Part 1 Class A or Class AA for more space

Class A gives you space on the ground and needs no application in most cases. Class AA gives you a whole extra storey and always needs prior approval. If you are choosing between them for a typical two-storey house, the question is usually not which is more generous, but whether your house qualifies for Class AA at all, because the age condition and the designated land exclusions rule out a large proportion of the housing stock.

Both are classes in Part 1 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015. Both are hedged with limits and conditions, and failing any one of them means you need planning permission instead.

What Class A covers at ground level

Class A permits the enlargement, improvement or other alteration of a house. The limits that matter in practice:

• no more than half the area of land around the original house may be covered by buildings
• the extension must not be higher than the highest part of the existing roof
• a single-storey rear extension may project 4 metres from the original rear wall on a detached house, or 3 metres on any other house, with a maximum height of 4 metres
• a rear extension of more than one storey may project 3 metres, and must be at least 7 metres from the rear boundary
• within 2 metres of a boundary, the eaves height of the extension must not exceed 3 metres
• a side extension must be single storey, no more than 4 metres high, and no wider than half the width of the original house
• materials must be similar in appearance to the existing house
• no verandas, balconies or raised platforms above 300 millimetres

"Original house" means as built, or as it stood on 1 July 1948. Previous extensions count against the allowance, which is the single most common reason a householder's calculation goes wrong.

What Class AA permits above

Class AA permits the enlargement of a house by constructing additional storeys on the principal part of the building. A detached house with two or more storeys may add up to two storeys. A terraced or semi-detached house with two or more storeys may also add up to two. A single-storey house of either type may add one.

The height controls are the binding ones. The extended house must not exceed 18 metres overall. Where one additional storey is added, the height of the extended house must not exceed the existing by more than 3.5 metres. Where two are added, the limit is 7 metres. Internal floor to ceiling height in the new storey is capped at 3 metres, or the existing floor to ceiling height, whichever is lower.

The new storeys have to sit on the principal part of the house, so you cannot build above a rear or side extension. Engineering operations other than those needed to strengthen existing walls and foundations are excluded.

Height, storey and age conditions on Class AA

The age condition catches most people. The house must have been built between 1 July 1948 and 28 October 2018. A Victorian terrace does not qualify. Nor does a house completed after that date.

Other conditions in the class:

• the house must not be a listed building, in the curtilage of one, or a scheduled monument
• the land must not be article 2(3) land, which covers conservation areas, National Landscapes, National Parks, the Broads and World Heritage Sites
• the materials must be of a similar appearance to the existing house
• windows in a wall or roof slope forming a side elevation must be obscure-glazed and non-opening below 1.7 metres above the new floor
• the external appearance must not include a veranda, balcony or raised platform

Where any of those fails, the route is a full planning application. A refusal of prior approval can also be appealed, and the householder route is faster than the standard one, as our comparison of the householder appeal and the fast-track commercial route explains.

Prior approval matters for an upward extension

Class AA is not automatic. You apply to the council for prior approval, with a fee and a drawn submission, and the council considers a defined list of matters rather than the whole planning merits.

Those matters include the impact of the additional storeys on the amenity of neighbouring premises, covering overlooking, privacy and loss of light, the external appearance of the house, including the design and architectural features of the principal elevation and any side elevation fronting a highway, air traffic and defence asset impacts, and whether the land is in a protected view. The council also has to consider the provision of adequate natural light in all habitable rooms.

That amenity test is where upward extensions fail. On a tightly spaced estate, a two-storey addition can take light from a neighbour's rear windows, and a Daylight and Sunlight Assessment prepared to the recognised building research guidance is often the only way to settle the point.

Submit it with the application rather than waiting to be asked. A prior approval application refused on amenity grounds you could have modelled is an expensive lesson.

Designated land exclusions

Both classes are restricted on article 2(3) land, but differently. Class AA is excluded altogether. Class A continues to apply in conservation areas and the other designated areas, with side extensions and cladding removed, and rear extensions limited to a single storey.

Article 4 directions bite further. A direction can withdraw householder rights in a defined area, which is common across conservation areas, and the effect is that work otherwise permitted needs an application. Check the council's register of directions and the designations dataset for the address before you design anything.

A Lawful Development Certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that proposed works are lawful without permission. On anything marginal, it is worth the fee.

Using both on the same house

The classes are cumulative in principle. You can have a Class A rear extension at ground level and a Class AA storey above the principal part of the house, provided each complies with its own limits.

Two interactions to watch. The 50 per cent curtilage limit in Class A counts all buildings, so a large outbuilding reduces what is left for an extension. And the Class AA storey cannot sit over the Class A extension, so the plan form of the new upper floor is constrained by the original footprint.

Sequence matters for evidence. Keep dated photographs and drawings of the house before each phase, because the "original house" baseline and the existing roof height are the two facts an enforcement officer will test years later.

Deciding which gives more usable space

Measure it rather than assuming. A 3 metre single-storey rear extension across the full width of a semi typically adds a usable room plus circulation. A full additional storey adds more floor area, but a good part of it goes to a staircase, a landing and the head height losses at the eaves.

Compare on these grounds:

1. Net usable area after circulation, not gross footprint.
2. Cost per square metre, which is usually higher for an upward extension because of structural work, scaffolding, roof removal and temporary weather protection.
3. Disruption, since an upward extension often means moving out.
4. Certainty, because Class A needs no application in most cases while Class AA needs a prior approval decision.
5. Resale logic, where an extra bedroom may be worth more than a larger kitchen, or the reverse, depending on the street.

Space standards are a useful sanity check on the result, and our comparison of the nationally described space standard and local minimum sizes sets out the figures rooms are measured against.

My own view, having seen both go wrong: for most two-storey houses built in the qualifying window, Class A gives better value per pound, and Class AA earns its keep only where another bedroom is the specific objective and the neighbouring daylight position is comfortable.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. Enter your postcode on PlanScope to see which reports your project is likely to need, and draft the Planning Statement if you end up applying for permission. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.