How Much Does It Cost To Build An Annex Or Granny Flat?

With rising property prices and limited housing options, more homeowners are considering whether building an annex or granny flat could provide better value and added flexibility, compared to moving to a larger home or purchasing a separate property.

Annexes are often designed to accommodate an older relative downsizing but still wanting to live close by, or to give a younger family member extra independence while remaining part of the household. Whatever the need, this raises important questions around cost and feasibility.

So, in this article, we examine the cost of building an annex in the UK in 2026, providing a clear benchmark for contractors and architects planning projects.

Building An Annex In The UK

In our calculations of building an annex, our costs comprise a ground floor constructed from brick and block with a pitched roof. The annex features an open-plan living area with a kitchenette, a bathroom, a bedroom, a boiler cupboard, and a cloakroom. 

 

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 annex according to all three specifications. 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 annex, so it’s important to align them with your budget and the finish you’re aiming for.

 

Cost of Building An Annex – Basic Spec

Here’s a table showing the average costs of building an annex or granny flat to a basic specification:

 

Annex – Basic Spec	Total Average Build Cost			Average Cost Per Sq Metre
	Low Benchmark	Mid Benchmark	High Benchmark	Low Benchmark	Mid Benchmark	High Benchmark
East Anglia	£104,239	£116,559	£127,930	£2,606	£2,914	£3,198
London	£107,906	£122,957	£125,747	£2,698	£3,074	£3,144
Midlands	£  99,116	£110,829	£121,642	£2,478	£2,771	£3,041
North England	£103,256	£115,459	£126,724	£2,581	£2,886	£3,168
Northern Ireland	£100,800	£108,454	£120,584	£2,456	£2,711	£3,015
Scotland	£103,346	£115,560	£126,834	£2,584	£2,889	£3,171
South East	£107,258	£119,495	£131,635	£2,681	£2,987	£3,291
South West	£  99,958	£115,096	£126,347	£2,574	£2,877	£3,159
Wales	£  96,966	£108,426	£116,652	£2,424	£2,711	£2,916
Average	£102,539	£114,759	£124,899	£2,565	£2,869	£3,122

All costs exclude VAT, overheads, and profit.

 

Here’s what the data shows for the cost of building an annex or granny flat to a basic specification:

 

• The minimum cost is £96,966, and the maximum cost is £131,635.
• The total average cost is £114,066, or £2,852 per square metre.

 

Housebuilding Cost Breakdown By Project Stage – London

 

Here’s an example of the associated costs for the different stages of building an annex or granny flat to a basic specification – when using a mid-benchmark contractor in London:

 

Total	£122,957	100.0%
Preliminaries	£3,947	3.2%
Foundations and substructure	£19,484	15.8%
Floor structure and lining	£5,012	4.1%
Steel and structural supports	£6,318	5.1%
External wall structure and lining	£20,313	16.5%
Internal wall structure and lining	£4,303	3.5%
Roof structure and coverings	£14,731	12.0%
Ceiling structure and lining	£807	0.7%
External windows and doors	£5,846	4.8%
Internal doors and frames	£2,269	1.8%
Drainage and pipework	£10,368	8.4%
Heating and cooling	£4,935	4.0%
Wiring and fuseboards	£1,425	1.2%
Plumbed appliances	£1,784	1.5%
Wired appliances	£4,273	3.5%
Ceiling preparation and finishes	£692	0.6%
Wall preparation and finishes	£4,971	4.0%
Floor preparation and finishes	£4,139	3.4%
Units, worktops and appliances	£3,230	2.6%
Joinery	£571	0.5%
Decoration	£3,539	2.9%

 

Cost Breakdown By Room – London

 

Here’s a breakdown of the associated costs by room for building an annex or granny flat to a basic specification – when using a mid-benchmark contractor in London:

 

Total	£122,957	100%
Bathroom	£8,799	7.2%
Bedroom	£6,058	4.9%
Boiler Cupboard	£3,134	2.5%
Boot Room / Cloakroom	£5,664	4.6%
G | Ground Floor	£69,771	56.7%
Kitchenette	£9,064	7.4%
Living room / family room	£4,865	4.0%
R | Pitched roof	£15,602	12.7%

 

Cost of Building An Annex – Standard Spec

Here’s a table showing the average costs of building an annex or granny flat to a standard specification:

 

