Loft Conversion Cost in London (2026): Dormer, Velux, Hip-to-Gable and Mansard Prices
A loft conversion in London costs from around £18,000 for a simple Velux room to £50,000 to £85,000 for a dormer, the most common type, and up to £165,000 for a period mansard in 2026. See scenario-by-scenario prices, planning and party wall rules, VAT, timelines and a fair-quote checklist before you hire a builder.
Last updated: September 2026. Written by the loacally editorial team, with pricing aggregated from specialist London builders' 2026 cost guides and planning, tax and consumer facts drawn from the Planning Portal, GOV.UK, legislation.gov.uk and Citizens Advice.
TL;DR A loft conversion in London in 2026 typically costs from around £18,000 for a simple Velux room to £50,000 to £85,000 for a dormer, the most common type, and up to £165,000 for a period mansard. These are market-observed ranges, not official figures, so always get several itemised written quotes.
A loft conversion is one of the most valuable ways to add a bedroom or a home office to a London house without moving. It is also structural building work, which means the headline price is only part of the story: planning rules, party wall agreements, professional fees and VAT all shape what you finally pay. This guide breaks down 2026 London costs by conversion type, explains what a fair quote should itemise, and sets out the planning and Building Regulations facts you need before you commit.
How much does a loft conversion cost in London in 2026?
A typical London loft conversion in 2026 costs between roughly £18,000 for a basic Velux room and £165,000 for a premium period mansard, with a standard rear dormer, the most common London case, generally landing in the £50,000 to £85,000 range. Cost per square metre runs at roughly £2,600 to £4,200/m² across London, materially above the UK-wide average because of higher labour rates, restricted site access and a greater incidence of conservation-area rules.
These figures are market-observed ranges aggregated from specialist London builders' published 2026 price guides, not government or survey data, because no official body publishes loft-conversion cost figures. Treat them as a planning guide and confirm the real number with three or more written quotes for your specific roof.
Loft conversions sit at the higher end of home-improvement spending, so it is worth reading them alongside our London trade costs index for 2026 to see where they rank against smaller jobs.
A fair loft conversion quote: what to check first
Before you compare any prices, know what a proper quote looks like, because the biggest cost risk on a loft conversion is a vague lump sum that hides extras. Citizens Advice draws a clear line between a quote, which is a fixed price the trader cannot change without good reason, and an estimate, which is only a rough guess, and recommends getting at least three written quotes before you decide [6].
A fair written loft conversion quote should itemise:
- A fixed total price, not just a day rate or hourly rate.
- Labour and materials broken down by task, so you can see what the steels, staircase, insulation, electrics, plastering and glazing each cost.
- Whether VAT at 20% is included in the headline figure or added on top [4].
- Professional fees stated separately: architect or designer, structural engineer's calculations, Building Control sign-off, and a party wall surveyor where one is needed.
- How long the quote is valid for, and the specific circumstances under which the price could still rise.
- Start and finish dates, payment schedule, deposit terms, and who is responsible for skips and rubbish removal.
Also confirm the builder holds public liability insurance, ask for two or three recent references, and check whether they are registered with a recognised scheme. TrustMark describes itself as the only UK Government-Endorsed Quality Scheme for home improvements, operating under a licence from the Department for Business and Trade, and checking for TrustMark or Trading Standards approval is a sensible verification step [7]. For a fuller walkthrough of vetting a builder, see our guide on how to spot and avoid cowboy builders.
Loft conversion cost by type (London, 2026)
The single biggest driver of price is the type of conversion, which is dictated by your roof shape and what planning allows. The table below sets out the market-observed 2026 London ranges by type; the sections beneath explain each one.
| Conversion type | Typical London 2026 range | Best suited to |
|---|---|---|
| Velux / rooflight | £18,000 to £48,000 | Roofs with existing head height, no roofline change |
| Dormer (most common) | £50,000 to £85,000 | Terraced and semi-detached houses |
| Hip-to-gable (+ dormer) | £45,000 to £75,000 | Semis and end-terraces with a hipped roof |
| L-shaped dormer | £65,000 to £97,500 | Victorian terraces with a rear addition |
| Mansard | £62,000 to £165,000 | Period terraces, often in conservation areas |
| Per square metre | £2,600 to £4,200/m² | All types, as a cross-check |
All ranges are market-observed 2026 figures aggregated from specialist London builders' cost guides, not official pricing. Professional fees of roughly £6,000 to £15,000 are often quoted separately, on top of the build cost.
