Class AA upward extension of a house: the real limits

Class AA lets some houses gain an extra storey without a full planning application, but it is far narrower than the headlines suggested when it arrived in 2020. Most houses fail one of its conditions before you get to the design. If your house was built before 1 July 1948 or after 28 October 2018, sits in a conservation area, is listed, or already stands close to 18 metres, the class is closed to you.

Where it does apply, it is not a free pass either. You still need prior approval from the council, which means an application, drawings and a written decision, and the council can refuse on grounds that include the effect on your neighbours' light and privacy.

What Class AA permits

Class AA of Part 1 of Schedule 2 to the General Permitted Development Order 2015 permits the enlargement of a house by adding storeys on its principal part. Two additional storeys where the house already has two or more, or one additional storey where the house has a single storey.

The new storeys have to sit on the principal part of the house, which is the main body of it rather than a rear outrigger or a single storey side addition. Engineering operations are permitted only so far as they strengthen existing walls and foundations for the new work.

What Class AA does not do is create a new flat. Adding dwellings on top of an existing building sits under Part 20, which has its own conditions and its own prior approval process.

The age and terrace conditions

The construction date window is the condition that ends most enquiries. The house must have been built between 1 July 1948 and 28 October 2018. A Victorian terrace, an Edwardian villa and a 1930s semi are all outside it, which removes a large share of the English housing stock in one line.

Then come the land conditions. Class AA is not available on article 2(3) land, which covers conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites. It is not available on a listed building or within its curtilage, on a scheduled monument or within its curtilage, or on a site of special scientific interest.

And it can only be used once. A house already extended upwards under the class cannot use it again.

Height limits against the original roof

Three separate height tests apply, and the lowest one governs.

That last row catches designers who plan a generous top floor. If the existing first floor has a 2.4 metre ceiling, the new storey cannot exceed it, whatever the 3 metre cap says.

Measure from the existing roof rather than from the ridge of a later extension, and take the terrace comparison from the adjoining house as built rather than as you remember it. On a stepped terrace on a slope, that comparison is where the scheme usually falls over.

Prior approval matters the council assesses

Prior approval is a real application with a defined scope. The council cannot refuse because it dislikes the principle, but it can and does refuse on the listed matters, which are:

• The effect on the amenity of adjoining premises, including overlooking, privacy and loss of light
• The external appearance of the house, particularly the design and architectural features of the principal elevation and of any side elevation fronting a highway
• Air traffic and defence asset impacts
• Protected views, where the site lies within one of the designated view corridors

Loss of light and overlooking are the grounds that generate most refusals. A new storey with a rear window looking straight into a neighbouring garden 9 metres away is vulnerable, and the fix is usually obscure glazing, a repositioned opening or a set-back at the rear. Our note on how the prior approval procedure actually works covers the mechanics common to all these classes.

Construction hours and the required statement

Class AA carries an obligation other householder classes do not. The application must include a report for the management of the construction, covering the proposed hours of operation and how the effects of noise, dust, vibration and traffic on neighbours will be limited.

Write it properly rather than as a paragraph of assurances. Give the working hours you actually intend, the access route for deliveries, where materials will be stored, how the scaffold will be erected on a shared boundary, and how dust and debris will be controlled. Neighbours read this document, and an application that treats it as a formality invites objections on the amenity ground the council can refuse under.

Allow eight weeks for determination, and wait for the written decision before starting work rather than relying on any assumption about what happens when a period expires.

Why most houses do not qualify

Run the tests in this order and you will save yourself a wasted week.

1. Was the house built between 1 July 1948 and 28 October 2018
2. Is it a house rather than a flat or maisonette
3. Is it outside article 2(3) land, and not listed, scheduled or within the curtilage of either
4. Will the finished building stay under 18 metres
5. Does the increase over the existing roof stay within 3.5 or 7 metres
6. Does the new storey stay within 3.5 metres of the adjoining house in a terrace or pair
7. Can the new storey work within the existing floor to ceiling height
8. Has the class already been used on this house

Fail any one and the class is unavailable. It is not a balancing exercise.

Remember too that permitted development is a legal question, not a council opinion, and the only formal way to confirm that works are lawful without permission is a lawful development certificate.

Alternatives when the class does not apply

A full planning application is the obvious route, and for a pre-1948 house in a conservation area it is the only one. The advantage is that the council assesses the whole design in the round rather than against a fixed list, so a well-designed additional storey on a suburban street can succeed where the class would not have applied at all.

Before that, look at the other householder classes. Class B permits a loft conversion with a rear dormer within a volume allowance of 40 cubic metres for a terrace and 50 cubic metres for other houses, with nothing extending beyond the roof plane on the highway side and nothing above the highest part of the existing roof. Class A covers rear and side extensions and interacts with your existing extensions in ways set out in our notes on Class A rear extension limits and how previous extensions count against your allowance.

Two further costs are worth budgeting now. An upward extension creates floorspace, so the Community Infrastructure Levy may be engaged, with a residential extension exemption that has to be claimed before work starts. And work on a shared wall or close to a boundary brings the Party Wall etc. Act 1996 into play, which is a separate process from planning.

As an illustrative example, a 1960s semi-detached house with a 2.5 metre first floor ceiling height, an adjoining neighbour whose ridge sits 400 mm lower, and no protected designations could gain one storey adding perhaps 2.8 metres, but not two. The binding constraints are the neighbour comparison and the existing ceiling height rather than the 18 metre cap.

Where a full application turns out to be the route, a Planning Statement setting the design against local plan policy does more work than most householders expect. 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.