House extensions – what you’re paying for

A house extension might sound like a lot of work but it’s often a much more convenient alternative than moving.

You’ll not only inject a new lease of life into your home and save on the hefty costs that come with relocating, but you could add value to your humble abode too.

How much you pay for your extension depends on the scale of work you’re planning and the materials you wish to use.

If you’re looking to keep expenses down, timber or pre-fabricated house extensions are options to consider.

Timber extensions are built on-site and are very well-insulated – delivering eco-friendly thermal efficiency.

Going down this pre-fabricated route will also mean less disruption to your home, as these structures are mainly assembled off-site. And despite their simple, low-hassle construction, effectively pre-made extensions provide impressive heat and sound insulation.

At the opposite end of the scale are more traditional options – extensions built from bricks or glass. The former can be expensive, but there’s a reason this method has stood the test of time - because brick and blockwork buildings are arguably more robust. Glass extensions are a popular choice in modern homes as they flood living spaces with natural light.

If you’re looking for ideas or advice, check out our ideas page for more on home extensions.

How much does a kitchen extension cost?

The average cost of converting an extension into a kitchen is £10,000.

Planning the layout of a kitchen extension is crucial to ensure that it will blend into your home and won’t look like an afterthought. The key is to consider every square inch of your proposed kitchen’s internal layout or floorplan.

Think about cupboard space, room for a kitchen table and chairs and whether you’d like an island or a breakfast bar. By taking the time to think through how you’ll use your kitchen and how much space you really need, you’ll ensure you’re not wasting space or paying for additions you don’t actually require when it comes to converting your existing extension.


How much does a single-storey extension cost?

The average cost of a single-storey extension is £30,000.

Single-storey extensions at the rear of a house are popular choices, with full-length sliding doors fitted to allow natural light to spill into your living spaces. This type of extension is more affordable than it once was. If you’re seeking to create a relatively simple space, without lots of fixtures and fittings, a single-storey extension will typically set you back £1,765 per square metre.

Most single-storey extensions (projecting less than 4 metres from the rear of your home) will be classed as a permitted development, meaning you shouldn’t need to obtain additional planning permission. You can check standard permitted development rights on the planning portal website and on our information page. It’s always worth confirming with your local planning authority that your development rights haven’t been restricted, or applying for planning permission for larger extensions before going too far down the road of obtaining quotes.

Multi-storey extensions will usually require planning permission. As the height of the extension increases, expect the overall bill for your project to rise. Whereas a single-storey option can be fitted with a simple flat roof, multi-storey extensions usually require tiled roofing to be installed. By doubling the square meterage of the extension, you’ll be adding to the cost you can expect to pay.


How much does a rear or side extension cost?

The average cost of a rear or side extension is £1,500 per sqm.

Rear and side extensions shouldn’t require planning permission provided they adhere to limits set out by your permitted development rights. You can find out what the permitted development rights for your home are likely to be by visiting the planning portal website and on our information page.

Rear extensions are the go-to choice if you’re looking to add a kitchen or make your living or dining room bigger.

Side extensions are often overlooked, but they’re well worth considering too.

You may be surprised by the difference that extending a few metres either side of your home can make.

Even a modest side extension can open up space and transform your lounge, kitchen or study into something more versatile.

While rear or side extensions are typically smaller-scale projects, structural work will still be required. That means your builder will need to partially or completely remove an existing external wall which is going to be load-bearing. Steel support beams (lintels) will usually need to be inserted. Then there’s the removal and re-routing of any existing pipework and drainage too.

The level of structural and plumbing work required can increase the cost of your extension.


How much does a planning application cost?

One of the first things you’ll need to think about when considering a house extension is your planning permission application. This isn’t always required but it may be needed depending on the size of your extension, the number of storeys, and where the new structure will sit (at the front, rear or side).  If your building is either listed or in a conservation area, you may find planning officers are stricter with what they’ll approve. The permission you’re granted may also have certain conditions which make your extension trickier (and therefore more expensive).

The planning fee for a residential extension starts at £206.

The likes of tree preservation orders and historic building reports, may be required, and are additional costs.

You’ll need an architectural designer to draw up plans and construction drawings for your extension and these services we offer to our clients. We will deal with the council right the way through to approvals for you.


Summary – Costs


General house conversion costs

                                                                                               Low           Average          High

Average cost of a house extension per sqm                £1,100          £1,500          £2,000

Single storey 20-sqm extension                                    £24,000        £30,000        £40,000

Double storey 20-sqm extension                                 £48,000         £60,000        £80,000

Specific conversion costs

Basement conversion per sqm                                      £2,070           £3,100           £4,010

Typical cost of kitchen within new extension              £6,000           £10,000        £20,000

Typical cost of bathroom within new extension         £5,000           £8,000          £11,000

Typical cost of shower room within new extension   £4,500           £8,000          £11,000