Velux / rooflight conversion: from around £18,000
A Velux or rooflight conversion installs roof windows into the existing slope without changing the roofline, which makes it the cheapest and structurally simplest option. A basic room sits at the lower end of roughly £18,000 to £30,000, while a rooflight conversion with an en-suite runs nearer £28,000 to £48,000. Because it does not alter the roof shape, it usually falls within permitted development, which keeps both cost and paperwork down.
Dormer conversion: £50,000 to £85,000 (the lead figure)
A rear dormer, which builds a box-shaped extension out from the roof slope to create full standing height, is the most common loft conversion in London's terraced and semi-detached streets, and it is the figure most homeowners should budget around. Specialist 2026 London guides put a full design-and-build dormer at roughly £50,000 to £85,000 for a single dormer with standard finishes and no en-suite. One survey-based source quotes a higher typical London figure of around £88,000, which is worth noting as an upper data point rather than an outlier to ignore.
Hip-to-gable (plus dormer): £45,000 to £75,000
A hip-to-gable conversion replaces a sloping (hipped) roof side with a vertical gable wall to gain internal volume, and it suits semi-detached and end-terrace houses. The structural conversion with a rooflight and standard finish is quoted at roughly £45,000 to £55,000, while the common London combination of a hip-to-gable plus a rear dormer runs nearer £55,000 to £65,000. Broader London guides give an overall band of £45,000 to £75,000 for this type.
L-shaped dormer: £65,000 to £97,500
An L-shaped dormer combines a rear dormer with a second dormer over a side-return or rear addition, which is typical of Victorian terraces and creates the most floor area of the dormer family. London 2026 guides quote roughly £65,000 to £97,500 for this type. It costs more than a single rear dormer because it forms two new structural roof elements and usually adds more headroom and floor space.
Mansard conversion: £62,000 to £165,000
A mansard rebuilds the rear roof as a near-vertical wall up to the ridge, which maximises space but effectively reconstructs the roof structure, so it is the most expensive and most planning-heavy option. London 2026 figures span roughly £62,000 to £165,000, with a premium central London conservation-area mansard of around 35m² quoted near £122,500. Mansards are most associated with period terraces and conservation areas, and they almost always need full planning permission rather than permitted development.
Adding an en-suite
Adding a bathroom to a loft conversion increases the price, though no source publishes a clean add-on figure. Every quoted number found is a whole-project price for a conversion that happens to include a bathroom, rather than a clean delta on top of a comparable dry-room conversion. As an editorial estimate only, budget roughly £8,000 to £15,000 above a comparable room without a bathroom, and remember an added bathroom may require a new or upgraded boiler, which is a separate cost you can size using our guide to new boiler costs in London.
What is included in a loft conversion price?
A full design-and-build loft conversion quote normally covers the structure, services and finishes needed to turn a cold roof void into a habitable room. Understanding the inclusion list helps you check that a quote is complete rather than artificially cheap because it has left something out.
A standard full design-and-build quote usually includes:
- Structural steels (RSJs) to carry the new floor and roof loads.
- A new staircase, including the structural opening and balustrade.
- Insulation to the roof and floor to meet Building Regulations.
- First-fix and second-fix electrics, which you can benchmark against our house rewire cost guide if the wider home also needs work.
- Plastering and decoration to a standard finish, in line with our plastering cost guide.
- Roof windows or dormer glazing.
- En-suite plumbing where specified.
Professional fees are frequently priced separately, typically £6,000 to £15,000 on top of the build cost, covering the architect or designer, the structural engineer's calculations, Building Control fees and, where triggered, a party wall surveyor. A fair quote lists each of these as its own line, not as a single bundled sum.
