Loft Conversions London
Unlock your home's hidden potential with a professionally built loft conversion, adding valuable living space without extending your footprint.
Loft conversion specialists in London
Alstruct designs and builds loft conversions across London: rooflight (Velux), rear dormer, L-shaped dormer, hip-to-gable and mansard. Whether you call it a loft conversion, a loft extension or an attic conversion, the aim is the same: a properly built new floor, usually a bedroom with an en-suite, finished to the standard of the rest of the house.
Every project is priced as a fixed-price, itemised written quote after a site visit and technical assessment, built under a JCT contract and covered by our 10-year workmanship warranty. We are CHAS Elite, Constructionline Gold and Federation of Master Builders (FMB) accredited, and all our work is signed off by building control.
At a glance: a loft conversion in London costs £25,000–£100,000+ depending on type: Velux £25,000–£45,000, dormer £45,000–£75,000, L-shaped dormer £55,000–£85,000, hip-to-gable £55,000–£80,000 and mansard £60,000–£100,000+, including VAT. Time on site runs from 6–10 weeks for a Velux conversion to 14–20 weeks for a mansard. Most loft conversions qualify as permitted development; mansards almost always need planning permission. Full breakdown: loft conversion cost guide.
Types of loft conversion we build
The roof you have decides most of this: its shape, the ridge height and whether the house is terraced, semi-detached or detached. The prices below are the London ranges from our cost guide, including VAT, and the durations are time on site.
Rooflight and dormer loft conversions
Velux (rooflight) loft conversion
The only type that leaves the roof shape unchanged: the existing structure is strengthened to form the floor and roof windows are fitted into the slope. It suits lofts with good standing height. In a typical Victorian terrace the central standing area may be only 1.5–1.8m wide, so it works best as a study, guest room or child's room. £25,000–£45,000; 6–10 weeks.
Dormer loft conversion
A box structure built out from the rear roof slope for full headroom and more floor area. It is the most popular type in London and the one we carry out most often; a full-width rear dormer on a Victorian terrace can give one large room, or two rooms with an en-suite. £45,000–£75,000 at mid specification; 10–14 weeks.
L-shaped dormer loft conversion
Two dormers, one on the main rear slope and one over the rear outrigger, meeting at a right angle. It suits Victorian and Edwardian terraces with an outrigger and frequently qualifies as permitted development, provided the volume and height limits are met. £55,000–£85,000; 12–16 weeks.
Hip-to-gable and mansard loft conversions
Hip-to-gable loft conversion
The hipped end of the roof is rebuilt as a vertical gable wall, almost always combined with a rear dormer. It suits semi-detached and detached houses with hipped roofs, which are common in the interwar and postwar suburbs. On a semi, party wall notices to the adjoining neighbour are typically required. £55,000–£80,000 with the rear dormer; 10–16 weeks.
Mansard loft conversion
The rear roof slope is rebuilt almost vertically, typically at 70 degrees or steeper, under a shallow-pitch flat roof. It gives the greatest volume of any type and is particularly prevalent on central London's period terraces. £60,000–£100,000+, with high-specification schemes in prime central London regularly reaching £120,000–£150,000; 14–20 weeks. It almost always needs planning permission.
What a loft conversion costs in London
The table repeats the summary from our 2026 cost guide. Use it to set a budget; your quote will be a fixed price based on a survey of your roof, not an assumption.
| Type | Typical cost | Typical duration |
|---|---|---|
| Velux (rooflight) | £25,000–£45,000 | 6–10 weeks |
| Dormer | £45,000–£75,000 | 10–14 weeks |
| L-shaped dormer | £55,000–£85,000 | 12–16 weeks |
| Hip-to-gable | £55,000–£80,000 | 10–16 weeks |
| Mansard | £60,000–£100,000+ | 14–20 weeks |
Indicative ranges for London in 2026. Figures include VAT at 20% and a standard allowance for staircase, insulation, plastering and basic decoration. En-suite bathrooms and bespoke joinery are additional.
Within each range, specification decides where you land. The upper end of the dormer range reflects a larger dormer with a fully tiled en-suite shower room, timber sash or casement windows to match the house and storage built into the eaves. A GRP, zinc or copper roof finish adds £2,000–£6,000 over a standard felt-and-batten roof, and Juliet balconies or French doors add £2,500–£6,000.
