In the UK, there is a lot of scope to extend a house without going through the planning process. A question that architects are frequently asked is: how big can I build without planning permission?

If your property benefits from permitted development rights, you can build a single-storey rear extension up to 4 metres long for detached houses or 3 metres for attached properties. In England, this can be extended to 8 metres (detached) or 6 metres (attached) with prior council approval. The size limits vary depending on your location within the UK, but generally, you can add a two-storey rear extension up to 3 metres from the original house and a dormer roof extension up to 50 cubic metres (40 for terraced houses).

Continue reading to discover more.

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Permitted development rights

Permitted development (PD) rights are a national planning consent granted by Parliament, allowing households (and businesses) to carry out certain developments without planning permission. These rights generally apply to most single dwelling houses.

However, they are restricted for properties on designated land, such as Conservation Areas, National Parks, Areas of Outstanding Natural Beauty, the Broads, and World Heritage Sites, where planning permission may still be needed.

PD rights do not apply to flats, maisonettes, houses in multiple occupancy, homes created through permitted development (e.g. a converted shop), or properties with legal restrictions.

For listed buildings, you need listed building consent, meaning PD rights rarely apply.

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The original house rule

Under PD rules, extensions are measured from the 'original house', meaning the property as it was first built or as it stood on 1 July 1948.

If you already have an extension and plan to build further, you measure from the wall of the original house, not the extension. For example, if you live in a Victorian terrace with an extension built after 1 July 1948, and you want to replace it with a larger one, the depth will be measured from the back wall of the original Victorian structure, not the current extension.

However, if the existing extension was built before 1 July 1948, it counts as part of the original house, and you would measure from its back wall.

Additionally, new extensions cannot cover more than 50% of the land around the original house. Any extensions built after 1 July 1948 count towards this limit and must be included in your site calculations.

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PD rules for single-storey extensions

In most parts of the UK, single-storey extensions can be up to:

• 4 metres long from the original back wall for detached houses, or 3 metres for attached houses.
• 4 metres high if more than 2 metres from a boundary.
• 3 metres high if within 2 metres of a boundary.

In England, larger single-storey rear extensions are allowed – up to 8 metres long for detached houses and 6 metres for attached homes – with Prior Approval from the council. In Northern Ireland, rear extensions must be at least 3.5 metres from the rear boundary if backing onto a road.

Additional rules:

• Larger extensions are generally not permitted on designated land.
• ‘Wrap around’ extensions are not allowed.
• Extensions cannot exceed 50% of the land around the original house.
• Verandas and balconies are not permitted.

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PD rules for two-storey extensions

The rules for two-storey extensions are:

• The roof eaves and ridge must not be higher than the original house, and the roof pitch must match the existing house. Eaves height is limited to 3 metres if within 2 metres of a boundary.
• The rear wall of the extension can’t project more than 3 metres from the rear wall of the original house if it is more than 7 metres from the rear boundary in England, 10 metres in Scotland, or 10.5 metres in Wales.

Additional rules:

• Two-storey extensions are not permitted on designated land or to the side or front of the house.
• Verandas and balconies are not allowed.
• First-floor side windows must be obscure glazed and non-opening unless they are over 1.7 metres above the internal floor level.

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PD rules for side extensions

Side extensions must:

• Be a single storey.
• Extend no more than 3 metres from the original house.
• Be no taller than 4 metres, or 3 metres to the eaves if within 2 metres of a boundary.
• Be no wider than half the width of the original house.

Other rules:

• Side extensions are not allowed on designated land.
• They cannot extend beyond the house’s front elevation.
• Materials must match the existing house.

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PD rules for roof dormers

Roof dormers are limited to:

• 40 cubic metres of additional roof volume for terraced houses.
• 50 cubic metres for semi-detached or detached houses.

Other rules:

• Dormers cannot exceed the existing ridge height.
• They cannot extend beyond the front roof slope.
• The rear wall must be inset by 0.2 metres from the existing eaves.
• Side windows must be obscure glazed and non-opening unless 1.7 metres above floor level.
• Balconies and roof terraces are not allowed, but Juliet balconies are permitted.
• Materials must match the appearance of the existing house.

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PD rules for outbuildings

Permitted development rules for outbuildings, including garden rooms, are:

• The footprint can cover up to 50% of the original site, minus any extensions.
• The roof can be 4 metres high if dual pitched, or 3 metres for other roof shapes, if more than 2 metres from a boundary.
• If within 2 metres of a boundary, the height must not exceed 2.5 metres, regardless of roof shape.

Other rules:

• On designated land, outbuildings are limited to 10 square metres if more than 20 metres from any house wall.
• The outbuilding must be single storey and positioned behind the house's front elevation.
• Verandas, balconies, and raised platforms above 300mm are not permitted.

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READ: Do Garage Conversions Need Planning Permission?

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Prior Approval for larger house extensions

Most house extensions that meet permitted development rules can be built without consulting the local planning authority. However, in England, if you’re planning a larger single-storey extension, you’ll need to seek 'Prior Approval' from your council before starting work.

To do this, you must apply for a Larger Home Extension. This process is similar to a householder planning application but more streamlined. The council assesses the impact on neighbouring properties and consults neighbours. If there are no valid objections, the extension can proceed at the maximum permitted size.

If your plans 'require prior approval', you may need to adjust the design or apply for planning permission.

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Other considerations before you build

Even if your extension doesn't need planning permission, other approvals are required before construction.

Building regulations approval ensures the extension is safe and meets technical standards. Simple projects can be done under a Building Notice, while more complex ones need a Full Plans application.

If you're building near a boundary, a party wall agreement with neighbours might be necessary. Understanding the Party Wall Act can help with planning.

Hiring professionals like an architect, structural engineer, and party wall surveyor ensures smooth project management. They’ll produce drawings and calculations to meet building regulations and secure approvals before work begins.

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The information provided in this article is intended for general guidance and educational purposes only. We advise you appoint a skilled residential architect to provide specific expert advice for your project.

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About the author

Aron Coates is an architect with over two decades of experience, encompassing projects ranging from the restoration of historic buildings to the design of contemporary homes.

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