House extension: increase your space and your home's value

A house extension is new building work added on to an existing building, this can be on the roof; to the front; side or back of your home; or underneath it, if you’re installing a basement.

Examples of house extensions:

• A structure attached to the sides of an existing building to create a new space.
• The addition of a new storey to an existing building.
• Raising the roof of an attic to convert it into a habitable space.

The following are not house extensions:

• The transformation of an existing attic into a habitable space that doesn't alter the home’s exterior.
• The creation of an intermediate floor within a building.
• The enclosing of a former open space, such as a corridor or a porch.
• The renovation of an existing space for a new purpose, for example, transforming a garage into a gym.

There are four categories of extension and each suit different purposes and property configurations:

A single-storey extension might enlarge your kitchen or living room or add a new room, such as an office or playroom. It extends on a single level at the front, rear or side of your property.

Using large bay windows or bi-folding doors, a single-storey extension can help your indoor space flow into your outdoor space, bringing more natural light into your home and making it more energy efficient.

A multi-level home extension adds two or more storeys to your home, increasing your living space and, perhaps, bedrooms and bathrooms. It can help you make the most of a scenic view and add that wow-factor.

This scheme requires detailed planning to ensure that its design complements your existing structure and complies with building regulations.

A side extension makes use of the typically narrow space at the side of a property. Often called a side-return extension on Victorian properties, it's ideal for terraced or semi-detached homes where rear expansion is limited.

A side extension can create a wider kitchen, an additional utility room or even a small office. It can unlock the hidden potential of your home's footprint.

A wraparound extension combines a rear and side extension to form an L-shaped space, dramatically increasing your floor area.

It's perfect for creating an open-plan living space integrating your kitchen, dining and entertaining space and offering a modern layout that optimises natural light and flexible living.

1. Personalisation: A house extension lets you tailor your home to meet your lifestyle needs. It could provide a larger kitchen for entertaining, a quiet home office or a playroom for your children, for example.
2. Adapting to changing circumstances: Life rarely stands still — whether you're growing your family, welcoming an elderly relative or making space to enjoy a new hobby, an extension helps your home evolve with you.
3. Add value to your home: A newly modernised, extended home is more attractive to potential buyers, which means it can command a higher price.
4. Energy efficiency: When your house extension features roof windows, energy-efficient heating systems and high-quality insulation you optimise its thermal comfort and reduce your energy bills.
5. Improved comfort and well-being: When you add living space, your home is less cluttered and better organised and, if your extension is well-designed, it will also maximise your natural light and improve your ventilation, which contribute to a healthy home.

With high-quality building work, improved functionality and fantastic interior design, your extension can be a clever investment that significantly increases your home’s value.

Depending on where you live, a house extension can add significant value to your home when you come to sell. For a rough guide of what this could be, estimate the cost of your extension per square metre then ask a professional property valuer to value your home and ask them how much value they think you can add by extending it. You can then roughly judge whether adding an extension will increase its value.

Unpredictable external factors can impact your property’s worth too and you may have to pay capital gains tax on any profits when you sell.

An extension can become a source of rental income, depending on what it can be used for. You could, for example, rent out an additional bedroom and bathroom, either as a short-term holiday let or as a longer-term tenancy, or you could rent out your garage to a local craftworker.

You may have to pay tax on any income from this, however.

Here are the three most common house extension styles:

Classic and elegant, this extension seamlessly integrates with the architecture and materials of your existing structure, you should find it hard to see where the new build starts and the original ends. This style is particularly well-suited to historic or period homes.

This minimalist extension style features clean lines, large expanses of glass and modern materials, such as steel and concrete. It can contrast beautifully with the architecture of an older home or make a house built in the last 50 years feel uber modern.

Contemporary, glass-box style extensions maximise the flow of natural light into your home and can brighten up your whole living space.

This style combines traditional materials with modern elements to create a striking visual contrast. A glass extension added to a stone building, for instance, provides an interesting mix of old and new.

Choosing the right design starts with understanding your space and how best to enhance it. From assessing your home's layout and potential build areas to selecting styles and materials that complement your property, here’s how to plan a well-integrated, functional extension.

