Class AA upward extension of a house: limits and prior approval

Class AA of Part 1 lets you add up to two storeys to a house without a full planning application, but only if the house was built between 1 July 1948 and 28 October 2018, sits outside a conservation area and every other designated area, and stays under 18 metres once extended. Those three tests knock out most of the houses people ask about. If yours survives them, the route is a prior approval application, and the council may only refuse it on a short list of grounds.

The right is narrower than the coverage suggests. It is also useful on a 1960s or 1970s two-storey house on an ordinary suburban street, which is exactly the stock it was written for.

What Class AA permits and on which houses

Class AA allows additional storeys on the principal part of a house. Where the house already has two or more storeys, you may add up to two. Where it has one storey, you may add one.

The house has to have been built after 1 July 1948 and before 28 October 2018. That single condition removes every Victorian terrace, every interwar semi and every 1930s bungalow, which is a large share of the housing people want to extend upward. It also removes anything built in the last few years.

The new storeys go on the principal part of the house, which means the main roof rather than an existing single-storey rear extension or an outrigger. And the house must not already have had storeys added under this right, so you cannot use it twice.

The storey and height limits

Where the house adjoins another building, a further test applies: the extended roof must not exceed the height of the adjoining building's highest roof by more than 3.5 metres. On a terrace or a semi that test usually bites before the 7 metre allowance does, which in practice limits many adjoined houses to a single additional storey.

Measure from the existing roof rather than from the ridge you intend to build. Getting the baseline wrong is the commonest reason a Class AA scheme turns out not to be permitted development at all after the builder has quoted.

Houses the class excludes

The right does not apply on article 2(3) land, which covers conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites. It does not apply to listed buildings or buildings within the curtilage of a listed building, to scheduled monuments, or on sites of special scientific interest, safety hazard areas and military explosives storage areas.

Check the designation before you spend anything. A conservation area boundary can run down the middle of a street, and the free constraint checks on planning.data.gov.uk and the National Heritage List for England will answer the question in minutes.

Two further traps. An Article 4 direction can remove permitted development rights in a specific area, and it will not show on a national map. And a planning condition on the original permission, common on estates built in the 1970s and 1980s, may have removed householder rights entirely.

The prior approval matters

The council cannot refuse a Class AA application because it dislikes the idea. It may only consider the matters the order lists, which are the impact on the amenity of adjoining premises including overlooking, privacy and loss of light, the external appearance of the house including the design of the principal elevation and any side elevation fronting a highway, air traffic and defence asset impacts, whether the siting makes the additional storeys impractical or undesirable, and protected views where those apply.

Amenity is where most refusals happen, and the argument turns on the same tests neighbours raise on any extension: whether habitable room windows are overlooked at close range, whether the addition breaches a 45 degree line taken from the nearest neighbouring window, and whether afternoon sunlight to a garden is lost. Sectional drawings showing the relationship to next door's windows do more good than any amount of written argument.

External appearance is the second battleground. A new storey with a different window rhythm, a flat roof on a street of pitched roofs, or a rendered top floor above brick will attract an objection on appearance, and the council is entitled to refuse on it. Designing the addition to read as part of the house is not an aesthetic preference here. It is the test.

Neighbour notification and the response period

The council notifies the occupiers of adjoining premises, displays a site notice and allows 21 days for representations. Anyone may comment, and comments that raise the listed matters carry weight while comments about property values, construction noise or the applicant's character do not.

The order sets the period within which the authority must determine the application, and you must not begin work before you have the approval or that period has run out. Read paragraph AA.2 of Part 1 on legislation.gov.uk for the exact procedure, because it differs from the shorter procedure used for other classes and it has been amended. The application itself needs a written description of the works, plans and elevations of the existing and proposed house, and the developer's contact details, and it carries a fee set in the current fee schedule on the Planning Portal. The procedural mechanics are the same family as those set out in the piece on how the prior approval procedure actually works.

Engineering and construction conditions

Class AA comes with conditions attached, and ignoring them makes the development unlawful even though the approval was granted.

1. Materials must be of a similar appearance to those used on the existing house.
2. No window may be installed in a wall or roof slope forming a side elevation, other than a roof light.
3. Before work begins, a report for the management of construction must be provided to the authority, covering hours of operation and how noise, dust, vibration and traffic effects on occupiers of the building and adjoining premises will be reduced.
4. Engineering operations are limited to strengthening existing walls and foundations.
5. The development must be completed within three years of the approval date, and the authority must be notified of completion.

The construction management report matters more than it looks on a house that stays occupied during the works, which is most of them. So does the engineering limit: if the structural engineer concludes that the existing foundations need underpinning beyond strengthening, the scheme falls outside the class.

Building Regulations apply separately and in full. A third storey usually triggers a protected stairway and fire escape requirements under Part B that change the internal layout, and a party wall award under the Party Wall etc. Act 1996 will be needed on a semi or a terrace.

Where an application is the better route

If the house predates 1948, sits in a conservation area, adjoins a taller neighbour, or you want a rear projection as well as the extra storey, apply for planning permission instead.

A householder application takes eight weeks rather than the prior approval period, costs more and exposes the design to the full range of policy. But it lets you argue the case, adjust the scheme during determination and combine the works in one consent. It also produces a permission that covers exactly what you drew, rather than a class you may later be found to have stepped outside. The supporting text for that route sits in a short Planning Statement alongside the drawings.

Where the position is arguable either way, a lawful development certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that the works would be lawful, and it is worth the fee before you exchange on a builder's contract. The same principle applies to any allowance question, including how previous extensions count against your allowance.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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