If you’re thinking of expanding your home, you’ll probably first ask, “How much does an house extension cost?” House extensions are increasingly becoming a popular alternative to moving, as property prices rise and space becomes more valuable. But as exciting as it seems, knowing the true cost is important to making the right decision.
In this blog, we will shed light on everything you need to know about how much does an extension cost, with average costs, how much it costs per square metre, and what to expect as far as overall costs. Whether you are planning a single-storey or double-storey extension, it will give you an idea so that you can plan your budget accordingly.
Understanding House Extension Cost in the UK
House extension costs in UK can differ significantly depending on your location, the size of your project, and the type of extension you want to build. The average cost for a standard extension ranges from £1,500 to £2,500 per square metre and typically includes all aspects of the build, such as construction, materials, and basic finishes.
When you are asking how much does an extension cost, do consider that a small 20m² single-storey extension will typically cost you around £30,000–£50,000; however, more complicated builds or high-end finishes can take this to well over £70,000 or more.
What’s the Extension Cost Per Square Metre?
The most useful method for working out costs is to calculate the extension cost per square metre. In 2025, the average costs for extensions in the UK are as follows:
- Basic quality: £1,500/m²
- Mid-range: £1,800–£2,200/m²
- High-end: £2,500+/m²
While these are just estimates and do not include extras such as kitchen fittings, underfloor heating, structural changes, etc., it is a good starting point when figuring out how much does an extension cost.
Single Storey Extension Cost UK
A single storey extension cost UK typically falls between £30,000 and £60,000, depending on finishes and materials.
Popular single-storey extensions include:
- Kitchen and dining room extensions
- Utility rooms
- Rear or side return extensions
- Garden rooms
These are the easiest to construct, which keeps labour and design costs lower. However, adding features like bi-fold doors or skylights can increase expenses, so be sure to get the right quotes from multiple contractors when thinking about how much does an extension cost.
Double Storey Extension Cost UK
A double storey extension cost UK is more expensive, but it gives greater value per square metre since you are expanding both ground and upper levels at once.
Typically, this can range from £60,000 to £100,000+, with per m² costs usually being 20-30% lower than building two separate single storey extensions.
If you are extending both your living space and adding an extra bedroom or bathroom, this can greatly increase your home’s value in terms of sale. When estimating how much does an extension cost, a double storey project may give better ROI over time.
What Influences the Total Cost?
When learning how much does an extension cost, many factors can impact the final figure:
- Location: The cost of buildings or extensions in London or the South East is vastly more than in the North.
- Size/Complexity: Extensions that are larger or are of non-standard shape increase costs.
- Access: Difficulty may increase labour and equipment hire costs.
- Planning and Permissions: architectural drawings, planning permission, and building regulation costs may total several thousand pounds.
- Foundations and Soil Conditions: Poor ground conditions or sloping sites may require more groundwork.
- Finishes and Fixtures: High-spec finishes and custom fittings can add tens of thousands to the project total.
You must always keep these in mind when calculating how much does an extension cost, so you do not under-budget.
Average Cost of a House Extension
To give you a quick reference, here is a summary of the average cost of a house extension based on current market data in the UK:
For a small, single-storey extension (approximately 15–20 square metres), costs range from £25,000 to £45,000, which generally include the basic build, materials, and finishes.
A medium-sized extension of 25 to 30 square metres may cost between £40,000 and £60,000 if you are adding some extra features.
Double-storey extensions are generally much pricier than single-storey builds but frequently offer a superior cost-per-metre value. Double-storey extensions can cost anywhere from £60,000 to £100,000 or more, depending on the configuration and internal specifications.
Home Extension Costs UK vs Moving
In many parts of the country, house extensions in the UK are more affordable than the costs of moving house, especially when you consider estate agent fees, stamp duty, legal fees, and removal fees.
A typical move can cost somewhere in the region of £10,000-£30,000 in fees alone, not to mention the stress and disruption you would also face. Extending a current space allows you to stay in the home and area you love and refit your space to suit your lifestyle.
So, when thinking about how much does an extension builder charge, you should also ask, “How much will it increase value?” Often, extensions enhance comfort and provide considerable market value increases, especially for extra bedrooms, larger kitchens, or improved layouts.
Building Costs Per Sq Metre UK
To get a wider view of UK construction prices, look at the building prices per sq. metre in the UK. These are averages around the country:
- Basic build: £1,400–1,800/m²
- Standard build: £1,800–2,200/m²
- Luxury build: £2,500–3,000+/m²
These amounts are reflective of extension costs and serve as a useful comparator when thinking about how much does an extension cost for new builds and existing properties.
Conclusion
So, what is the cost of an extension in the UK? There is no definitive answer, as each extension is individual. The majority of homeowners spend between £30,000 and £100,000 on extensions, depending on the size, scope, and complexity of the design. If you understand all the variables involved, the cost of your location, the size of the extension, the chosen materials, and the necessary permissions, you can then budget accurately and ensure your project adds long-term value.
Extending your home is still one of the most popular and effective ways to expand living space, increase property value, and adapt it to a changing lifestyle.