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?

The first thing to note is, you can only build without planning permission if your property benefits from permitted development rights. If it does, then the amount you can build varies depending on where you live in the UK.

In most parts of the UK, if you have permitted development rights you can build a single-storey rear extension that measures 4 metres long from the original back wall of the house if detached, or 3 metres if attached, by the full width of the original house by 4 metres in height, reducing to 3 metres high if you are building within 2 metres of a boundary.

In England, it is possible to increase the depth of the extension to 8 metres if your house is detached, or 6 metres if attached, but you will need Prior Approval from the council.

A two-storey extension can project up to 3 metres from the back wall of the original house and as high as the existing eaves.

A dormer roof extension can be up to 50 cubic metres in volume for a semi-detached or detached house, or 40 cubic metres for terrace housing.

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Permitted development rights

Permitted development rights are a national grant of planning consent by Parliament which allows households (and businesses) to carry out certain types of development without the need for planning permission.

Full permitted development rights apply to most single dwelling houses.

Rights are restricted on houses built on designated land – Conservation Areas, National Parks, Areas of Outstanding Natural Beauty, the Broads and World Heritage Sites– which means certain types of work may require planning permission. 

Permitted development rights do not apply to flats, maisonettes, multiple occupancy houses, houses formed under permitted development (for example a converted shop), and certain properties with legal restrictions.

Extending a listed building requires listed building consent, which means permitted development rights are largely void.

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The original house rule

Under permitted development rules, new extensions are measured from the ‘original house’ which refers to the house as it was first built or as it stood on 01 July 1948.

This means that if you already have an existing extension on your house and are looking to extend out further, you measure from the wall the existing extension is built off, not from the extension itself.

For example, if you live in a Victorian terrace house which has an existing extension built after 01 July 1948 and you want to replace it with a larger extension, the depth of the new extension will be measured from the back wall of the Victorian structure, not the back wall of the existing extension.

However, if the existing extension was built before 01 July 1948, it would qualify as an original part of the house, and you would measure out from the back wall of the existing extension. 

In addition to the above, new extensions cannot exceed 50% of the land around the original house. Any extensions built after 01 July 1948 will contribute to your site allocation and need to be deducted from the final calculation.

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Permitted development 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 of the original house if detached, or 3 metres if the house is attached.
• 4 metres high if more than 2 metres from a boundary.
• 3 metres high if within 2 metres of a boundary.

In England, larger extension rights apply. Single-storey rear extensions can be up to 8 metres long if the house is detached, or 6 metres long if attached, but require Prior Approval from the council before work can start on site. More on this below.

In Northern Ireland, a single-storey rear extension must be at least 3.5m from the rear boundary if the house backs onto a road.

Additional rules to note:

• Larger extensions are generally not permitted on designated land.
• ‘Wrap around’ extensions are not permitted.
• The footprint of the extension must not exceed 50% of the land around the original house.
• Verandas and balconies are not permitted.

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Permitted development rules for two-storey extensions 

The rules for two-storey extensions are as follows:

• The roof eaves and ridge must not be higher than the original house and the roof pitch should match the original house. The eaves height is limited to 3 metres if the extension is within 2 metres of a boundary.
• The rear wall of the extension must not exceed 3 metres from the rear wall of the original house if it is more than 7 metres from the rear boundary opposite the rear wall inEngland, 10 metres in Scotland or 10.5 metres in Wales.

Additional rules to note:

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

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Permitted development rules for side extensions

Side extensions must not exceed:

• A single storey in height.
• 3 metres from the original house.
• 4 metres in height, or 3 metres to the eaves if the extension is within 2 metres of a boundary.
• Half the width of the original house.

Other rules to note:

• Side extensions are not permitted on houses on designated land.
• They cannot extend beyond the elevation of the house.
• Materials must be similar to the existing house.

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Permitted development rules for roof dormers

Roof dormers are limited to:

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

Other rules to note:

• The height of the dormer cannot exceed the existing ridge height of the house.
• The dormer cannot extend beyond the existing roof slope that forms the principal front elevation.
• The rear wall needs to be inset 0.2 metres from the existing eaves.
• Side windows must be obscure glazed and non-opening unless they are 1.7 metres above finished floor level.
• Balconies and roof terraces are not permitted.
• Juliet balconies are permissible.
• Proposed materials must be similar in appearance to the existing house.  

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Permitted development rules for outbuildings

Another great way to add floor area to your home is to build an outbuilding. Permitted development rules for outbuildings, which include garden rooms, are as follows:

• The footprint of an outbuilding can be up to 50% of the total external area of the original site, less the area of any extensions.
• The roof can be 4 metres high if it is dual pitched, or 3 metres for any other roof shape if the outbuilding is more than 2 metres from a boundary.
• If the outbuilding is within 2 metres of a boundary, the height must not exceed 2.5 metres, regardless of the shape of the roof.

Other rules to note:

• If on designated land, the building area is restricted to 10 square metres if it is more than 20 metres from any wall of the house.
• It must be single storey.
• The building must not be positioned forward of the front elevation of the house.
• No verandas or balconies are permitted.
• Raised platforms above 300mm are not permitted. 

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Prior Approval for larger house extensions

Most types of house extension that comply with permitted development rules can be built without consulting the local planning authority. However, if you live in England and are proposing a larger single-storey extension, you will need to seek ‘Prior Approval’ from your council before works can take place on site.

To seek this form of consent you need to submit an application to determine if prior approval is required for a proposed: Larger Home Extension.

In many ways the process is similar to householder planning application, albeit in a slightly more streamlined format. The council evaluates the impact of the proposals on surrounding properties and consults neighbours for comment. The maximum permitted size is only possible if no valid objections are received.

If the council deems your proposals ‘do not require prior approval’, it means they are permissible, and you are free to build.

If your plans ‘require prior approval’, it means you will need to modify your design or apply for planning permission to build out the proposals.

More information can be found at Permitted development rules for house extensions.

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Other considerations before you build

Even if you can build your extension without planning permission, you will need to gain other approvals before starting construction.

Building regulations approval is required to ensure your extension is safe for habitation and meets current technical standards. A simple extension can be carried out on a simple Building Notice while a Full Plans application is more appropriate for complex building work.

If you are building close to a boundary you may need to form a party wall agreement with your neighbour(s) before starting work on site. Having a broad understanding of the Party Wall Act will help you plan your project.

Appointing professionals, such as an architect, structural engineer and party wall surveyor will ensure your project runs smoothly. They will produce drawings and calculations to demonstrate your proposals meets building legislation and obtain the necessary approvals to allow building work to take place.

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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.

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

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