August 28th, 2025

A dormer window offers a straightforward way to improve spatial efficiency, natural light, and overall property value, while also improving kerb appeal. When planning your project, it’s important to think beyond the design and consider the cost implications. Location, roof type, and finish choices can all swing the budget significantly.

 

But how much does it cost to build a dormer window on an existing property? In this guide, we break down typical installation costs to help you weigh up your options. We’ve included example figures for both flat-roof and pitched-roof dormers to give a clear starting point for budgeting.

 

Dormer Window Cost Calculations

In our dormer window cost calculations, we’ve included the cost of fitting new radiators under each window to help with heat distribution and reduce cold spots. We also include making good internally with plastering and painting, along with a professional clean once the work is complete.

 

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 a dormer window 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, so it’s important to align them with your budget and the level of finish you’re aiming for.

 

Cost Of Building A Dormer Window With Flat Roof

The table below shows the average cost of building a dormer window with a flat roof to a standard specification:

 

Dormer Window – Flat Roof	Total Average Build Cost			Average Cost Per Sq Metre
	Low Benchmark	Mid Benchmark	High Benchmark	Low Benchmark	Mid Benchmark	High Benchmark
East Anglia	£7,494	£8,379	£9,197	£1,873	£2,095	£2,299
Greater London	£7,714	£8,752	£9,335	£1,928	£2,188	£2,334
Inner London	£7,463	£8,486	£8,809	£1,866	£2,122	£2,202
Midlands	£7,009	£7,837	£8,602	£1,752	£1,959	£2,150
North England	£7,525	£8,415	£9,236	£1,881	£2,104	£2,309
Northern Ireland	£7,234	£7,793	£8,767	£1,786	£1,948	£2,192
Scotland	£7,324	£8,190	£8,989	£1,831	£2,047	£2,247
South East	£6,974	£7,755	£8,652	£1,895	£2,107	£2,326
South West	£7,042	£8,062	£8,850	£1,803	£2,015	£2,213
Wales	£6,872	£7,684	£8,301	£1,718	£1,921	£2,075
Average	£7,265	£8,135	£8,874	£1,833	£2,051	£2,235

All costs exclude VAT.

 

Here’s what the data shows for building a dormer window with a flat roof:

 

• The minimum cost is £6,872, and the maximum cost is £9,335.
• The total average cost is £8,091, or £2,040 per square metre.

 

Cost Breakdown By Element – Inner London

The table below shows a detailed breakdown of the typical costs involved in building a dormer window in Inner London. It highlights the main elements, including materials, labour, finishing touches, and contractor overheads and profits.

Elements	Cost	%
Total	£7,684.04	100%
External wall finishes	£1,677.39	21.8%
External wall lining	£951.34	12.4%
Hoarding	£717.07	9.3%
Professional cleaning	£683.91	8.9%
External wall structure	£585.23	7.6%
Radiator under window	£481.91	6.3%
Roof structure	£362.89	4.7%
Paint to ceiling	£291.83	3.8%
Ceiling preparation	£249.13	3.2%
Roof coverings	£233.85	3.0%
Rainwater drainage	£229.47	3.0%
Roof lining	£210.35	2.7%
Scaffolding	£209.10	2.7%
Flashing	£203.20	2.6%
Paint to walls	£177.40	2.3%
Internal wall preparation	£158.18	2.1%
Heavy demolition	£121.74	1.6%
Ceiling lining	£101.58	1.3%
Medium demolition	£  38.47	0.5%

 

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Cost Of Building A Dormer Window With Pitched Roof

The table below shows the average cost of building a dormer window with a pitched roof to a standard specification:

 

Dormer Window – 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	£8,050	£9,002	£9,880	£2,013	£2,250	£2,470
Greater London	£8,359	£9,487	£10,067	£2,090	£2,372	£2,517
Inner London	£8,034	£9,125	£9,486	£2,009	£2,281	£2,371
Midlands	£7,560	£8,454	£9,278	£1,890	£2,113	£2,320
North England	£8,087	£9,043	£9,925	£2,022	£2,261	£2,481
Northern Ireland	£7,787	£8,391	£9,440	£1,923	£2,098	£2,360
Scotland	£7,883	£8,815	£9,675	£1,971	£2,204	£2,419
South East	£7,539	£8,384	£9,252	£2,049	£2,278	£2,514
South West	£7,592	£8,694	£9,544	£1,944	£2,173	£2,386
Wales	£7,407	£8,283	£8,948	£1,852	£2,071	£2,237
Average	£7,830	£8,768	£9,550	£1,976	£2,210	£2,407

All costs exclude VAT.

