How Much Does A Loft Conversion Cost In London?
For many London properties, a loft conversion represents one of the most efficient ways to add space without extending the building’s footprint. But behind every well-executed loft conversion is a delicate balance where cost drives many of the structural and design decisions.
This article breaks down the main cost components, offering a framework architects and developers can use to explain pricing to clients, compare basic, standard, and premium specifications, and benchmark costs across different-sized contractors to guide early design decisions.
Main Types of Loft Conversions
In this article, we’ll be reviewing three loft conversion types: a standard loft conversion, a dormer loft conversion, and a Mansard loft conversion:
- Standard Loft Conversion – Predominantly internal work without major changes like roof lifts or complete rebuilds.
- Dormer Loft Conversion – Extending the roof outward to create additional space and headroom; they can be full-width or smaller, depending on the space requirements.
- Mansard Loft Conversion – Altering the roof structure to create a near-vertical wall with a flat roof, providing maximum space and headroom.
Other popular loft conversion types include the hip-to-gable and gable-to-gable conversions, which extend the roof to maximise space; shell conversions, which provide just the structural shell for homeowners to finish; roof lift conversions, which raise the entire roof for additional headroom; modular conversions, using prefabricated sections assembled on-site; and piggyback conversions, where a completely new storey is added on top of the existing roof.
Loft Conversion Cost Calculations
For reference, a fully finished attic generally matches the floor’s square footage. We have based our calculations on 20m² of floor space. In each example, we provide costs per square metre, so you can easily adjust the figures if your project is larger or smaller.
All our calculations are based on a loft conversion comprising the following:
- Master bedroom
- En-suite bathroom
- Eaves storage
- Landing including stairs
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 loft conversion 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 a loft conversion, so it’s important to align them with your budget and the level of finish you’re aiming for.
Cost Of Building A Standard Loft Conversion In London
The table below shows the average cost of building a standard loft conversion in London (20m2):
| Standard Loft Conversion | Total Average Build Cost | ||
| Low Benchmark | Mid Benchmark | High Benchmark | |
| Basic Spec | £45,061 | £51,257 | £55,862 |
| Mid Spec | £48,216 | £54,639 | £59,508 |
| Premium Spec | £53,831 | £61,063 | £66,628 |
| Average | £49,036 | £55,653 | £60,666 |
The table below shows the average cost per square metre of building a standard loft conversion in London (20m2):
| Standard Loft Conversion | Average Cost Per Sq Metre | ||
| Low Benchmark | Mid Benchmark | High Benchmark | |
| Basic Spec | £2,253 | £2,563 | £2,793 |
| Mid Spec | £2,411 | £2,732 | £2,975 |
| Premium Spec | £2,692 | £3,053 | £3,331 |
| Average | £2,452 | £2,783 | £3,033 |
All costs exclude VAT.
Here’s what the data shows for building a standard loft conversion in London:
- The minimum cost is £45,061, and the maximum cost is £66,628.
- The total average cost is £55,653, or £2,783 per square metre.