Why London costs more
London loft conversions run roughly 15 to 25 per cent above the UK average per square metre, and some sources cite up to 40 per cent, driven by a consistent set of local factors. Recognising these helps you understand why two similar houses can carry very different prices.
The main London premium drivers are:
- Higher labour rates across the trades.
- Restricted site access in dense terraced streets, which complicates scaffolding, skip placement, deliveries and parking.
- Conservation areas and Article 4 directions, which remove permitted development rights and force full planning applications.
- Party wall agreements, which are the norm rather than the exception in terraced and semi-detached streets.
- Head height and extra steelwork, often needed in Victorian and Edwardian roofs with lower pitches.
- Staircase repositioning, which is more disruptive in narrower period floor plans.
Borough-specific access constraints can add further cost, so two identical designs can differ purely on where they are built.
Do I need planning permission for a loft conversion in London?
Many London loft conversions do not need planning permission because they qualify as permitted development, but a significant minority do, and getting this wrong is expensive. A loft conversion is permitted development, with no planning application required, provided it meets the volume and design limits in the Town and Country Planning (General Permitted Development) (England) Order 2015 [1].
The key permitted development limits are that the added roof volume must not exceed the original roof space by more than 40 cubic metres for terraced houses or 50 cubic metres for detached and semi-detached houses, the work must not sit higher than the existing roof, and it must not extend beyond the roof slope on the elevation facing a highway [1]. Side-facing windows must be obscure-glazed, and any that open must sit at least 1.7 metres above the floor [1].
Permitted development rights do not apply everywhere. Homes in conservation areas do not qualify and need a full householder planning application, and a council can remove permitted development rights on specific streets through an Article 4 direction, commonly used where a uniform roofline defines the street's character [1]. Always check your own borough's planning pages for an active Article 4 direction before assuming your scheme is permitted development, because this varies street by street even within one borough.
Building Regulations, party walls and VAT
Beyond planning, three separate legal requirements shape a London loft conversion, and each is distinct from planning permission. A scheme can be permitted development and still need all three.
Building Regulations approval is always required, because a loft conversion involves structural work, fire safety and escape routes, insulation and electrics [2]. You obtain it through a local authority building control team or a private registered building control approver, and the resulting completion certificate matters when you later sell, since missing sign-off can stall a sale and non-compliance can lead to enforcement [2]. From 1 October 2026, certain residential building work may also become subject to a Building Safety Levy, so ask your builder whether it applies [2].
A party wall agreement is very likely on a London terraced or semi-detached house. The Party Wall etc. Act 1996 applies where work affects a shared wall, including steels bearing into a party wall, dormers, hip-to-gable or mansard conversions, and chimney-breast removal [3]. You must serve written notice on affected neighbours at least two months before work starts; the notice is valid for one year, and neighbours have 14 days to respond, with a dissent triggering the appointment of a party wall surveyor, usually at your expense [3].
VAT is charged at the standard 20 per cent rate on a loft conversion to an existing, lived-in home [4]. Reduced rates apply only in narrow cases, such as a property that has been empty for two years or more, so a standard occupied-home conversion does not qualify for a discount [4]. Always confirm whether a quoted figure includes VAT.
You also have a legal backstop on the quality of the work. Under Section 49 of the Consumer Rights Act 2015, every contract to supply a service includes an implied term that the trader performs it with reasonable care and skill, and if they do not, you can require them to put it right at no extra cost [5].
How long does a loft conversion take?
Most London loft conversions run three to four months from instruction to completion, with the build phase itself a smaller part of that timeline. Planning how long the work will take matters as much as the budget, especially if you are living in the house throughout.
Typical build-phase durations by type are:
- Velux / rooflight: around 6 weeks [9].
- Rear dormer: 6 to 8 weeks [9].
- L-shaped dormer: 8 to 10 weeks [9].
- Mansard or complex designs: up to 12 weeks [9].
Add a pre-construction phase of roughly 6 to 10 weeks for design, any planning application, Building Regulations submission and the party wall process [9]. The party wall notice period alone is a minimum of two months, so it can set the pace regardless of how quickly the build runs [3].