 

Here’s what the data shows for the cost of building a dormer window with a pitched roof:

 

• The minimum cost is £7,407, and the maximum cost is £10,067.
• The total average cost is £8,716, or £2,198 per square metre.

 

Cost Of Building A Double Dormer Window

The table below shows the average cost of building two dormer windows with pitched roofs to a standard specification:

 

Double Dormer Windows – Pitched Roofs	Total Average Build Cost			Average Cost Per Sq Metre
	Low Benchmark	Mid Benchmark	High Benchmark	Low Benchmark	Mid Benchmark	High Benchmark
East Anglia	£13,993	£15,646	£17,173	£  1,749	£  1,956	£  2,147
Greater London	£14,578	£16,546	£17,549	£  1,822	£  2,068	£  2,194
Inner London	£13,996	£15,889	£16,469	£  1,750	£  1,986	£  2,059
Midlands	£13,211	£14,772	£16,213	£  1,651	£  1,847	£  2,027
North England	£14,104	£15,771	£17,310	£  1,763	£  1,971	£  2,164
Northern Ireland	£13,822	£15,455	£16,963	£  1,728	£  1,932	£  2,120
Scotland	£13,794	£15,424	£16,929	£  1,724	£  1,928	£  2,116
South East	£13,148	£14,619	£16,312	£  1,644	£  1,827	£  2,039
South West	£13,267	£15,211	£16,697	£  1,658	£  1,901	£  2,087
Wales	£12,928	£14,456	£15,597	£  1,616	£  1,807	£  1,950
Average	£13,684	£15,379	£16,721	£  1,711	£  1,922	£  2,090

All costs exclude VAT.

 

Here’s what the data shows for the cost of building two dormer windows with pitched roofs:

• The minimum cost is £12,928, and the maximum cost is £17,549.
• The total average cost is £15,261, or £1,908 per square metre.

 

Average Cost Of Building A Dormer Window

So, how much does it cost to build a dormer window in the UK? Based on the three types of installations covered in this article, it costs on average (when using a mid-benchmark contractor):

• Dormer window – flat roof: £8,315, or £2,051 per square metre. 
• Dormer window – pitched roof: £8,768, or £2,210 per square metre.
• Double dormer windows – pitched roofs: £15,379, or £1,922 per square metre. 

It’s also worth noting that these projects were based on a standard specification, so your final cost could be lower or higher depending on the quality of materials you choose.

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Please remember that all costs exclude VAT.

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Frequently Asked Questions About Building A Dormer Window

Here are some answers to frequently asked questions about installing dormer windows.

How long does it take to install a dormer window?

Installing a dormer window usually takes around two to three weeks from start to finish. The main build involves fitting the dormer frame and window into the roof, followed by moving any central heating or radiator pipework.

Once the window is in place, the interior is plastered, left to dry for several days, and then decorated before the radiator is hung. While the construction itself can take about a week, drying times and decoration mean it’s usually a couple of weeks before the space is ready to use.

What are the different types of dormer windows?

Dormer windows come in several styles, each with its own unique look and purpose:

1. Gable-fronted dormer – pitched roof with two sloping sides forming a peak.
2. Hip roof dormer – three sloping sides that blend into the roofline.
3. Flat roof dormer – flat top, maximising interior space and headroom.
4. Shed dormer – single sloping roof extending from the main roof.
5. Eyebrow dormer – curved, wave-like roof for a decorative look.
6. Arched dormer – rounded top that softens the roofline.

The main types you see in the UK are gable-fronted, flat roof, and shed dormers because they are the most cost-effective and provide extra space.

Do I need guttering on my dormer window?

Guttering is usually recommended for large dormer windows to properly manage rainwater runoff, but they’re not essential on smaller windows as water naturally drains without causing problems. Flat roof dormers are more likely to need guttering to prevent water pooling. 

Is scaffolding required to build a dormer window?

Scaffolding is almost always necessary, even if some of the work can be done from the inside. This is because building the dormer involves external tasks like removing roof tiles, cutting rafters, and fitting lead flashing. 

UK regulations also mandate scaffolding for prolonged work at height to ensure safety. Additionally, scaffolding makes handling heavy materials like timber and glazing much easier and safer.