Class AA upward extension: measuring the roof correctly

Class AA measures your new roof against the highest part of your existing roof, not against the eaves, not against the main ridge if a rear outrigger happens to be taller, and not against your neighbour's roof unless the house is attached to something. One additional storey may add up to 3.5 metres to that highest point. Two additional storeys may add up to 7 metres. The finished house must not exceed 18 metres overall. Those three figures decide most Class AA upward extension measurement questions before any design work starts.

Class AA sits in Part 1 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015, as amended in 2020. It is permitted development, but only with prior approval, so you still submit an application and still wait.

What Class AA permits on a dwellinghouse

Class AA allows a dwellinghouse to be enlarged by building additional storeys above the principal part of the 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.

Several gates have to be passed before any of it applies:

• The house must have been built between 1 July 1948 and 28 October 2018. A Victorian terrace is out. So is anything completed after that October date.
• The house must not already have been extended upwards above the original dwellinghouse, whether under Class AA or by planning permission.
• The right does not apply where the dwelling exists only because of a prior approval conversion under Part 3, so an office or barn converted to a house cannot then go up under Class AA.
• Flats are excluded. Upward extension of a purpose-built block sits in a separate class in Part 20, with its own limits.

The additional storeys must go on the principal part of the house, which means the main body of the building rather than a rear addition, a garage wing or a later single-storey extension.

Measuring from the highest part of the existing roof

The comparison the Order asks for is between the height of the highest part of the roof of the house as enlarged and the height of the highest part of the roof of the existing house. Two consequences follow, and they catch people out in opposite directions.

A house with a tall main ridge and a lower rear outrigger is measured from the main ridge. That is helpful, because the datum is the highest point you have rather than the part you are building over.

A bungalow with a shallow hipped roof has a low datum, and 3.5 metres above a 4.5 metre ridge gives you very little usable headroom once the new floor structure and roof build-up are accounted for. Many bungalow owners discover that a Class AA storey is geometrically possible but produces rooms with a ceiling they cannot stand up under at the perimeter.

Heights in Part 1 are taken from ground level, measured from the highest part of the surface of the ground next to the building. On a sloping plot that matters: measure from the high side, not from the low side where the number flatters you.

Chimneys, flues and aerials are not normally treated as part of the roof for this purpose. If your datum depends on a chimney stack, ask the council in writing before you rely on it.

The relationship to adjoining properties

This is the limb that stops most terraced and semi-detached schemes. Where the house is not detached, the highest part of the enlarged roof must not exceed the highest part of the roof of the highest adjoining building by more than 3.5 metres.

Read that carefully. It is not 3.5 metres above your own roof. It is 3.5 metres above the tallest building joined to yours. On a uniform two-storey terrace the two tests produce roughly the same answer and one storey works. Where your neighbour's house is already a storey taller, you get more. Where you are the tall one in the row, the adjoining-building test bites first and the permitted increase shrinks.

Two additional storeys are therefore realistically a detached-house proposition. A terraced house attempting a second new storey breaches the adjoining-building limit.

Each new storey is separately constrained as well: no more than 3 metres floor to ceiling, and no more than the floor to ceiling height of any existing storey, whichever is lower. A 1960s house with 2.4 metre ceilings cannot have a 2.8 metre new one.

Engineering operations and structural strength

Class AA permits the engineering operations reasonably necessary to build the storeys, and those operations are confined to works within the curtilage to strengthen existing walls and foundations. Underpinning and pier strengthening inside your own boundary are within the right. Works that reach beyond the curtilage are not.

This is the same drafting approach taken to conversions elsewhere in the Order, and the test of what is reasonably necessary is read the same way, so the reasoning in Class Q structural works and what is reasonably necessary transfers directly.

Get a structural engineer to confirm the existing foundations and wall construction can take two extra floors before you spend anything on drawings. Cavity walls built for two storeys frequently cannot, and the cost of the strengthening is what kills otherwise viable Class AA schemes rather than the planning process.

Party wall matters sit entirely outside planning. A terraced or semi-detached upward extension will engage the Party Wall etc. Act 1996, which runs on its own timetable.

Prior approval matters including external appearance

The council cannot refuse your Class AA proposal on general planning grounds. It can only consider the prior approval matters set out in the class, and the list is longer than people expect:

1. Impact on the amenity of adjoining premises, including overlooking, privacy and loss of light.
2. The external appearance of the house, specifically the design and architectural features of the principal elevation and of any side elevation fronting a highway.
3. Air traffic and defence asset impacts.
4. Whether a protected view identified in the relevant Direction would be affected.
5. The provision of adequate natural light in all habitable rooms of the house.

That last one is easy to fail. Putting a new storey over a house with a deep plan can leave a stairwell-lit inner room with no window, and the officer is entitled to say so. Show window positions and room layouts for every habitable room, including the ones you are not changing.

On amenity, a daylight and sunlight appraisal to the BRE guidance is not required by the Order, but on a tight urban plot where a neighbour's rear window sits close to the party boundary, submitting a short Daylight and Sunlight Assessment is often what turns a contested prior approval into an approved one.

Materials must be of similar appearance to those used on the existing exterior. Any window in a wall or roof slope forming a side elevation must be obscure-glazed, and non-opening unless the openable parts are more than 1.7 metres above the floor of the room.

Excluded buildings and designated land

Class AA 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 on a site of special scientific interest. It does not apply to a listed building or to a building within the curtilage of a listed building.

Check the designation before the design, using the national planning data service and the council's own constraints map. The same logic that governs outbuilding limits in the 50 per cent curtilage rule for garden outbuildings applies here: designation removes the right outright rather than merely tightening it.

If the right is unavailable, a full householder planning application is the route, and that needs a Planning Statement addressing local plan design and amenity policies rather than a prior approval checklist.

Construction management requirements

Class AA carries a condition that no development begins until the developer has provided the council with a report for the management of the construction, setting out the proposed hours of operation and how any adverse effect of noise, dust, vibration and traffic on occupiers of the building and on owners and occupiers of adjoining premises will be mitigated.

Write it properly. It is a short document, it is the only part of the process where you control the narrative about disruption, and a council that has a sensible construction report in hand is much less likely to find an amenity objection persuasive. State working hours, delivery arrangements, scaffold position, dust suppression and a contact point.

Prior approval applications under Class AA are determined within 56 days, and the development must be completed within three years of the approval date.

A prior approval is not a certificate of lawfulness. If you need certainty that what you have built is lawful, a Lawful Development Certificate under section 192 or section 191 of the Town and Country Planning Act 1990 is the only formal way to establish it.

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

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