Assess your home's current layout and the space available for you to build on to work out the best option for extending – building out, up or under your home – and check with an architect whether this is feasible.

Also, think about your home's position and how your extension will impact access to natural light and the flow between spaces.

Planning for daylight is essential at this stage—incorporating roof windows early in the design process can significantly enhance brightness, energy efficiency, and the overall comfort of your new space. Roof windows also complement façade windows by bringing in balanced, even light from above, reducing the need for artificial lighting. Beyond comfort, they add value by creating a brighter, more desirable living environment.

To choose the architectural style, materials and finishes for your extension, look at similar projects for inspiration.

Explore design trends in homes and interior magazines and use Pinterest to create a decor mood board.

With computer-aided design (CAD) software you can experiment with different layouts to optimise the space and functionality of your extension.

An architect will guide you through any technical, planning and design challenges you have with your extension and ensure that it optimises your use of space and natural light and meets building regulations. They can also apply for planning permission, deal with any Party Wall Awards and supervise the build.

Can I build under permitted development rights? Will I get planning permission for my extension? What you need to know before beginning:

Your local government website offers guidance on when planning permission is required for an extension.

It’s likely you will need planning permission if you:

• Make a significant change to your home’s structure or planned use.
• Make a change that affects your neighbours.

Here are the main aspects to consider when building your extension:

• Safety

  • Structure: your extension’s structure and foundations must support its structural loads, which includes the weight of the building, furniture, people, etc. and environmental forces, such as wind and snow.

  • Fire protection: construction materials and the building design must meet fire safety standards.

• Energy efficiency:

  • Insulation: insulation of the walls, roof and floor must meet building regulations and will also help to minimise heat loss and reduce heating and cooling bills.

  • Ventilation: good ventilation ensures optimal indoor air quality and removes moisture from the air to avoid damp and mould. Building regulations may require you to include extractor fans or natural ventilation, such as windows or roof windows.

  • Natural lighting: maximising the use of daylight through windows, roof windows, sun tunnels or other openings will make your home more comfortable and inviting and cut your lighting bills.

• Accessibility:

  • Access for all: if your extension includes a studio or office to which you invite the public, it may need to be accessible for people with reduced mobility. Your architect can guide you on this.

It’s important to consider how your extension may impact your neighbours. If you’re building close to your neighbour’s property or on to a shared wall, you will probably have to issue them with a Party Wall Notice and consult with them to draw up a Party Wall Award.

If the extension could obstruct your neighbours’ views or block light from their gardens, for example, they may object to your planning application. Early and open communication with your neighbours can help prevent conflict and ensure your project progresses smoothly.

The cost of a house extension varies depending on many factors, including its size, the materials used and the complexity of the design. As, mentioned earlier, you can get a rough estimate of build costs by looking at the latest build costs per square metre. Checkatrade provides average build costs per square metre.

To get an accurate estimate, get quotes from several building companies. These should cover everything from the preparatory work to finishing. And, remember to add indirect costs, such as architect and building control fees.

When budgeting for a home extension, allow a safety margin of about 20% for unforeseen costs.

These tips will help you save on extension costs without compromising on quality.

1. Accurate planning: detailed planning of your project should identify unnecessary costs and optimise material usage.
2. Modular design: consider a modular or prefabricated design to reduce construction costs, material use and build time.
3. Choice of materials: select economical building materials. Certain types of wood or composite materials are cheaper than others, for example.
4. Bulk buy: take advantage of volume discounts; using the same paint colour throughout, for example, helps cut costs.
5. Reclaimed materials: scour reclamation yards for floorboards, doors and windows, etc. and online marketplaces for tiles and paint that people sell on when they over buy.
6. Take on some of the work yourself: if you have DIY skills and want to reduce labour costs, consider doing some of the work, like painting and wallpapering.
7. Choosing a contractor: Get several quotes from recommended building contractors and negotiate before going ahead.
8. Search for grants and financing: There are grants and financial support available for elements of a build that make your home more sustainable, such as heat pumps and insulation

In addition to using your personal savings to finance your home extension, there are other options, including:

• Consumer loans and mortgages can provide extra funds, but there are additional fees, and they will need to be paid back with interest.
• Renegotiate your current home mortgage to free up more funds.

