A loft conversion in London in 2026 typically costs between £28,000 and £95,000 or more, depending on conversion type, roof shape and whether the project needs full planning permission rather than falling within permitted development. A rooflight or Velux conversion, which keeps the existing roof structure unchanged, sits at the cheaper end from around £28,000. A dormer conversion, the most common type fitted to London terraces, typically runs £45,000 to £65,000. A hip-to-gable conversion, suited to semi-detached and detached houses with a hipped roof, costs more again, and a mansard conversion, which replaces most of the roof structure, is the most expensive option at £65,000 to £95,000 or more. This guide breaks down what drives that range, including the staircase and floor strengthening cost common to every conversion type, and the party wall surveyor fees many homeowners don't budget for.
Loft conversion cost in London by conversion type
Loft conversion cost in London varies more than most home improvement projects, because the type of conversion you choose changes both the structural work involved and the space it delivers. A rooflight or Velux conversion is the cheapest route because it leaves the existing roof structure and pitch untouched, adding windows into the existing slope alongside the staircase and floor strengthening every conversion needs. It suits a loft with reasonable headroom at the ridge already, typically 2.2 to 2.3 metres or more, and works well for a home office, a single bedroom or extra storage where the budget doesn't stretch to altering the roof shape. A dormer conversion, the type fitted most often to London terraces, adds a vertical-walled box projecting from the rear roof slope, increasing both headroom and floor area compared with a rooflight conversion alone, and usually creates enough space for a double bedroom with a small ensuite. A hip-to-gable conversion suits semi-detached and detached houses with a hipped roof end, extending that hipped section out to a vertical gable wall and adding significant floor area along that side of the roof, sometimes combined with a rear dormer on the same project. A mansard conversion is the most substantial option, replacing most of the existing roof slope with a near-vertical wall and a shallow flat section at the top, and it's common on London terraces specifically because it recovers close to the full footprint of the floor below.
The table below sets out broad cost ranges for each conversion type, and the gap between them reflects how much roof structure is actually being rebuilt rather than added to. A rooflight conversion is closest to a straightforward internal job with a roof-window addition. A dormer sits in the middle, since it's a genuine new structure bearing on the existing roof and, in most terraces, onto the party wall. A hip-to-gable conversion involves rebuilding an entire hipped roof section into a vertical gable wall, which is a bigger structural undertaking than it first appears from outside the house. A mansard conversion is closer to a partial roof rebuild than an addition, which is both why it delivers the most space and why it's the most expensive option on this list. These figures include the staircase, floor strengthening and a standard fit-out; an ensuite, covered further down, adds to each figure separately.
London loft conversion cost by conversion type (2026 guide)
Item
Typical range
Notes
Rooflight/Velux conversion
£28,000–£45,000
Existing roof structure unchanged, staircase and floor strengthening included
Dormer conversion (single rear dormer)
£45,000–£65,000
Most common type on London terraces, often includes a small ensuite
Hip-to-gable conversion
£50,000–£70,000
Semi-detached or detached houses with a hipped roof end
Hip-to-gable combined with rear dormer
£60,000–£85,000
Common combination for maximum floor area
Mansard conversion
£70,000–£95,000+
Near-full new roof structure, usually needs planning permission
Figures are general London market guidance only, not a fixed Lian Construction quote. Request a free survey for pricing specific to your loft.
The staircase and floor strengthening cost that applies to every conversion type
Whichever conversion type you choose, the staircase and floor strengthening cost sits underneath the headline figure and applies equally across all four types, since it's driven by the existing structure of the house rather than by the roof alteration itself. Loft joists are almost never sized for a habitable room. They're designed to carry stored boxes and the odd person walking across them, not the furniture, people and daily loads a bedroom or bathroom brings, so nearly every conversion needs the floor strengthened, either by adding new joists alongside the existing ones or introducing steel beams to carry the new loads. Cutting the staircase opening into the floor below removes structural support that has to be replaced with trimmer joists around the new opening, and the stairs themselves are rarely a like-for-like drop-in: a straight flight is the simplest and cheapest option, while a winding or space-saver staircase, often the only way to fit a compliant staircase into a tight London terrace floor plan, costs more, with some loss of floor area on the level below unavoidable either way.
Across a typical London loft conversion, the combined staircase and floor strengthening package, covering the structural floor, the new staircase itself, and the trimmer joists and any steel needed around the opening, typically costs £8,000 to £16,000, depending on whether steel beams are needed and how much the staircase design has to work around an existing tight stairwell. This figure is already folded into the conversion-type ranges above rather than sitting on top of them, but it's worth understanding as its own line item, since it's the reason even the cheapest rooflight conversion isn't a cheap job.
