House Extension Cost: An In-Depth Look at the Price of Adding Living Space
House extensions are the most comprehensive home improvement option.
By extending, you can add significant value to your home while creating new space and design opportunities.
Extending enables homeowners to increase the size of their property without moving, which saves time, money and stress!
Extensions also rate as an excellent investment, boosting a
home’s value by some 20% and showing an
ROI of approximately 70%.
Here, we’ll answer the all-important question: how much does a house extension cost?
Average house extension costs
Most house extensions cost between £1,350 to £3,000/m2 of new extended floor space. The upper-end of the price bracket applies to high-end house extensions, especially those in London and the South East or Home Counties.
Some standard house extension sizes are as follows:
- Small extension: 15m2
- Medium-sized extension: 25m2
- Large extension: 35m2
- Extra-large extension (two-storey): 60m2
So, house extensions can cost as little as £20,000 for a smaller extension to over £180,000 for larger extensions. Prices in London and other areas where the cost of living is much higher than average can soar, reaching in excess of £5,000 per square metre.
Generally speaking, £2000/m2 is considered the current cost benchmark for most house extensions in the UK.
The following table shows the average costs of house extensions by the square metre and by the project:
| Average house extension conversion prices | Rear or single-storey | Side return or wrap around | Double/ second-storey |
|---|---|---|---|
| Per metre cost (standard) | £1,250 to £1,800 | £1,350 to £1,900 | £1,350 to £1,900 |
| Per metre cost (high-end) | £1,750 to £3,000 | £1,850 to £3,000 | £1,750 to £3,000 |
| — | |||
| Small 15m2 extension (standard) | £18,750 to £27,000 | £20,250 to £28,500 | £20,250 to £28,500 |
| Small 15m2 extension (high-end) | £26,250 to £45,000 | £27,750 to £46,000 | £27,750 to £45,000 |
| — | |||
| Average 25m2 extension (standard) | £31,250 to £45,000 | £33,750 to £47,500 | £33,750 to £47,500 |
| Average 25m2 extension (high-end) | £43,750 to £75,000 | £46,250 to £77,500 | £46,250 to £77,500 |
| — | |||
| Large 35m extension (standard) | £43,750 to £64,000 | £47,250 to £66,500 | £47,250 to £66,500 |
| Large 35m extension (high-end) | £61,250 to £105,000 | £64,750 to £108,500 | £64,750 to £108,500 |
In short, expect to pay around £25,000 to £75,000 or so for a straightforward 15m2 to 25m2 extension. For most, it’s sensible to budget around £50,000 for a straightforward extension, but every case is different!
Factors affecting the cost of a house extension
There are many factors affecting the average cost of house extensions. Key factors include:
- The type of extension (e.g. rear extension, wrap-around, side return, etc).
- The number of storeys.
- The size of the extension.
- The build quality, especially with regard to windows, doors, insulation, soundproofing and interior.
- Additional interior design costs and fittings.
- Roof type and material.
- Any added features, like kitchens, bathrooms, walk-in wardrobes, etc.
- Access.
- Location.
- Any bespoke ideas or features.
Admin and planning costs you’ll need to factor in include:
- Architects fees (approximately 7% of construction costs).
- Plans and drawings (Fast Plans provides a cost-effective property survey, drawings and engineer calculations service, which rolls several costs into one affordable package).
- Planning Permission (approximately £238 in England. Use the Planning Portal calculator here).
- Party Wall Agreements (see our ultimate guide here).
- Lawful Development Certificate (currently £108).
- Extension and unoccupied home insurance. (varies depending on how long the project will take, the cost, and whether you’ll remain at home or not).
Here’s a detailed breakdown of some of the above costs:
Design and planning
Designing and planning the extension usually involves a combination of site surveys, architectural designs and drawings and engineer’s calculations.
This is where
Fast Plans’ house extension service comes into play. We offer an inspection, survey, professional architectural drawings and engineer’s calculations for most popular types of house extensions.
We also manage the Planning Permission process, where applicable, to take the load off your shoulders. This accelerates the house extension process and gives you the best chance of navigating the planning stage quickly and easily.
Extension type
Not all extensions are built the same. For example, rear extensions are probably the simplest, costing marginally less than wrap-around or side return extensions. Any knocking through of interior walls will also add to the cost. Double-storey extensions are the most expensive, but building a second storey on top of an existing storey is cheaper.
Materials
High-end materials such as weathered, aged or red brick, flint and other natural stones cost more. It’s usually necessary to match the original building materials (especially in some areas such as National Parks). If your home has a more niche or high-end exterior and roof, you’ll probably need to match it.
Groundwork and site access
Building on flat surfaces is easier than building on gradients. Site access also affects cost, especially when it’s necessary to crane building materials over a garage, house or other structure.
Windows, doors and insulation
Windows, doors and insulation all greatly impact the cost of an extension. High-end insulative doors and windows cost three to four times more than their inexpensive counterparts. Insulation and soundproofing are also highly variable.
Extra features such as lantern windows, skylights, and double and bifold doors also significantly affect cost.
Fixtures and fittings
Interior fixtures and fittings like lights, handles, skirting, and cornices all affect cost. Higher-end interior features increase the overall cost.
Extra features
House extensions often include extra features like bathrooms, kitchens, bi-folding doors, walk-in wardrobes, etc. Here are some typical extra costs for factor in for specific projects:
| Standard | High-End | |
|---|---|---|
| Bathroom | Add £2,500 to £5,000 | Add £4,000 to £15,000 |
| New kitchen | Add £8,000 to £25,000 | Add £12,000 to £50,000 |
| Bifolding doors | Add £3,000 to £8,000 | Add £5,000 to £12,000 |
| Walk-in wardrobe | Add £750 to £1,500 | Add £1500 to £3,000 |
Summary: How much does an extension cost?
Extensions in the UK usually cost at least £20,000 for a smaller 15m2 extension, extending up to £75,000 or more for larger 25m2+ extensions.
This doesn’t include the costs of extra features, whether that be a kitchen, bathroom or something else.
Despite the initial layout, house extensions rate as an extremely solid investment for the present and the future. Extending has to be factored against the cost of moving if you need additional space and are weighing up the two. There’s a good chance that your extension will pay for itself from the eventual sale of your home.
And, of course, in the meantime, you can enjoy your extra space without uprooting from your location at all!
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