
| Conservation area | Not available | Available, judged against the area's character |
| Side windows on the new floor | Not allowed | Negotiable |
| What the council can weigh | The listed prior approval matters only | The whole development plan |
| Council fee | £249 | £548 |
For the London terrace, the realistic choice is between a dormer loft under Class B and a mansard under a full application. Our comparison of dormer and mansard roof extensions covers that decision. If you are not sure whether your house keeps its permitted development rights, a Lawful Development Certificate or a short call before survey will save a wasted set of drawings.
Our published fee for a mansard or upward roof extension drawing package starts at £1,575, and a loft conversion set starts at £1,225. Both are listed on our pricing page, and you can request a fixed quote for an upward extension with your postcode and the year the house was built.
Written by Benyamen Shaukat, Technical Architect & Platform Founder. Fact-checked against Schedule 2, Part 1, Class AA of the Town and Country Planning (General Permitted Development) (England) Order 2015 and Approved Document B Volume 1.
