How Much Does It Cost To Build An Extension In 2026? A UK Guide

Planning and costing an extension requires careful consideration, especially given the variables that can affect timelines, materials, and labour. Project costs can vary widely depending on the type of extension, location, design complexity, and specification of materials.

In this guide, we provide a detailed overview of the average costs for ground-floor and double-storey extensions across the UK. We break costs down by region, project stage, and room type, giving architects, contractors, and project managers the data needed to plan accurately, manage budgets, and advise clients with confidence.

Building An Extension In The UK

In our calculations, the costs of building an extension 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.

 

Also, the rooms in the calculation for the extension comprise:

 

• 20m² single-storey rear extension: dining room, utility room, WC.
• 40m² double-storey rear extension: dining room, utility room, WC, one bedroom, an en-suite bathroom, storage room, small landing.

 

The rooms you choose ultimately affect the overall cost of the extension. For instance, if we were adding a kitchen instead of a dining room, we would need to factor in the cost of fitting a kitchen and the appliances.

 

Calculations reflect the cost of fitting out the new rooms only. They do not include the remodelling of existing rooms or changes to the layout.

 

In terms of build specification, BuildPartner gives you three broad options to choose from:

 

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

 

In this article, we’ve provided a breakdown of the cost of building an extension to a standard specification. In addition, we have provided three different benchmarks to show the range of pricing you can expect among 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.

 

The specification and benchmark you choose will have a major impact on the overall cost of building an extension, so it’s important to align them with your budget and the finish you’re aiming for.

 

Cost of Building A Ground-Floor Extension – Standard Spec

Here’s a table showing the average costs of building a single-storey rear extension for a standard-spec project in the UK (20m2):

 

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	£63,312	£70,795	£77,702	£3,166	£3,540	£3,885
London	£64,815	£75,531	£79,886	£3,241	£3,777	£3,994
Midlands	£60,462	£67,607	£74,203	£3,023	£3,380	£3,710
North England	£62,652	£70,056	£76,891	£3,133	£3,503	£3,845
Northern Ireland	£61,398	£66,314	£73,800	£3,007	£3,316	£3,690
Scotland	£62,662	£70,068	£76,904	£3,133	£3,503	£3,845
South East	£64,773	£71,876	£79,494	£3,239	£3,594	£3,975
South West	£60,788	£69,636	£76,443	£3,114	£3,482	£3,822
Wales	£59,289	£66,296	£71,220	£2,964	£3,315	£3,561
Average	£62,239	£69,798	£76,283	£3,113	£3,490	£3,814

All costs exclude VAT, overheads, and profit.

 

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

 

• It costs 9% more to build an extension in London than in other regions.
• The minimum cost is £59,289, and the maximum cost is £79,886.
• The total average cost is £69,440, or £3,472 per square metre.

 

Extension Cost Breakdown By Project Stage – London

 

Here’s an example of the associated costs for the different stages of building an extension for a standard-spec project – when using a mid-benchmark contractor in London:

 

Total	£75,531	100.0%
External wall structure and lining	£11,329	15.0%
Foundations and substructure	£10,631	14.1%
Roof structure and coverings	£9,323	12.3%
Steel and structural supports	£6,571	8.7%
Floor preparation and finishes	£4,820	6.4%
Units, worktops and kitchen appliances	£3,600	4.8%
Heating and cooling	£3,391	4.5%
External windows and doors	£3,219	4.3%
Decoration	£3,173	4.2%
Preliminaries	£2,879	3.8%
Floor structure and lining	£2,872	3.8%
Wired appliances	£2,616	3.5%
Internal wall structure and lining	£2,190	2.9%
Drainage and pipework	£1,961	2.6%
Wall preparation and finishes	£1,747	2.3%
Internal doors and frames	£1,226	1.6%
Plumbed appliances	£1,109	1.5%
Demolitions	£1,088	1.4%
Wiring and fuseboards	£513	0.7%
Ceiling structure and lining	£470	0.6%
Ceiling preparation and finishes	£458	0.6%
Fixtures and fittings	£344	0.5%

 

Cost Breakdown By Room – London

 

Here’s a breakdown of the associated costs by room for building a standard-spec extension – when using a mid-benchmark contractor in London:

 

Total	£75,531	100%
Ground Floor Rear Extension with pitched roof	£48,766	64.6%
Utility	£11,166	14.8%
Dining room	£8,807	11.7%
WC	£6,392	8.5%
P | Preliminaries	£400	0.5%