Annex – Standard Spec	Total Average Build Cost			Average Cost Per Sq Metre
	Low Benchmark	Mid Benchmark	High Benchmark	Low Benchmark	Mid Benchmark	High Benchmark
East Anglia	£112,019	£125,258	£137,478	£2,800	£3,131	£3,437
London	£116,040	£131,010	£134,830	£2,901	£3,275	£3,371
Midlands	£106,895	£119,529	£131,190	£2,672	£2,988	£3,280
North England	£111,036	£124,159	£136,272	£2,776	£3,104	£3,407
Northern Ireland	£108,580	£116,941	£130,132	£2,651	£2,924	£3,253
Scotland	£111,126	£124,259	£136,381	£2,778	£3,106	£3,410
South East	£115,038	£127,842	£141,183	£2,876	£3,196	£3,530
South West	£107,738	£123,795	£135,895	£2,768	£3,095	£3,397
Wales	£104,746	£117,125	£125,846	£2,619	£2,928	£3,146
Average	£110,357	£123,324	£134,356	£2,760	£3,083	£3,359

All costs exclude VAT, overheads, and profit.

 

Here’s what the data shows for the cost of building an annex or granny flat to a standard specification:

 

• The minimum cost is £104,746, and the maximum cost is £141,183.
• The total average cost is £122,679, or £3,067 per square metre.

 

Cost of Building An Annex – Premium Spec

Here’s a table showing the average costs of building an annex or granny flat to a premium specification:

 

Annex – Premium Spec	Total Average Build Cost			Average Cost Per Sq Metre
	Low Benchmark	Mid Benchmark	High Benchmark	Low Benchmark	Mid Benchmark	High Benchmark
East Anglia	£127,574	£142,651	£156,568	£3,189	£3,566	£3,914
London	£132,301	£150,110	£153,015	£3,308	£3,753	£3,825
Midlands	£122,450	£136,921	£150,280	£3,061	£3,423	£3,757
North England	£126,591	£141,551	£155,361	£3,165	£3,539	£3,884
Northern Ireland	£124,134	£133,910	£149,222	£3,103	£3,348	£3,731
Scotland	£126,680	£141,652	£155,471	£3,167	£3,541	£3,887
South East	£130,593	£145,587	£160,273	£3,265	£3,640	£4,007
South West	£123,292	£141,188	£154,985	£3,082	£3,530	£3,875
Wales	£120,301	£134,518	£144,228	£3,008	£3,363	£3,606
Average	£125,991	£140,899	£153,267	£3,150	£3,522	£3,832

All costs exclude VAT, overheads, and profit.

 

Here’s what the data shows for the cost of building an annex or granny flat to a premium specification:

 

• The minimum cost is £120,301, and the maximum cost is £160,273.
• The total average cost is £140,052, or £3,501 per square metre.

 

Average Cost To Build An Annex In 2026

So, how much does it cost to build an annex in the UK? Based on the three different specifications covered in this article, you can expect building an annex to cost on average (when using a mid-benchmark contractor):

• Basic-spec annex – £114,759, or  £2,869 per square metre.

• Standard-spec annex – £123,324, or £3,083 per square metre.

• Premium-spec annex – £140,899, or £3,522 per square metre.

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

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

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FAQs About Annex Building Costs In The UK

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

Do I need planning permission to build an annex?

In most cases, you’ll need planning permission, especially if the annex is self-contained. Some annexes may fall under permitted development rights, but this depends on use and location. 

For example, a single-storey side extension with internal access from the main house with no separate entrance may fall within permitted development and not require planning permission.

Is a prefab annex cheaper than a traditional build?

Prefab annexes can appear cheaper upfront, often advertised from £35,000 to £60,000, but once you factor in groundworks, foundations, utilities, delivery, and installation, the total cost is usually much closer to a traditional build.

Prefab annexes also offer limited flexibility in layouts and interior finishes, giving homeowners far less choice compared to a traditionally built annex.

Does an annex need its own utilities?

Annexes can share utilities with the main dwelling, which helps keep costs down. Installing separate water, electricity, or gas meters allows for independent use and typically adds £2,000 to £5,000 to the overall build cost, depending on connection distances and local supplier charges.

Having separate utilities is better for independent billing for the occupant, providing clearer cost responsibility and making the setup more appropriate for long-term or semi-independent living.

What’s the difference between a granny flat and an annex?

In most cases, the terms are used interchangeably. Both refer to a self-contained living space added to a property. Some people use “granny flat” when the space is intended for an older relative, while “annex” refers more generally to a separate dwelling for any family member or occupant.