House Extension Cost: An In-Depth Look at the Price of Adding Living Space

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:

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!