Structure matters too. A full-width dormer needs larger structural elements, including a ridge beam, valley rafters and careful integration with the party wall, which adds time and cost. Beyond the build contract, our guide lists architect or designer fees of £2,000–£6,000, a structural engineer at £600–£1,500 and building regulations fees of £700–£2,000.
The loft conversion cost guide sets out what a quote should include, line by line, and what is usually extra.
A fixed price, not a range
Get a fixed-price quote for your loft conversion
We visit, measure and send an itemised quote on a JCT contract, with our 10-year workmanship warranty on the finished work.
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How a project runs with Alstruct
We treat a loft conversion as a structural and spatial design project, not just a construction task. Time on site depends on the type, from 6–10 weeks for a Velux conversion to 14–20 weeks for a mansard. Add 2–4 months before that for design, structural calculations and any planning permission.
- Site visit and technical assessment. Before we quote, we measure the existing loft volume, ridge height, rafter spacing and the ceiling height below. We review the party wall position and any planning constraints, and check that the staircase can be routed without an unacceptable loss of space on the floor below.
- Design and drawings. With in-house architectural support we prepare the drawings, or we work with your architect and structural engineer. Either way, the design and structural package is complete before work starts on site.
- Planning, permitted development and party wall. Most conversions are permitted development. Where an application is needed, a householder decision typically takes 8–10 weeks; for a mansard our guide suggests budgeting 10–16 weeks for planning. We manage the party wall process for you where it applies.
- Fixed-price quote and JCT contract. The quotation is fully itemised and based on a complete specification, with no provisional sums for items we have not properly scoped. We use JCT Minor Works contracts on all projects: fixed pricing, a defined programme and a transparent variation process.
- Structure and roof. Structural steelwork and the new floor structure go in first. Then the dormer or mansard is built, the roof is covered to match the existing, leadwork and flashings are fitted at every junction, and the windows and rooflights are installed.
- Stairs, services and finishes. A compliant staircase connects the new floor to the one below. Insulation to Part L, first and second fix electrics with smoke alarms linked to the existing system, plumbing for any en-suite, plastering and decoration follow.
- Building control sign-off. Building regulations approval is required for every loft conversion, whatever its planning status. It covers fire protection to the escape route, structural calculations and energy compliance, and the work is signed off by building control.
- Handover and warranty. Our 10-year workmanship warranty starts at practical completion, the date the works are handed over, and passes to the new owner if you sell. The full wording is in our terms and warranty.
Planning, building regulations and neighbours
In England, most loft conversions qualify as permitted development, so no planning application is needed, provided the conditions are met. The added volume must not exceed 40 cubic metres on a terraced house or 50 cubic metres on a semi-detached or detached house. The conversion must not extend beyond the plane of the roof slope on the front of the house or rise above the highest part of the existing roof. Side-facing windows must be obscure-glazed and non-opening below 1.7m above floor level, and materials must be similar in appearance to the existing house.
There are two common exceptions in London. Mansard conversions almost always need full planning permission, whether or not the house is in a conservation area. Article 4 Directions remove some or all permitted development rights in specific areas, and Camden, Islington, Westminster, Hackney and Southwark all have extensive coverage. We review your address and advise on the planning position as part of the initial consultation.
Conservation area properties in Islington, Hackney and Camden can often still use Velux-type roof windows under permitted development, provided they do not project above the plane of the roof slope facing the highway, although some boroughs with Article 4 directions require permission even for rooflight conversions. For a mansard in a conservation area, planners look closely at materials, window proportions and parapet detailing, so the quality of the drawings and heritage statement matters.
Building regulations apply to every loft conversion regardless of planning status. Our guide sets out the main items: a compliant staircase, typically with a 42-degree maximum pitch; fire protection to escape routes and smoke alarms integrated with the existing system; insulation to Part L; structural calculations for the floor and steelwork; and windows that meet the energy and fire-safety requirements. Head height is measured at the site visit, because it decides which types will work.
On a terraced or semi-detached house you will likely need to serve party wall notices, because the structural work to the roof and floor typically involves the party wall. A Party Structure Notice must be served at least 2 months before the start, and the neighbour has 14 days to respond. Surveyor costs in London are typically £800–£1,500 with a single agreed surveyor, or £800–£3,500 per neighbour where each side appoints its own, paid by the building owner. The party wall agreement cost guide has the detail.
