Velux Loft Conversion London 2026: Costs, Planning & Design Guide
A Velux, or rooflight-only, loft conversion is the cheapest and simplest way to convert a loft in London - no dormer, no added volume, and in most houses no planning permission. Here are the real 2026 costs, exactly when a rooflight conversion works, and how it compares to a dormer.
TL;DR - The Quick Answer
A Velux loft conversion in London typically costs £27,500-£40,000 for a basic finish, £38,000-£55,000 mid-range, and £50,000-£70,000 for high-spec work including VAT - the cheapest of all loft conversion types. It only works where the existing roof already gives around 2.2m or more of headroom at the ridge, because no dormer or extra volume is added, just rooflights set into the existing roof slope. Rooflights fall under permitted development Class C, so most houses need no planning permission, though flats, maisonettes, listed buildings and some conservation areas are exceptions. Builds typically take 6-10 weeks once on site.
What Is a Velux Loft Conversion?
A Velux loft conversion - also called a rooflight-only or roof-light conversion, after the market-leading rooflight brand - converts the existing loft into usable living space without altering the shape of the roof at all. Rooflights are cut into the existing roof slope, insulation and flooring are added, and the space is finished as a room. No dormer, no gable extension, no additional volume of any kind is added to the building.
This makes it the simplest, cheapest and fastest loft conversion route available, but it is also the most limited. Because nothing is added to the roof structure, the usable floor area is fixed by whatever headroom the existing roof pitch already provides. On a shallow-pitched postwar semi, that might not be enough to make the conversion worthwhile. On a steep-pitched Victorian or Edwardian roof, it can produce a genuinely comfortable room.
Velux conversions suit lofts being used as a home office, single bedroom, playroom or occasional guest room, where full head height across the whole floor is less important than in a bedroom used every night. They are a poor fit where you need a second full bathroom, built-in wardrobes at the eaves, or head height right up to the walls.
When a Velux conversion works
Velux vs Dormer vs Hip-to-Gable vs Mansard
There are four common loft conversion routes in London. Cost, space gained and planning route vary sharply between them:
| Type | Space added | All-in cost | Planning route |
|---|---|---|---|
| Velux / rooflight-only | None (uses existing volume) | £27.5k-£70k | Permitted development (Class C) in most cases |
| Rear dormer | 20-30m² | £45k-£90k | Permitted development (Class B) in most cases |
| Hip-to-gable | 15-25m² | £40k-£75k | Permitted development or planning |
| Mansard | 35-50m² | £65k-£180k | Full planning (almost always) |
| Type | Space added | All-in cost inc VAT | Planning route |
|---|---|---|---|
| Velux / rooflight-only | None (uses existing volume) | £27.5k-£70k | Permitted development (Class C) in most cases |
| Rear dormer | 20-30m² | £45k-£90k | Permitted development (Class B) in most cases |
| Hip-to-gable | 15-25m² | £40k-£75k | Permitted development or planning |
| Mansard | 35-50m² | £65k-£180k | Full planning (almost always) |
Source: Mayfair Studio project data • Data as of 2026
The trade-off is straightforward. A Velux conversion is the cheapest and fastest route, and it uses a different permitted development class from every other loft type - Class C rather than Class B - because it adds no volume at all. A dormer roughly doubles the budget but adds real floor area with full head height where it matters most. Hip-to-gable and mansard cost more again but suit hipped roofs or a need for maximum space respectively.
For a loft that already has good headroom and only needs to become a usable room rather than gain square metres, a Velux conversion is usually the right answer. For a loft that is currently too low to stand up in, a dormer or hip-to-gable conversion is the only way to create genuinely usable space.
2026 Costs by Spec
All figures below include VAT at 20%. Construction cost covers the structural and building works. All-in adds design fees, structural engineer where a beam is needed, building regulations, and a contingency.
| Spec level | Construction | All-in | What's included |
|---|---|---|---|
| Basic | £22k-£32k | £27.5k-£40k | 2-3 standard rooflights, ladder or fixed stair, plasterboard finish, no bathroom |
| Mid-range | £30k-£44k | £38k-£55k | 3-4 rooflights, proper staircase, engineered timber floor, small shower room |
| High-spec | £40k-£56k | £50k-£70k | Premium electric rooflights, full bathroom, built-in storage, quality joinery |
What adds to cost on a Velux conversion
All figures inc VAT at 20%. Inner London boroughs (zones 1-2) typically run 10-15% above these figures. A structural engineer is only needed where existing ceiling joists cannot carry the new floor loading without strengthening - common on older properties, less common where joists were already sized generously.