En-suite addition: the most common cost add-on
An ensuite bathroom is the most common cost add-on requested alongside a loft conversion, and it's worth pricing separately from the base conversion cost rather than assumed to be included. Adding an ensuite means a new waste run falling back to the existing soil stack, which is straightforward where the new bathroom sits reasonably close to the stack and considerably more involved where pipework has to travel further across the new floor to reach it, sometimes needing a raised section of floor to achieve the necessary fall. It also means extending hot and cold water supplies up into the roof space, additional electrics for lighting, extraction and often underfloor heating, and a mechanical extract fan to manage moisture in a room that often has no external wall for natural ventilation. Waterproofing around the shower area and full tiling add to the finish cost on top of the first and second fix work.
Adding an ensuite to a loft conversion typically costs £5,000 to £9,000, depending on how far the new bathroom sits from the existing soil stack and the sanitaryware and tiling specification chosen. Electrical work in a new ensuite is notifiable under Part P of the Building Regulations and is tested and certified by a qualified electrician as part of the same job, and Building Control checks the waterproofing and ventilation alongside the rest of the conversion at the relevant inspection stage.
The party wall cost factor: surveyor fees people don't budget for
Loft conversions on terraced and semi-detached houses very often involve the party wall, the wall shared with the house next door, and this is one of the most commonly underbudgeted costs on the whole project. New floor joists bearing onto the party wall, structural steels supporting a dormer, new masonry forming a dormer cheek wall built off the party wall, raising the height of a party wall's parapet, or altering a shared flue as part of the conversion can all bring the Party Wall etc. Act 1996 into play, and on most terrace loft conversions at least one of these applies. A Party Structure Notice generally needs serving at least two months before the affected work starts, and if a neighbour doesn't respond within 14 days or actively objects, a dispute is deemed to exist under the Act, which triggers a formal surveyor process before that work can lawfully begin. We identify at survey stage where a proposed conversion is likely to trigger the Act and build the notice period into the programme, but the party wall process itself, including agreeing and issuing the Award, is carried out by party wall surveyors, not by Lian Construction.
Cost varies considerably depending on how the process goes. Where both neighbours agree to appoint a single Agreed Surveyor acting impartially for both sides, which is usually the faster and cheaper route for a straightforward loft conversion, fees are typically in the region of £700 to £1,500 in total. Where a neighbour instead appoints their own separate surveyor, each surveyor charges their own fee, commonly £700 to £1,500 each, and by convention, the person carrying out the work pays both, taking the combined cost to somewhere in the region of £1,500 to £3,500 or more if a third surveyor needs to be brought in to resolve a disagreement between the two. These are general guidance figures rather than a quote, since actual surveyor fees vary by practice and by how straightforward the work is, but budgeting for at least the lower end of this range on any terrace or semi-detached loft conversion is sensible rather than assuming a neighbour's consent will be free.
Planning permission vs permitted development: cost and time implications
Many loft conversions can proceed under permitted development rather than needing a full planning application, though the rules have real limits worth understanding before assuming a project qualifies. Under the current permitted development rules, a terraced house can typically add up to 40 cubic metres of additional roof space, while a detached or semi-detached house can typically add up to 50 cubic metres, and that allowance covers the whole roof alteration, not just the visible dormer or extension. Materials need to be of a similar appearance to the existing house, verandas, balconies and raised platforms aren't normally permitted, roof extensions other than a hip-to-gable conversion usually need to be set back from the original eaves, and side-facing windows are typically required to be obscure-glazed and non-opening below 1.7 metres. Flats and maisonettes don't have the same permitted development rights as houses and almost always need planning permission. Some boroughs and conservation areas have an Article 4 direction removing permitted development rights for roof alterations specifically, and previous extensions or loft work can use up some or all of the volume allowance even where they weren't loft-specific, which is easy to overlook on a house that's already been extended.
Mansard conversions, because of the scale of roof alteration involved, most often fall outside permitted development and need a full planning application as a matter of course, which is worth factoring into both budget and timeline from the outset rather than discovered partway through design. A full planning application typically adds several weeks to a couple of months to the programme before a decision comes back, plus the cost of an architect or planning consultant preparing drawings and making the application, on top of the build cost itself. Permitted development avoids the planning application fee and the waiting period for a decision, but confirming the position with certainty, through a lawful development certificate, still carries its own modest council fee and processing time. Checking the specific position with the local planning department, or applying for a lawful development certificate, is worth doing before committing to a start date. Confirming the planning route and making any application is handled by you directly or by an architect working on your behalf, not something Lian Construction does itself.
Timeline expectations for a loft conversion
Programme length depends heavily on the type of conversion and how much of the roof structure is changing. A straightforward rooflight conversion, without any alteration to the roof shape, typically takes around four to six weeks from scaffold going up to final handover. A dormer conversion, the most common type on London terraces, usually takes six to ten weeks, since it involves more roof alteration, a larger structural floor and often an ensuite. A hip-to-gable or mansard conversion takes longer again, often ten to fourteen weeks or more, reflecting the scale of roof rebuild and the additional structural and finishing work that comes with a larger new floor area. None of this includes the lead-in period before building work starts, covering confirmation of the planning route and any party wall notices, both of which need running in parallel with structural design rather than left until the build is ready to start.
