In the ever-evolving landscape of construction and home improvement, the question of building costs is perennially at the forefront of any designer’s mind—materials, labour, compliance with the latest building regulations, and VAT of course.

Naturally, the cost of building an extension varies by size, type, and complexity, so in this article, we’ll break down the costs across various regions in the UK. We’ll also look at four types of extensions:

• Single-storey rear extension
• Double-storey rear extension
• Ground-floor wrap-around extension
• Brick porch extension

We’ll explore both the total project costs and costs per square metre. At the end of the article, we’ll summarise with a comparison to give you a holistic view of the costs of building an extension.

Building An Extension Construction Cost Calculations

In our calculations, we applied a consistent methodology across different regions. The cost comparisons are based on the following:

• The roof of the extension is pitched, not flat.
• The extension is constructed out of brick/block, not a timber frame.

As far as rooms are concerned, we included the following in our calculations:

• Single-storey rear extension – dining room, utility room, WC.
• Double-storey rear extension – dining room, utility room, WC, two bedrooms, an en-suite bathroom, a storage room, small landing.
• Ground-floor wrap-around extension – dining room, utility room, WC, boot/cloakroom.
• Brick porch extension – vacant space.

If you’d like to try a demo of our pricing tool, sign up here.

 

Which rooms you choose ultimately affects the overall cost of the extension. If we were adding a room for a kitchen instead of a dining room in these extensions, we would need to factor in the cost of fitting a kitchen and the appliances – which would bump up the cost.

Important! To keep things simple, we’ve only included the costs to build the extension, not the costs to refurbish and remodel the existing space.

So, when somebody asks “How much does it cost to build an extension?” it depends entirely on the nature of the project.

There’s no single answer.

But by sharing with you the specifications for these examples, you’ll leave with a much clearer idea of the range and average costs. When using our BuildPartner pricing tool, our pricing calculator gives you three broad options (which can be revised by item at a later point):

• Low spec is a basic finish; e.g. Leyland, Everest, Wickes.
• Mid spec is a standard finish; e.g. Dulux, Hamilton, Slim Line.
• High spec is a high-spec finish; e.g. Farrow & Ball, Fine Line, Siemens.

To keep this comparison simple, we have chosen mid spec in all our cost estimations. In addition to this, we have also provided three different benchmarks to show the range you can expect amongst different contractors. These are:

• Low benchmark – smaller companies are generally lower cost but less specialised and with low project management function; good for low/mid-spec projects and tight budgets.
• Mid benchmark – medium-sized companies offer a balanced mix of cost-effectiveness and specialisation; good for mid-range projects for effective execution within moderate budgets.
• High benchmark – larger companies generally have a higher cost base but can handle a wide variety of specialist tasks; good for mid/high-spec projects and project management.

In short, contractor pricing varies due to factors such as expertise, location, and project intricacies.

 

Cost Of Building A Single-Storey Rear Extension

Here’s a table showing the average building costs of a single-storey rear extension with a gross external area of 30 square metres:

Single-Storey Rear Extension	Total Average Build Cost			Average Cost Per Sq Metre
	Low Benchmark	Mid Benchmark	High Benchmark	Low Benchmark	Mid Benchmark	High Benchmark
East Anglia	£70,600	£77,018	£83,436	£2,353	£2,567	£2,781
London	£81,986	£92,188	£102,408	£2,733	£3,073	£3,414
Midlands	£68,963	£75,232	£81,502	£2,299	£2,508	£2,717
North England	£65,568	£74,576	£80,790	£2,186	£2,486	£2,693
Northern Ireland	£66,730	£71,528	£77,489	£2,186	£2,384	£2,583
Scotland	£67,892	£74,064	£80,236	£2,263	£2,469	£2,675
South East	£74,199	£82,968	£91,062	£2,473	£2,766	£3,035
South West	£69,801	£78,943	£86,645	£2,353	£2,631	£2,888
Wales	£69,003	£77,157	£82,847	£2,300	£2,572	£2,762
Average	£70,527	£78,186	£85,157	£2,350	£2,606	£2,839

All costs exclude VAT.

Here’s what the data shows for the cost of building a single-storey rear extension:

• It costs 21% more to build an extension in London than in other regions.
• The minimum cost is £65,568 and the maximum cost is £102,408.
• The total average cost is £77,957, or £2,598 per square metre.