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Cost By Stages Of Loft Conversion In London
The table below shows the average cost by project stage for a standard loft conversion in London (20m2):
| Total | £54,639 | 100% |
| Preliminaries | £10,581 | 19.4% |
| Steel and structural supports | £6,854 | 12.5% |
| External wall structure and lining | £5,025 | 9.2% |
| Internal wall structure and lining | £3,770 | 6.9% |
| Ceiling structure and lining | £3,215 | 5.9% |
| External windows and doors | £3,085 | 5.6% |
| Wall preparation and finishes | £2,847 | 5.2% |
| Floor structure and lining | £2,790 | 5.1% |
| Floor preparation and finishes | £2,733 | 5.0% |
| Plumbed appliances | £2,588 | 4.7% |
| Decoration | £2,354 | 4.3% |
| Wired appliances | £1,945 | 3.6% |
| Heating and cooling | £1,541 | 2.8% |
| Demolitions | £1,440 | 2.6% |
| Wiring and fuseboards | £1,307 | 2.4% |
| Internal doors and frames | £1,147 | 2.1% |
| Drainage and pipework | £632 | 1.2% |
| Fixtures and fittings | £427 | 0.8% |
| Ceiling preparation and finishes | £358 | 0.7% |
Cost Of Building A Mansard Loft Conversion In London
The table below shows the average cost of building a mansard loft conversion in London (20m²):
| Mansard Loft Conversion | Total Average Build Cost | ||
| Low Benchmark | Mid Benchmark | High Benchmark | |
| Basic Spec | £66,783 | £76,558 | £81,924 |
| Mid Spec | £69,938 | £79,940 | £85,570 |
| Premium Spec | £75,553 | £86,365 | £92,690 |
| Average | £70,758 | £80,954 | £86,728 |
The table below shows the average cost per square metre of building a mansard loft conversion in London (20m²):
| Mansard Loft Conversion | Average Cost Per Sq Metre | ||
| Low Benchmark | Mid Benchmark | High Benchmark | |
| Basic Spec | £3,339 | £3,828 | £4,096 |
| Mid Spec | £3,497 | £3,997 | £4,279 |
| Premium Spec | £3,778 | £4,318 | £4,635 |
| Average | £3,538 | £4,048 | £4,336 |
All costs exclude VAT.
Here’s what the data shows for the cost of building a mansard loft conversion in London:
- The minimum cost is £66,783, and the maximum cost is £92,690.
- The total average cost is £80,954, or £4,048 per square metre.
Cost Of Building A Dormer Loft Conversion In London
The table below shows the average cost of building a dormer loft conversion in London (20m2):
| Dormer Loft Conversion | Total Average Build Cost | ||
| Low Benchmark | Mid Benchmark | High Benchmark | |
| Basic Spec | £56,780 | £65,012 | £69,988 |
| Mid Spec | £59,935 | £68,394 | £73,634 |
| Premium Spec | £65,549 | £74,819 | £80,754 |
| Average | £60,755 | £69,408 | £74,792 |
The table below shows the average cost per square metre of building a dormer loft conversion in London (20m2):
| Dormer Loft Conversion | Average Cost Per Sq Metre | ||
| Low Benchmark | Mid Benchmark | High Benchmark | |
| Basic Spec | £2,839 | £3,251 | £3,499 |
| Mid Spec | £2,997 | £3,420 | £3,682 |
| Premium Spec | £3,277 | £3,741 | £4,038 |
| Average | £3,038 | £3,470 | £3,740 |
All costs exclude VAT.
Here’s what the data shows for the cost of building a dormer loft conversion in London:
- The minimum cost is £56,780, and the maximum cost is £80,754.
- The total average cost is £69,408, or £3,470 per square metre.
Average Cost Of Loft Conversion In London
So, how much does a loft conversion cost in London? Based on the three types of builds covered in this article, it costs on average:
- Standard Loft Conversion: £55,653, or £2,783 per square metre.
- Mansard Loft Conversion: £80,954, or £4,048 per square metre.
- Dormer Loft Conversion: £69,408, or £3,470 per square metre.
It’s also worth noting that 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.
Frequently Asked Questions About Loft Conversions In London
Here are some answers to frequently asked questions about building loft conversions in London.
What is the typical cost breakdown for a loft conversion in London?
On average, labour accounts for the largest portion of a loft conversion budget in London: around 52% of total costs. Building materials make up roughly 31%, while finishing materials such as flooring, paint, and fixtures represent about 17%.
In our standard loft conversion example, a total project costing £68,394 would equate to approximately £36,106 for labour, £20,800 for building materials, and £11,488 for finishes.
How long does a loft conversion take?
A standard dormer loft typically takes 6 to 8 weeks from the start of construction. More complex designs such as mansard or L-shaped conversions can take up to 10 to 12 weeks. Design, approvals, and structural calculations usually add another 4 to 6 weeks before the build begins.
Do I need a Party Wall Agreement?
Yes, if your property shares a wall with a neighbour. Under the Party Wall etc. Act 1996, you must notify adjoining owners in writing before work begins. We can guide you through this process or recommend a local party wall surveyor.
Older London homes (particularly Victorian and Edwardian terraces) often require additional planning and structural consideration, especially where roof spaces are shared or party walls are involved.