 

Cost of Building A Double-Storey Extension – Standard Spec

Here’s a table showing the average costs of building a double-storey extension for a standard-spec project in the UK (40m2):

 

Double-Storey Rear Extension (Pitched Roof)	Total Average Build Cost			Average Cost Per Sq Metre
	Low Benchmark	Mid Benchmark	High Benchmark	Low Benchmark	Mid Benchmark	High Benchmark
East Anglia	£97,366	£107,826	£117,749	£2,434	£2,696	£2,944
London	£111,841	£130,399	£139,045	£2,796	£3,260	£3,476
Midlands	£104,544	£116,899	£128,304	£2,614	£2,922	£3,208
North England	£108,316	£121,116	£132,933	£2,708	£3,028	£3,323
Northern Ireland	£106,182	£114,843	£127,812	£2,604	£2,871	£3,195
Scotland	£108,221	£121,011	£132,817	£2,706	£3,025	£3,320
South East	£111,814	£123,868	£137,226	£2,795	£3,097	£3,431
South West	£101,540	£120,409	£132,176	£2,692	£3,010	£3,304
Wales	£95,381	£106,653	£114,378	£2,385	£2,666	£2,859
Average	£105,023	£118,114	£129,160	£2,637	£2,953	£3,229

All costs exclude VAT, overheads, and profit.

 

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

 

• The minimum cost is £95,381, and the maximum cost is £139,045.
• The total average cost is £117,432, or £2,940 per square metre.

 

Average Cost To Build An Extension In 2026

So, how much does it cost to build an extension in the UK? Based on building to a standard specification, you can expect building an extension to cost on average (when using a mid-benchmark contractor):

 

• Ground-floor rear extension – £69,798, or £3,490 per square metre.
• Double-storey rear extension – £118,114, or £2,953 per square metre.

 

Please remember that these costs exclude a contractor’s overheads, profit, and VAT. There may be other additional costs, such as obtaining planning permission, conducting structural surveys, making unexpected repairs, and completing custom finishes.

---

Start your free trial with BuildPartner today to get the most accurate estimate of building an extension based on your own dimensions and specifications.

---

FAQs About Building An Extension In The UK

Here are some frequently asked questions about building extensions in the UK.

Do I need planning permission for an extension? 

Many single-storey rear extensions fall under Permitted Development Rights, meaning you won’t need full planning permission if you meet specific size and design limits. Anything outside those rules requires an application to your local planning authority.

 

Here are some examples:

Extension Scenario	Planning Permission Needed?	Notes
Single-storey rear extension	Sometimes	Allowed if it projects less than 3m on a terrace/semi or 4m on a detached house and uses materials similar in appearance to the existing property.
Double-storey rear extension	Sometimes	Allowed if not deeper than 3m, roof pitch matches house, and within height/boundary rules.
Side extension (single storey)	Sometimes	Must be no higher than 4m, max half the width of the original house, and not forward of the main façade.
Side extension (two storeys)	Needed	Two-storey side extensions are not Permitted Development.
Wraparound extension	Needed	Wraparounds always exceed permitted development limits because they combine rear and side extensions.
Extension on a listed building	Always needed	Listed building consent also required.
Extension in a conservation area	Sometimes	Permitted Developments still applies, but restrictions are tighter. Large or visible alterations typically need permission.
Porch under 3m high and <3m²	Not needed	As long as it’s not within 2m of the road.

 

Every build is different, so if you’re ever unsure, it’s best to check with your local planning authority before starting any work.

 

Do I need a Party Wall Agreement?

If your extension involves a shared wall, boundary, or structure with a neighbour, the Party Wall Act 1996 sets out steps you must follow:

 

• Serve notice to your neighbour at least 2 months before work on a shared wall.
• Serve notice at least 1 month before excavations near a boundary.
• Clearly describe the work, timeline, and potential impact.
• Obtain your neighbour’s consent or appoint a Party Wall Surveyor if they dissent.

The Party Wall Act applies to works such as building on the boundary, cutting into shared walls, excavations, and installing structural elements. Complying helps you avoid legal disputes, injunctions, or claims for damages.

Does the extension need to match my existing house materials?

For planning or permitted development compliance, materials usually need to be “similar in appearance”. However, modern contrasting designs can be approved through full planning permission.