Do Velux Windows Need Planning Permission?
In most London houses, no. Rooflights fall under a different permitted development class from dormers, hip-to-gable conversions and mansards. Dormers and other roof extensions that add volume are assessed under Class B of the General Permitted Development Order, which caps the additional roof space at 40m³ for terraced houses and 50m³ for semi-detached and detached houses. A Velux conversion adds no roof space at all - the rooflights sit within the existing roof plane - so it falls under Class C: alterations to the roof, and the Class B volume limits simply do not apply.
Class C permitted development allows rooflights subject to a small number of conditions. Get these wrong and the installation is unlawful, regardless of how modest it looks from the street.
Class C conditions for rooflights
No part of the rooflight may protrude more than 150mm beyond the plane of the existing roof slope, measured perpendicular to the slope. Standard flush and low-profile rooflights comply easily; some older raised-kerb designs do not.
No part of the installation may be higher than the highest part of the existing roof. Because rooflights sit within the roof plane rather than above it, this is rarely an issue, but it rules out any raised surround or upstand that lifts above the ridge line.
Materials used in the alteration must be of a similar appearance to the existing house, so far as practicable. This mainly affects flashing and surround materials rather than the glazing itself.
Any window inserted at ground, first floor, or (where relevant) second floor level in a wall or roof slope forming a side elevation, and facing a neighbouring boundary, must be obscure-glazed and non-opening (or opening only above 1.7m). Rooflights facing directly onto a side boundary are the case to check carefully.
Flats and maisonettes: permitted development rights under Class C apply only to houses. Flats and maisonettes - including leasehold flats within a converted house - have no permitted development rights whatsoever for roof alterations. Every rooflight installation in a flat needs a full planning application, regardless of how small it is.
Listed buildings: Class C permitted development rights do not apply to listed buildings. Any alteration to a listed roof, however minor, needs listed building consent, and often full planning permission alongside it.
Conservation areas
Class C generally continues to apply inside conservation areas - unlike Class B, it is not automatically restricted there. In practice this means most rear and side rooflights in a conservation area still qualify as permitted development. Two things still need checking before you proceed:
- Front-elevation rooflights (facing a highway) are far more likely to be resisted by conservation officers, even where they are technically permitted development, because they change the appearance of the principal roof slope seen from the street
- Some conservation areas and estates sit under an Article 4 direction that specifically withdraws permitted development rights for roof alterations - always check whether an Article 4 direction applies to your property before assuming Class C is available
Where an Article 4 direction removes Class C rights, or where the property is a flat or listed building, a householder planning application is required - typically an £548 council fee, on top of design costs.
Lawful Development Certificate vs Planning Application
Even where a Velux conversion clearly qualifies as permitted development, it is worth applying for a Lawful Development Certificate (LDC) rather than simply proceeding without any paperwork. An LDC is a council-issued document confirming that the work you carried out was lawful at the time - useful proof for solicitors and buyers when you come to sell, and a safeguard if a neighbour or the council later queries the work.
| Route | Council fee | When to use it |
|---|---|---|
| Lawful Development Certificate | £274 | Recommended evidence when the conversion clearly meets Class C - confirms lawfulness, no design judgement involved |
| Householder planning application | £548 | Required where the property is a flat, listed, under an Article 4 direction, or the rooflights exceed Class C limits |
An LDC is not a legal requirement for permitted development work, but it removes ambiguity. Without one, you are relying on your own assessment of the rules being correct - and Class C, while simpler than Class B, still has enough conditions (protrusion limits, ridge height, side-elevation glazing rules) that a formal certificate is worth the modest fee, particularly on a property you plan to sell within the next few years.
Building Regulations Requirements
Planning permission and building regulations are separate approvals. A Velux conversion may need no planning permission at all, but it still needs full building regulations sign-off before it can be used as habitable space.
Floor strengthening
Ceiling joists in most houses are sized to carry a loft used for storage, not a habitable room. Building control will require either an assessment confirming the existing joists are adequate, or new floor joists (often installed alongside the existing ones) sized by a structural engineer to carry residential loading.
