How Much Does a Side Extension Cost for a Semi-Detached House?

Adding a side extension to a semi-detached house is an increasingly popular way to create more living space without moving home. Whether you want a bigger kitchen, a spacious dining area, or an extra bedroom, a side extension can transform your property to better suit your lifestyle. However, one of the first questions homeowners ask is: how much will it cost?

Understanding Side Extensions: What Are They?

Side extensions are additions built on the left or right side of your home, typically taking advantage of available space beside your property. For semi-detached houses, which share one wall with a neighbour, side extensions can be a smart way to increase floor area without affecting the front or rear garden.

These extensions can vary in size and design complexity — from a simple single-storey extension to a more elaborate double-storey build. The choice depends on your needs, budget, and planning permissions.

One of the key advantages of side extensions is their versatility. Homeowners can transform these spaces into a variety of functional areas, such as a spacious kitchen, a home office, or even a playroom for children. By extending sideways, you can create a seamless flow between the existing structure and the new space, enhancing the overall functionality of your home. Additionally, side extensions often allow for larger windows or bi-fold doors, which can flood the new area with natural light and provide a connection to the outdoors.

Moreover, side extensions can significantly increase the value of your property. In many cases, a well-designed extension can yield a substantial return on investment, particularly in urban areas where space is at a premium. Potential buyers often look for homes with additional living space, making side extensions a desirable feature. However, it is crucial to consider local building regulations and consult with professionals to ensure that your extension not only meets aesthetic goals but also adheres to safety and zoning laws.

Typical Cost Range for Side Extensions

The cost of a side extension can vary widely depending on several factors, including the size of the extension, the complexity of the design, the materials used, and the location of your property.

According to the National Association of Building Contractors, side extensions generally cost between £1,000 and £2,000 per square metre. However, more recent data from 2025 suggests that single-storey extensions across the UK typically range from £1,800 to £3,500 per square metre, while double-storey extensions are often more cost-efficient per square metre, ranging from £1,650 to £3,250.

For side-return extensions, which involve extending into the narrow space between the side of your house and the boundary wall, costs tend to be higher due to structural complexities, averaging between £2,400 and £3,500 per square metre.

What Does This Mean in Practice?

To put this into perspective, a small side extension outside London might start at around £37,500, while a larger, more complex extension could cost up to £161,000. London and other high-cost areas will typically see prices at the upper end of these ranges.

Additionally, the choice of materials can significantly influence the overall cost. For instance, opting for high-quality brickwork or bespoke glazing can elevate the aesthetic appeal of your side extension but will also increase expenses. Furthermore, energy-efficient features, such as triple-glazed windows or eco-friendly insulation, may require a higher initial investment but could lead to long-term savings on energy bills. Homeowners should also consider the potential for increased property value that a well-designed extension can bring, often recouping a significant portion of the initial outlay upon resale.

Another factor to consider is the planning permission and regulatory requirements that may apply to your project. Depending on the scale and nature of the extension, you may need to navigate local council regulations, which can add both time and cost to the project. Engaging with an experienced architect or builder early in the process can help ensure that your design meets all necessary guidelines, potentially saving you from costly modifications later on. Understanding these nuances can empower homeowners to make informed decisions that align with their budget and vision for their property.

Factors Influencing the Cost of Your Side Extension

Size and Scale

Unsurprisingly, the larger the extension, the higher the cost. A modest single-storey side extension will be less expensive than a double-storey build, which involves more materials, labour, and often more complex planning.

Design Complexity

Simple rectangular extensions are generally cheaper than those requiring unusual shapes, multiple rooflines, or additional structural work. Side-return extensions, for example, often need extra support and careful design, which drives up the price.

Materials and Finishes

The choice of building materials significantly affects costs. Brick matching your existing home, high-quality windows, and premium flooring will all add to the budget. Conversely, more economical materials can reduce costs but might impact the overall look and longevity of the extension.

Location

Building costs vary across the UK. Properties in London and the South East tend to have higher labour and material costs compared to other regions. Planning permission requirements and local building regulations can also influence the timeline and expenses.

Value Added: Is a Side Extension Worth It?

A well-designed and well-built side extension can do more than just increase your living space — it can significantly boost the value of your semi-detached house. Experts from Sussex Structural Engineers highlight that such extensions often offer excellent returns on investment, making them a smart choice for homeowners looking to improve their property’s market appeal.

Fluent Architectural Design Services Ltd also note that side extensions add functional space that enhances everyday living, which can be a strong selling point if you decide to move in the future.

Planning and Budgeting Tips

Get Multiple Quotes

Always obtain several quotes from reputable builders or contractors. This helps ensure you get a fair price and understand the range of costs involved.

Consider Hidden Costs

Remember to budget for planning permission fees, architect or structural engineer fees, and any unexpected costs that might arise during construction.

Think Long-Term

Investing in quality materials and professional design can save money in the long run by reducing maintenance and increasing your home’s value.

Conclusion

Side extensions offer a fantastic opportunity to expand your semi-detached home and improve its value and functionality. While the cost varies depending on size, complexity, materials, and location, typical prices in 2025 range from around £1,800 to £3,500 per square metre for single-storey extensions, with side-return extensions costing more due to structural challenges.

Careful planning, budgeting, and choosing the right design can help you make the most of your investment. Whether you’re dreaming of a larger kitchen, a new living room, or extra bedrooms, a side extension could be the perfect solution to enhance your home.