House extensions: how much does a house extension cost and do I need planning permission?

A well-designed and constructed house extension will not only increase your space and bring in more light, it could also increase your home's value

House extensions: how much does a house extension cost and do I need planning permission?

If you’ve outgrown your home, your first thought is to find somewhere bigger. But high house prices and moving costs mean climbing the property ladder is harder than ever, so many people are expanding and extending their homes instead.

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According to LV= General Insurance, nearly a third of homeowners have opted to improve their property with a house extension rather than move because it was too expensive to do so.

How much does a house extension cost?

You’ll typically pay £1,000 to £1,500 per square metre for an extension, depending on the complexity of the project, materials and location (London and the South-East are more expensive than other parts of the UK). A 4 x 6 metre extension will start from about £25,000 plus VAT, which includes foundations, new walls and subfloor, insulation, plumbing and electrics, but not decorating. Allow £25-plus per square metre for flooring, at least £6,000 for a kitchen, and don’t forget to budget for design, planning and building control fees. Once the work’s complete, your new extension could increase your home’s value by up to 15 per cent.

Do you need planning permission for a house extension?

Many builds are covered by permitted development. So, provided you don’t live in a listed property or conservation area and your proposed extension won’t take up more than half the garden, doesn’t exceed four metres in height or more than six metres beyond the original wall in a terraced or semi-detached house (eight metres in a detached house), it’s unlikely that planning permission will be required. But you will need building regulations approval, which deals with safety and energy efficiency. Planning and building control applications can be made online, but it’s worth picking up the phone to double-check exactly what’s expected.

What do you need to consider before starting the extension?

Before you go ahead, there are a number of factors to consider that could affect the cost, design and success of the project. When building on a wall shared with your neighbours in a terraced or semi-detached house, or close to the boundary of both properties, you’ll require a party wall agreement detailing the work. If your neighbours dispute it, you’ll have to fork out for separate surveyors to protect everyone’s interests.

You should also look into whether your boiler and electrical system can cope with the additional structure, and whether you’ll need extra insurance during the works. Think also about access to your property for deliveries, and where materials and contractors’ tools can be stored during the build.

Types of extension

Thanks to our love affair with open-plan layouts, many of us extend to accommodate an all-in-one kitchen, dining and family room opening into the garden. Depending on the size and type of the property, this is normally achieved by building into the side return, or at the back.

What is a side-return extension?

A side-return extension makes excellent use of the redundant area alongside a terraced house and doesn’t encroach into the garden.

What other types of extension are there?

A rear extension is ideal for detached houses or ones lacking a side return, while a wraparound extension is a combination of rear and side-return extensions to maximise space. Families requiring more bedrooms should look into the viability of a two-storey extension or building on top of an existing flat roof.

Who should design my house extension? Do I need an architect?

There are several routes you can take to get your extension designed and built. You could design a simple structure yourself and get plans for the builders to work from drawn up by an architectural technician; you could employ a specialist design and build company to take care of every stage from initial ideas through to the kit out; or you could ask an architect to create a bespoke extension, then find the builder and other sub-contractors yourself. For example, you may need a surveyor for a party wall agreement and a structural engineer to advise on stability. When hiring people you haven’t used before, do as much research as you can – ask for recommendations, look at examples of previous projects and agree costs in advance. Here’s some advice on how to find a trusted tradesperson

Should the extension blend in with the rest of the house or stand out?

Once you’ve decided on the extension’s position and footprint, you can either match the materials with the original building so that it blends in seamlessly or go for a complete contrast. Timber and metal cladding are very contemporary, and a glass box extension will flood the interior with light. That said, however, glazing is more expensive than solid walls and your privacy may be compromised in an urban area, so a more traditional brick build with full-width bi-fold doors across the back of the house might be more practical.

The roof of the extension may be flat, sloping, pitched, vaulted or even curved, but shouldn’t interfere with your home’s first-floor windows. Light can be introduced inside via roof lights and/or lanterns.

“Owners should know what they want to get out of the space, to understand and have a good feel for how they use the house, and how they want to make it better,” says James Bernard, company director of Plus Rooms.

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“They need to consider how the new space is going to connect with the original rooms of the existing property. This helps to avoid dark spots: a clever design will maximise light in areas of the original house that would otherwise have been compromised by the new structure.”