Fire safety and escape
Converting a loft into habitable space triggers the same fire safety requirements as any other loft conversion: 30-minute fire resistance to the new floor, a protected escape route from the loft down to the final exit, and mains-wired interlinked smoke alarms on every floor. Each habitable room needs an escape window with a minimum clear opening of 0.33m² and no dimension less than 450mm - many Velux and rooflight products are available in an escape-compliant configuration specifically for this purpose.
Insulation (2026 standards)
- Roof slope (between and over rafters): U-value 0.18 W/m²K or better
- Floor: U-value 0.18 W/m²K or better
- Rooflights: U-value 1.4 W/m²K or better
Staircase and headroom on the stair
A loft ladder is not acceptable for a habitable room - a proper fixed staircase is required, and it must provide adequate headroom (typically 2m, reducing to 1.8m at the stair edge in constrained cases) along its full length. On a Velux conversion, fitting a compliant staircase into the existing stairwell footprint is often the single biggest design challenge, because there is no additional volume available to absorb an awkward stair run.
Design Considerations
Rooflight placement and count
Because there is no dormer to bring in light and headroom together, rooflight placement does most of the work in making the room feel usable rather than cramped. The general rule is one rooflight per 4-5m² of floor area, positioned to avoid the lowest part of the ceiling where head height already falls away. On a typical Victorian terrace loft, 3-4 rooflights on the rear slope (and sometimes a smaller one on the front slope, subject to the conservation area check above) gives a good balance of daylight without over-glazing the room.
Fixed vs electric opening rooflights
Manual opening rooflight
- Lower cost (£900-£1,600 fitted)
- Simple, reliable, no wiring
- Hard to reach if positioned high or over furniture
Electric opening rooflight
- Remote or app-controlled ventilation
- Often includes rain sensors that auto-close
- Higher cost (£1,800-£2,500 fitted) plus wiring
Working with the existing eaves
Without a dormer to square off the walls, the low eaves at the edges of a Velux conversion define how the room can be used. Getting this right matters more here than in any other loft type, because there is no scope to change it later without moving to a dormer conversion:
- Use the sub-1.5m headroom zone at the eaves for built-in storage rather than leaving it as unusable dead space
- Position the bed, desk, or main furniture piece under the highest section of the ceiling, near the ridge
- Keep the staircase landing at the point of maximum headroom, not tucked under the lowest part of the slope
Realistic Timeline
Design and drawings
2-3 weeksBuilding regulations drawings and structural coordination. First drawings typically within 2 weeks of instruction.
LDC or planning application
4-8 weeksLDC applications are typically determined within 8 weeks but often issued sooner given the low complexity. A householder planning application (where PD does not apply) runs the standard 8-week determination period.
Building regulations
2-4 weeksFull plans or building notice submission with structural calculations where needed. Can run in parallel with the LDC or planning process.
Construction on site
6-10 weeksBasic spec with no bathroom: 6-7 weeks. Mid-range or high-spec with a shower room and full staircase replacement: 8-10 weeks. No temporary roof covering is needed, since the roof structure itself is not opened up beyond the rooflight cut-outs - a significant time and cost saving over a dormer or mansard.
Common Mistakes
Not measuring ridge height before committing to the brief
A Velux conversion only works where the existing roof gives enough headroom. Committing to a two-bedroom brief before checking the ridge height against building regulations often means discovering, after design work has started, that a dormer is unavoidable. Measure first.
Assuming a flat has the same permitted development rights as a house
Permitted development Class C applies to houses only. A leaseholder in a converted Victorian house who assumes their loft can be treated the same as next door's freehold house is very often wrong - flats and maisonettes have no permitted development rights at all, and a full planning application is required regardless of how modest the rooflights are.
Ignoring the Article 4 check in a conservation area
Class C generally survives inside conservation areas, unlike Class B - but some conservation areas and estates sit under an Article 4 direction that removes it specifically. Proceeding on the general assumption that conservation areas are fine for rooflights, without checking for a local Article 4 direction, is a common and avoidable error.
Skipping the Lawful Development Certificate
Proceeding without an LDC saves £274 upfront but creates a gap in the paper trail that a solicitor will flag at sale. Buying back that certainty retrospectively, after work is complete, is more expensive and slower than applying before you start.
