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What Size Extension Can I Build Without Planning Permission

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  • 25-07-2022
What Size Extension Can I Build Without Planning Permission

What size extension can I build without planning permission? Find out more about extending without planning permission as well as the restrictions to extending without planning permission?

Bob Trimble is a chartered architect registered with RIBA with 30 years experience in the industry. Trimble Architects work throughout Hounslow, Twickenham, Richmond, Kingston Upon Thames, Teddington and the surrounding areas of London.

So you're looking to build a house extension but aren't sure about the restrictions? It is vital that before any building process begins, you are aware of the limitations, from planning permission to budgeting.

In this article, we discuss some of the restrictions when planning to build a new extension. Information regarding extensions on Listed Buildings is also discussed.

What size extension can I build without planning permission

Most single-storey extensions, like conservatories or garages, are covered by 'permitted development rights under a few circumstances. With permitted development, you won't require planning permissions so long as the proposed extensions remain in the guidelines listed below.

The below legislations cover all houses in England, apart from those restricted by Article 4 directions (Town and Country Planning Order 1995: General Permitted Development).

Attached Extension: Do not extend beyond the rear wall of the original build by 3 metres.

Detached Extension: Do not extend beyond the rear wall of the original build by 4 metres.

Height: Extension roofs and ridges shouldn't exceed the height of the original build. Eaves should not exceed 2 metres in height of the 3 metres boundary.

Single-Storey Extension: No higher than 4 metres, width no greater than half the size of the original build.

Side Or Front Of House Extensions: Should be closer to the original property than any public highway. 

Benefits of extending without Planning Permission

Applying for and receiving the correct type of planning permissions for an extension can be time-consuming and costly. Any permissions should be dealt with in the earliest stages of planning, as without the correct permits, building work cannot begin, or any work started may be halted and reversed.

Some extensions built on a residential property can be regarded as permitted development, this type of extension doesn't require planning permission applications.

A few obvious benefits of having a permitted development extension built are avoiding the twelve-week long wait for planning permission, reducing stress from construction hold-ups and allowing the project to get underway quicker. 

Moreover, without a need for planning permissions, you could save hundreds of pounds on application fees and reduce time spent during the earlier stages of planning!


What Size Extension Can I Build Without Planning Permission?

Restrictions to extending without planning permission

There are a few restrictions when building an extension without planning permission, albeit much less than with planning permission. Below we discuss these restrictions, so you can be aware and prevent the project from being unexpectantly affected. 

 The extension should be single-storey and no higher than 4 metres.

 Does not extend past any wall facing a road to the front or side of the property. Extensions built to the rear of the property are generally the most popular, such as conservatories.

 Similar exterior materials should be used to create a sense of flow between the original build and the new extension, or it should be finished to match the existing property.

 Side extensions should be no wider than half of the original properties' width. 

Terraced & semi-detached homes

Following the guidelines when building an extension on a terraced or semi-detached home, without any planning permissions or neighbour permission, the build should be single storey, no taller than 4 metres and no wider than 3 metres.

An exception is when the affected neighbours have agreed to the extension using the 'neighbour consultation scheme', allowing the build to extend as far as 6m. Once this scheme is applied to affected neighbours, they have 21 days to object; after this local authority has another 21 days to give the all-clear.

If planning permissions are not required for the proposed extension, but you still want to be certain that the use of the existing building and extension is lawful, you can apply for a 'Lawful Development Certificate (LDC)'. This certificate can take up to 8 weeks to be issued, so ensure you apply for this in the early planning stages to avoid expensive and timely hold-ups. 

Detached homes

Similarly to other restraints, the rules for building an extension on a Detached house reiterate the extension shouldn't extend over 4 metres to the rear of the property without the necessary permissions. However, when using the Neighbour Consultation Scheme, all affected parties can reach as far as 8m if agreed upon.

Rural & Listed homes

For those lucky few living in beautiful areas of nature conservation, building an extension comes with many more setbacks than other property types. Firstly, as with more general extensions, the new building must be no taller than 4 meters and should be single-story. Whatsmore it's important that natural and locally sourced exterior materials are used, the new extension cannot be clad with certain materials.

Realistically the new building should closely match the exterior materials of the existing building for a sense of flow and authenticity. No matter what house extension project you're planning, you must receive 'Listed Building Consent' from local planning authorities if the original building being extended is Listed in the UK. The local authorities may even contact Historic England or the National Amenity Society for historic properties that are considered more treasured and historically significant than other listed buildings.

Furthermore, any property close to or bordering a Listed building may still require more specific planning permissions, even after building consent.

Carrying out any kind type of work on Listed buildings without the correct permissions is a criminal offence and can lead to prosecution of anyone involved. If you decide to begin work without suitable permissions, you can be ordered to reverse the processes, costing a lot of time, money and patience. If you plan to sell your Listed property, any illegal works with no Listed Building Consent can cause issues and a reduction in the selling price. 

Emergency works for Listed Properties

If you believe a Listed Property needs Emergency work, you are able to complete this under the Planning (LB&C) Act 1990 for emergency works (Section 9). However, you are requested to contact a Built Heritage Team prior to commencing works for advice and assistance.

If you cannot contact the Local Planning Authority before work commences, may be owed to a national holiday or out of hours, you should contact them at the earliest opportunity you get. As completing work on a Listed building without consent is a criminal offence, the reasoning for emergency work must have a real sense of urgency, and be convincing and clear.


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If you are looking for architectural services throughout Hounslow, Twickenham and the greater London area, then get in contact with our team today. We can offer advice and begin guiding you towards making the right decision.


Bob Trimble is a chartered architect registered with RIBA. Bob Trimble has 30 years of experience working with residential and commercial property projects. For 4 years, Bob Trimble has worked from his housing association and private architectural practice for clients throughout Hounslow, Twickenham, Richmond, Kingston Upon Thames, Teddington and the surrounding areas of London.