What is included
One contract covers the work from survey to sign-off. Fitted wardrobes, bathroom sanitary ware and tiling are typically priced separately unless you ask us to specify them, so those choices stay yours.
- Free site visit and consultation
- Loft feasibility assessment
- Architectural drawings and 3D visualisations
- Planning applications (where required)
- Building regulations approval
- Structural engineering and steel beams
- Party wall process managed (where required)
- Roof construction (dormer, mansard or hip-to-gable)
- Staircase design and installation
- Full insulation and fire protection
- Electrics, plumbing and heating
- En-suite bathroom installation (if required)
- Plastering, decoration and flooring
- Full project management throughout
Recent loft projects
Two of our published case studies include loft conversions, both delivered as part of a wider refurbishment; more work is in our projects portfolio.
Areas we cover
We build loft conversions across London. L-shaped dormers are popular on the Victorian terraces of Hackney, Islington and Walthamstow; hip-to-gable conversions suit the hipped-roof semis of Wimbledon, Wanstead, Richmond and Chiswick; and mansards are typical of central London's period terraces, in areas such as Kensington, Notting Hill and Chelsea. Each area page has a section on loft conversions with local planning notes; see all areas.
Frequently Asked Questions
How much does a loft conversion cost in London in 2026?
A loft conversion in London costs £25,000–£100,000+ depending on type: a Velux (rooflight) conversion £25,000–£45,000, a dormer £45,000–£75,000, an L-shaped dormer £55,000–£85,000, a hip-to-gable £55,000–£80,000 and a mansard £60,000–£100,000+. Figures include VAT at 20% and a standard allowance for the staircase, insulation, plastering and basic decoration; en-suite bathrooms and fitted joinery are extra. We give a fixed-price, itemised quote after a site visit.
Do I need planning permission for a loft conversion in London?
Most loft conversions qualify as permitted development, provided the added volume stays within 40m³ for a terraced house or 50m³ for a semi-detached or detached house and the other conditions are met. Mansard conversions almost always need planning permission. Article 4 directions, common in many inner London boroughs, can require an application even for simpler types. We review your address and advise on the planning position at the initial consultation.
How long does a loft conversion take?
On site, a Velux conversion typically takes 6–10 weeks, a dormer 10–14 weeks, an L-shaped dormer 12–16 weeks, a hip-to-gable 10–16 weeks and a mansard 14–20 weeks. Add 2–4 months beforehand for design, structural calculations and any planning permission. Where an application is needed, a householder decision typically takes 8–10 weeks; for a mansard, our guide suggests budgeting 10–16 weeks for planning.
Can we live in the house during a loft conversion?
In most cases, yes. Homeowners can usually remain in the property throughout a loft conversion, which is one of the least disruptive types of building work because most of it takes place above the existing living space. We seal off the work area from your living space, keep dust, noise and disruption to a minimum and maintain safe access to your home at all times.
Do I need a party wall agreement for a loft conversion?
If your house is terraced or semi-detached, you will likely need to serve party wall notices, because the structural work to the roof and floor typically involves the party wall. A Party Structure Notice must be served at least 2 months before the start. Surveyor costs in London are typically £800–£1,500 with a single agreed surveyor, or £800–£3,500 per neighbour if each side appoints its own. We manage the process for you.
What building regulations apply to a loft conversion?
Building regulations approval is required for every loft conversion, whether or not it needs planning permission. The main items are a compliant staircase, typically with a 42-degree maximum pitch, fire protection to escape routes with smoke alarms integrated with the existing system, insulation to Part L, structural calculations for the floor and steelwork, and windows that meet the energy and fire-safety requirements. We measure head height at the site visit, and building control signs off the work.
Will a loft conversion add value to my London home?
Our published guidance is that a loft conversion adding a double bedroom and an en-suite typically adds 15–20% to a London property's value, because it adds a bedroom without losing garden space. The return depends on the quality of the work and the usefulness of the space: a main bedroom with an en-suite adds more than a study, and the staircase should not take too much space from the floor below.
What contract and warranty do you work under?
We use JCT Minor Works contracts on all projects: fixed pricing, a defined programme and a transparent variation process. Quotations are fully itemised, with no provisional sums for items we have not properly scoped. Our 10-year workmanship warranty covers defects in our workmanship, starts at practical completion and passes to the new owner if you sell. We are CHAS Elite, Constructionline Gold and Federation of Master Builders accredited.