Cost Of Building A Double-Storey Rear Extension

Here’s a table showing the average building costs of a double-storey rear extension with a gross external area of 60 square metres:

Double-Storey Rear Extension	Total Average Build Cost			Average Cost Per Sq Metre
	Low Benchmark	Mid Benchmark	High Benchmark	Low Benchmark	Mid Benchmark	High Benchmark
East Anglia	£123,085	£134,276	£145,465	£1,827	£2,238	£2,424
London	£143,207	£160,796	£179,281	£2,387	£2,680	£2,988
Midlands	£120,286	£131,221	£142,156	£2,005	£2,187	£2,369
North England	£114,540	£130,152	£140,998	£1,909	£2,169	£2,350
Northern Ireland	£116,541	£124,952	£135,365	£1,909	£2,083	£2,256
Scotland	£118,542	£129,318	£140,095	£1,976	£2,155	£2,335
South East	£129,236	£144,509	£158,608	£2,154	£2,408	£2,643
South West	£122,295	£137,633	£151,060	£2,051	£2,294	£2,518
Wales	£121,504	£135,864	£145,812	£2,025	£2,264	£2,430
Average	£ 123,113	£136,525	£148,760	£2,027	£2,275	£2,479

All costs exclude VAT.

Here’s what the data shows for the cost of building a double-storey rear extension:

• The minimum cost is £114,540, and the maximum cost is £179,281.
• The total average cost is £136,132, or £2,268 per square metre.

Cost Of Building A Ground-Floor Wrap-Around Extension

Here’s a table showing the average building costs of a ground-floor wrap-around extension with a gross external area of 35 square metres:

Ground-Floor Wrap-Around Extension	Total Average Build Cost			Average Cost Per Sq Metre
	Low Benchmark	Mid Benchmark	High Benchmark	Low Benchmark	Mid Benchmark	High Benchmark
East Anglia	£84,035	£91,675	£99,314	£2,334	£2,547	£2,759
London	£97,577	£109,621	£122,056	£2,710	£3,045	£3,390
Midlands	£82,103	£89,567	£97,031	£2,281	£2,488	£2,695
North England	£78,096	£88,792	£96,191	£2,169	£2,466	£2,672
Northern Ireland	£79,458	£85,196	£92,295	£2,207	£2,367	£2,564
Scotland	£80,820	£88,167	£95,514	£2,245	£2,449	£2,653
South East	£88,296	£98,731	£108,363	£2,453	£2,743	£3,010
South West	£83,092	£93,967	£103,134	£2,308	£2,610	£2,865
Wales	£82,150	£91,858	£98,619	£2,282	£2,552	£2,739
Average	£83,959	£93,064	£101,391	£2,332	£2,585	£2,816

All costs exclude VAT.

Here’s what the data shows for the cost of building a ground-floor wrap-around extension:

• The minimum cost is £78,096, and the maximum cost is £122,056.
• The total average cost is £92,804, or £2,578 per square metre.

Cost Of Building A Brick Porch

Here’s a table showing the average building costs of a brick porch with a gross external area of five square metres:

Brick Porch	Total Average Build Cost			Average Cost Per Sq Metre
	Low Benchmark	Mid Benchmark	High Benchmark	Low Benchmark	Mid Benchmark	High Benchmark
East Anglia	£5,983	£5,984	£7,071	£952	£1,197	£1,414
London	£6,917	£7,735	£8,489	£1,383	£1,547	£1,698
Midlands	£5,861	£6,394	£6,927	£1,172	£1,279	£1,385
North England	£5,606	£6,344	£6,873	£1,121	£1,269	£1,375
Northern Ireland	£5,693	£6,116	£6,625	£1,121	£1,223	£1,325
Scotland	£5,780	£6,306	£6,831	£1,156	£1,261	£1,366
South East	£6,249	£6,987	£7,669	£1,250	£1,397	£1,534
South West	£5,926	£6,691	£7,343	£1,197	£1,338	£1,469
Wales	£5,868	£6,562	£7,033	£1,174	£1,312	£1,407
Average	£5,851	£6,569	£7,207	£1,170	£1,314	£1,441

All costs exclude VAT

Here’s what the data shows for the cost of building a brick porch:

• The minimum cost is £5,606, and the maximum cost is £8,489.
• The total average cost is £6,584, or £1,317 per square metre.

Summary Of The Average Cost To Build An Extension

So, how much does it cost to build an extension in the UK? Based on the four different extensions we covered, you can expect it to cost on average (when using a mid-benchmark contractor):

• Single-storey rear extension – £78,186, or £2,606 per square metre.
• Double-storey rear extension – £136,525, or £2,275 per square metre.
• Ground-floor wrap-around extension – £93,064, or £2,585 per square metre.
• Brick porch – £6,569, or £1,314 per square metre.

With the exception of a brick porch, you’ll notice that the cost per square metre decreases as the project size increases due to more efficient resource utilisation and lower average costs, leading to economies of scale.

Please remember that these costs do not include VAT.

By Tommy Crabtree