Once building work starts, scaffolding goes up first, followed by opening up the roof structure, sequenced carefully to keep the property weathertight at the end of each working day. The most disruptive stage for the rest of the house is usually cutting the staircase opening into the existing floor, since it affects the room directly below and briefly opens a structural gap that needs supporting properly until the new stairs are in. Most households can continue living in the property throughout a loft conversion, since the work is largely contained to the roof space and the staircase area, though noise and dust are unavoidable during the structural stages.
Questions
Frequently asked questions
How much does a loft conversion cost in London in 2026?
A rooflight or Velux conversion, which keeps the existing roof structure unchanged, typically costs £28,000 to £45,000. A dormer conversion, the most common type on London terraces, typically runs £45,000 to £65,000. A hip-to-gable conversion typically costs £50,000 to £70,000, and a mansard conversion, the most substantial option, typically runs £65,000 to £95,000 or more. These figures include staircase and floor strengthening; an ensuite and any party wall surveyor fees add to the total separately. We give a fixed figure specific to your property once we've surveyed the loft.
What's the cheapest type of loft conversion?
A rooflight or Velux conversion is the cheapest option because it leaves the existing roof structure and pitch unchanged, adding windows set into the existing slope rather than rebuilding any part of the roof. It still needs the same staircase and floor strengthening as any other conversion type, which is why even the cheapest option isn't inexpensive, but it avoids the cost of a dormer structure, a hip-to-gable rebuild or a mansard's near-full roof replacement. It suits a loft with reasonable existing headroom at the ridge.
Why is a dormer conversion the most common choice for London terraces?
A dormer conversion adds a vertical-walled box projecting from the rear roof slope, which increases both headroom and usable floor area more than a rooflight conversion alone, usually creating enough space for a double bedroom and a small ensuite. Most Victorian and Edwardian London terraces have a roof shape well suited to a rear dormer without the scale of rebuild a hip-to-gable or mansard conversion needs, which is why it's the specification we quote most often for terraced houses across London.
Can I get a hip-to-gable conversion on a terraced house?
Generally no. A hip-to-gable conversion works by extending a hipped, sloping roof end out to a vertical gable wall, which suits semi-detached and detached houses that have a hipped roof end in the first place. Most London terraces have a roof that runs straight between two gable-ended party walls rather than a hipped end, so a dormer or mansard conversion is usually the relevant option for a terraced property. We'll confirm what your roof shape actually allows at survey stage.
Why is a mansard conversion the most expensive option?
A mansard conversion replaces most of the existing roof slope with a near-vertical wall and a shallow flat section at the top, which is closer to a partial rebuild of the roof than an addition to it. That's why it delivers the most usable floor area of any conversion type, recovering close to the full footprint of the floor below, and also why it costs the most and, in most cases, needs full planning permission rather than falling within permitted development.
Does every loft conversion need a new staircase and floor strengthening?
Yes. Loft joists are sized to carry storage loads, not the weight of a habitable room with furniture and people, so nearly every conversion needs the floor strengthened with new joists or steel beams, and a new staircase has to be fitted since a loft hatch and ladder don't meet Building Regulations for a habitable room. This combined package typically costs £8,000 to £16,000 across a rooflight, dormer, hip-to-gable or mansard conversion, and is already included in the headline cost ranges for each conversion type above.
How much does adding an ensuite to a loft conversion cost?
Adding an ensuite typically costs £5,000 to £9,000, depending on how far the new bathroom sits from the existing soil stack, the sanitaryware specification chosen, and whether mechanical extraction and underfloor heating are included. Where the new bathroom sits some distance from the stack, achieving the necessary waste fall sometimes needs a raised section of floor, which adds further cost. Electrical work in the new bathroom is notifiable under Part P and is certified by a qualified electrician.
How much do party wall surveyor fees cost for a loft conversion?
Where both neighbours agree to a single Agreed Surveyor, typical fees are in the region of £700 to £1,500 in total for a straightforward loft conversion. Where a neighbour appoints their own separate surveyor instead, each surveyor charges their own fee, commonly £700 to £1,500 each, and by convention the person carrying out the work pays both, taking the combined cost to roughly £1,500 to £3,500 or more if a third surveyor is needed to resolve a disagreement. These are general guidance figures rather than a quote, and it's a cost worth budgeting for on any terrace or semi-detached loft conversion.
Do I need planning permission for a loft conversion, or does permitted development cover it?
Many loft conversions fall within permitted development, allowing up to 40 cubic metres of additional roof space on a terraced house or 50 cubic metres on a semi-detached or detached house, subject to conditions on materials, window glazing and set-back from the eaves. Flats almost always need planning permission, some conservation areas have an Article 4 direction removing permitted development rights, and mansard conversions usually need full planning given the scale of roof alteration involved. We'll flag which route your project is likely to need at survey stage, though confirming the position and making any application is handled by you or your architect.
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