Evaluate the available options based on your financial circumstances and choose the one that minimises risk and meets your needs.

Look for builders who have experience building extensions like yours.

Ask friends or family for recommendations and check online reviews.

Once you have a list of potential candidates, review their portfolios of past projects and seek references.

Get detailed quotes from multiple builders to compare costs, timelines and approaches. These should include everything that your architect has outlined so there are no hidden surprises and you can compare them accurately.

Before settling on a builder check their qualifications, insurance and certifications.

Make sure they are registered with the relevant professional bodies and have professional indemnity insurance to cover any potential damage to your property and neighbouring properties during the work.

Once you have chosen a builder, draw up a detailed contract to specify the scope of work, deadlines, costs and payment terms. This protects both parties if a dispute arises.

How can you ensure that work progresses as planned and stays within budget? Planning, coordination and communication are key here.

Establish a clear timeline with your builder, including key milestones and deadlines.

This helps track progress and highlights potential delays.

Your extension project may require the hiring of specialised craftspeople, such as carpenters and plumbers.

Good coordination avoids scheduling conflicts and helps the work run smoothly.

You always have the final say, but it can be helpful to have your architect project manage your scheme

Even with the best planning, delays and problems can occur.

Communicate regularly with your builder to stay informed about your project’s progress and discuss potential solutions to problems as they arise.

Flexibility and cooperation between all parties will help keep the project on track.

Looks, energy efficiency or durability — which materials should you choose?

Classic brick looks great and offers good thermal insulation, strength and durability.

Bricks aren’t cheap and they require a lot of labour to install, but a brick extension offers superior insulation and can increase your home’s resale value.

Wooden frames are quick to construct, light and flexible, which can reduce labour costs.

Wood is an excellent natural insulator and creates a warm atmosphere.

To reduce your carbon footprint, choose wood from suppliers who buy from sustainably managed forests.

Often made of cement or wood, prefabricated panels are manufactured in factories and assembled on-site. This reduces construction times and minimises waste, which is great for the environment.

The initial cost may be higher, but the faster build times and precision manufacturing can offset this.

Lightweight, flexible, durable, energy efficient and low maintenance, aluminium has a striking contemporary look.

Plus, it's a sustainable and environmentally friendly investment as it can be recycled and requires minimal maintenance.

Sustainable construction methods have minimal environmental impact. They use environmentally friendly materials, optimise energy efficiency and reduce waste.

They can significantly reduce your energy bills and eco-building materials are also better for your health. Grants may be available for some of the materials.

You want to create a smooth transition between your extension and the existing parts of your home, so go for pieces that connect the new space with your home's existing style and use design elements and colours present in the existing scheme.

Here are some ideas for selecting furniture and decorations that complement new extensions:

• Furniture: Choose functional pieces with sculptural lines and a refined mix of materials — slim-profile metals, polished woods and soft-matte finishes that bring a sense of calm to your space.
• Decor: Your walls are an integral part of your extension’s design, opt for a simple neutral paint or wallpaper. Display carefully chosen ornaments on built-in shelving and use sculptural floor or wall-mounted lights to add warmth and architectural interest.

• Furniture: Opt for solid wood furniture with a visible grain or a lightly-aged finish. Pair with relaxed upholstery, natural textures and woven rugs to create a warm, lived-in feel.
• Decor: Layer in tactile details that add character — hand-thrown ceramics, ironwork and houseplants. These elements will ground your space and strengthen its connection to nature.

Will your extension be a living room, kitchen, bedroom or attic? Discover all our best decorating tips in the Decoration section of VELUX Magazine!

Once your extension is complete, you need to have building control sign off on the final inspections and obtain the necessary certifications on electrics, plumbing, etc.

A Building Control Completion Certificate certifies that your extension has complied with all applicable building codes, safety standards and environmental regulations.

It’s crucial for proving that any modifications made to your property are lawful and you will need to show this when you sell your home and possibly if you remortgage it.

If your home is in a Conservation Area or is listed, you may also need to certify that your extension meets those requirements. Your architect can advise you on this.