Treating the loft ladder as adequate for the finished room
A habitable room needs a proper fixed staircase meeting building regulations headroom and pitch requirements, not the folding ladder used for storage access. Budgeting for the staircase late in the process, after the floor plan is fixed, often means an awkward compromise on stair position.
A Velux loft conversion in London costs £27,500-£70,000 inc VAT depending on spec - the cheapest loft conversion route. It adds no volume to the house, so it falls under permitted development Class C rather than the Class B rules that govern dormers, and most houses need no planning permission at all. Flats, listed buildings and some Article 4 areas are exceptions. Builds take 6-10 weeks on site once instructed, and it only works where the existing roof already gives around 2.2m of headroom at the ridge.
Frequently Asked Questions
How much does a Velux loft conversion cost in London in 2026?
All costs inc VAT: basic spec £27,500-£40,000 all-in, mid-range £38,000-£55,000, high-spec with a full bathroom and premium electric rooflights £50,000-£70,000. It is the cheapest loft conversion type in London because no dormer or roof extension is built - only rooflights are cut into the existing roof slope.
Do Velux windows need planning permission?
In most London houses, no. Rooflights fall under permitted development Class C (alterations to the roof), separate from the Class B rules that apply to dormers and other volume-adding roof extensions. Conditions are that the rooflight protrudes no more than 150mm beyond the roof plane and is no higher than the ridge. Flats and maisonettes have no permitted development rights and always need planning permission. Listed buildings need listed building consent regardless.
What is the difference between a Velux conversion and a dormer loft conversion?
A Velux (rooflight-only) conversion adds no volume to the roof - rooflights are simply cut into the existing slope, so it relies entirely on whatever headroom the roof already provides. A dormer projects out from the roof slope, adding genuine floor area and full head height across a larger footprint. Velux conversions cost £27,500-£70,000 and rely on Class C permitted development; dormers cost £45,000-£90,000 and rely on Class B permitted development, with a 40-50m³ volume limit.
How much headroom do I need for a Velux loft conversion?
As a working guide, the existing roof needs to give around 2.2m or more of headroom measured from the top of the ceiling joists to the underside of the ridge beam. Below this, it becomes difficult to create a usable floor area that meets building regulations head-height requirements, since no dormer is added to compensate.
Do I need a Lawful Development Certificate for a Velux conversion?
It is not a legal requirement but is strongly recommended. An LDC costs £274 and provides council confirmation that the rooflight installation was lawful permitted development at the time - useful evidence for a solicitor or buyer at the point of sale, and a safeguard if the work is later queried. Where permitted development does not apply (flats, listed buildings, some Article 4 areas), a full householder planning application is required instead, at a £548 council fee.
Can I add a Velux loft conversion to a flat in London?
Only with full planning permission. Permitted development Class C, which allows most rooflight installations in houses without planning permission, applies to houses only - flats and maisonettes have no permitted development rights for roof alterations. Every rooflight in a flat, however small, needs a planning application before work starts.
Summary
A Velux loft conversion is the cheapest and fastest way to convert a London loft, but it only works within the limits of the existing roof. No volume is added, so the space you end up with is fixed by the headroom you already have - roughly 2.2m or more at the ridge is the practical minimum for a usable room.
Because no volume is added, rooflights sit under permitted development Class C rather than the Class B rules that apply to dormers - most houses need no planning permission at all, subject to the 150mm protrusion limit, the ridge-height limit, and side-elevation glazing rules. Flats, maisonettes, listed buildings, and properties under an Article 4 direction are the main exceptions, and always need checking before work starts.
Budget carefully for the full project including design, an LDC or planning fee, building regulations, and the staircase - not just the construction contract. Get the ridge-height check and the planning status confirmed before committing to a brief.
How Mayfair Studio Handles a Velux Conversion
Our fixed-fee design and planning package for a loft conversion is £1,880 all-in - design drawings, the Lawful Development Certificate or householder planning application submitted on your behalf, and building regulations drawings with structural calculations, all included in one fee. It is paid in two stages: £1,180 to submission, and £700 after approval, with a price match guarantee against any comparable quote.
First drawings land within 2 weeks of instruction. There is no wait for a measured survey to start - the initial design is produced from planning records, property data, and your own photos, with a measured survey carried out at the building regulations stage once the design is agreed.