Does a loft conversion add value to a London home?
A loft conversion can add meaningful value, and Nationwide's October 2025 research found that an extension or loft conversion adding a double bedroom and bathroom can increase a typical three-bedroom, one-bathroom home's value by up to 24 per cent [8]. The same research found only 4 per cent of homeowners regretted having improvement work done [8].
It is important to read that figure carefully: the 24 per cent headline is a national figure, not a London-specific one, and it applies to a specific house type rather than every property [8]. In practice, the value a conversion adds depends on your local market, the quality of the finish and whether the new space is a genuinely usable bedroom. Treat added value as a reasonable expectation, not a guarantee, and base your decision on the space you gain as much as any resale premium.
Frequently asked questions
How much does a loft conversion cost in London in 2026?
A basic Velux conversion typically starts around £18,000 to £48,000, while a rear dormer, the most common London type, generally runs £50,000 to £85,000. L-shaped dormers, hip-to-gable and mansard conversions cost more, up to about £165,000 for a premium period mansard. These are market-observed ranges, so always get itemised written quotes.
Do I need planning permission for a loft conversion in London?
Often not. Many loft conversions qualify as permitted development if they stay within volume limits of 40 cubic metres for terraced houses or 50 for detached and semi-detached homes, do not exceed the existing roof height, and do not extend beyond the roof slope facing a highway. Conservation areas and Article 4 directions always need full planning permission.
Do I need Building Regulations approval for a loft conversion?
Yes. Building Regulations approval is separate from planning permission and covers the structural work, fire safety, escape routes, insulation and electrics that every loft conversion involves. You obtain it through local authority building control or a private registered building control approver, and you will need the completion certificate when you come to sell the property.
Will I need a party wall agreement for my loft conversion?
Very likely, if your London home is terraced or semi-detached. The Party Wall etc. Act 1996 applies where work affects a shared wall, including steels bearing into a party wall, dormers, hip-to-gable or mansard conversions, and chimney-breast removal. You must give neighbours at least two months' written notice before work begins.
How long does a loft conversion take?
Build time varies by type: roughly 6 weeks for a Velux conversion, 6 to 8 weeks for a rear dormer, 8 to 10 weeks for an L-shaped dormer, and up to 12 weeks for a mansard. Add a pre-construction phase of 6 to 10 weeks, so most London projects run three to four months start to finish.
Is VAT charged on a loft conversion?
Yes. Loft conversions on an existing, lived-in home are charged at the standard 20 per cent VAT rate. Reduced rates apply only in narrow exceptions, such as a property empty for two years or more, which a normal occupied home does not meet. Always check whether a quote is inclusive or exclusive of VAT before comparing prices.
Find a trusted local builder for your loft conversion
A loft conversion is a big commitment, so the builder matters as much as the price. Get three or more itemised written quotes, confirm insurance and references, and make sure planning, Building Regulations and any party wall process are all covered before work starts. For more on vetting the right person, read our guide on how to find a trusted tradesperson in London, and when you are ready, find a trusted local builder near you on loacally.
Sources
- Planning Permission - Loft conversion -- Planning Portal, accessed 2026-09-15
- Building regulations approval -- GOV.UK, accessed 2026-09-15
- The Party Wall etc. Act 1996: explanatory booklet -- GOV.UK, Ministry of Housing, Communities and Local Government, accessed 2026-09-15
- Buildings and construction (VAT Notice 708) -- GOV.UK, accessed 2026-09-15
- Consumer Rights Act 2015, Section 49 -- legislation.gov.uk, accessed 2026-09-15
- Before you get home improvements done -- Citizens Advice, accessed 2026-09-15
- Protecting homeowners and improving the quality of home improvements -- TrustMark, accessed 2026-09-15
- Loft conversions boost home value by up to 24%, Nationwide research shows -- Nationwide Building Society research (Andrew Harvey, 13 October 2025), reported by The Intermediary, accessed 2026-09-15
- How long does a loft conversion take? -- Homebuilding & Renovating, accessed 2026-09-15
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