A Velux loft conversion — more precisely called a rooflight conversion — is the simplest and most affordable way to turn a loft into a habitable room. Rather than altering the roof structure by adding dormers or extending the gable, you work within the existing roof shape, fitting roof windows (the Velux brand name is widely used generically) to bring in light and ventilation. The result is a room with sloping ceilings and characterful angled windows, at roughly half the cost of a dormer conversion.
What Is a Velux Loft Conversion?
In a rooflight conversion, the roof structure itself is not changed. The work involves:
- Strengthening the floor — existing ceiling joists are typically 100 × 50 mm or 75 × 50 mm, designed to carry only plasterboard and insulation. New structural floor joists (usually 200 × 50 mm C16 or C24 timber, or engineered I-joists) are installed alongside or between the originals.
- Installing rooflights — typically two to four Velux or equivalent windows cut through the existing rafters, properly flashed and weather-sealed.
- Insulating the roof slope — between and over (or under) the rafters to Part L standard (U-value ≤ 0.18 W/m²K). This usually requires cutting back to exposed rafters and using a combination of rigid insulation board and a breathable membrane.
- Adding a staircase — the biggest space-consumer in any loft conversion. A straight flight needs roughly 2.6 m of floor below; space-saving alternating-tread stairs can reduce this to 1.5 m.
- Electrics, plasterboard, and finishing — first fix wiring, plasterboard to walls and sloping ceiling, plastering, and basic decoration.
The roof’s external profile is unchanged. No scaffolding drama for weeks — a rooflight conversion typically needs scaffolding for 1–2 weeks, versus 6–10 weeks for a dormer.
Cost
| Element | Typical Cost (2026) |
|---|---|
| Structural floor (new joists, trimming) | £3,000–£5,500 |
| Rooflights (2–4 units, supply and fit) | £2,000–£4,500 |
| Roof insulation (between/over rafters) | £2,500–£4,500 |
| Staircase (straight flight, basic) | £3,500–£6,000 |
| Electrics (lighting, sockets, smoke alarms) | £1,500–£2,500 |
| Plasterboard and plastering | £2,500–£4,500 |
| Decoration (basic) | £800–£1,500 |
| Building Regulations application | £900–£1,300 |
| Total (excluding VAT) | £18,000–£32,000 |
London pricing sits at the upper end or above; rural areas and the Midlands/North are typically 15–25% cheaper. VAT at 20% is applicable on most new-build loft conversion work.
The cost per m² of completed floor area is £900–£1,600/m², compared with £1,400–£2,200/m² for a dormer conversion — the saving reflects the absence of structural roof alteration.
Rooflight Sizes and Costs
Standard Velux units most commonly used in loft bedrooms:
| Window Model | Approx Size | Supply Cost (unit) |
|---|---|---|
| GGL C04 | 550 × 980 mm | £380–£450 |
| GGL MK04 | 780 × 980 mm | £430–£520 |
| GGL MK06 | 780 × 1,180 mm | £470–£570 |
| GGL PK10 | 940 × 1,600 mm | £600–£750 |
Installation (including cutting, structural trimming, flashing kit, and sealing) adds £400–£700 per window. For building regulation compliance, rooflights in habitable rooms must have an openable area of at least 1/20th of the floor area they serve, and meet minimum ventilation requirements under Part F.
When No Planning Permission Is Needed
In England, rooflight conversions almost always fall within Permitted Development under Class A or Class B of Schedule 2, Part 1 of the Town and Country Planning (General Permitted Development) (England) Order 2015.
The key conditions for rooflights:
- The rooflight must not project more than 150 mm from the existing roof plane when measured from the roof surface.
- On a principal elevation (the front of the house facing the highway), only flush-fitting rooflights are permitted — projecting units on the front face require planning permission.
- The house must not be in a Conservation Area, AONB, National Park, or subject to an Article 4 Direction removing PD rights.
- The property must not be listed.
For Conservation Areas, flush-fitting rooflights on rear slopes are still generally permitted, but front-elevation rooflights are not. Check with your local authority.
A Lawful Development Certificate (LDC) is not legally required but is strongly recommended (£258 application fee in England). It provides a formal record that the works are lawful, which is valuable at point of sale.
Pros and Cons vs a Dormer Conversion
| Factor | Rooflight Conversion | Dormer Conversion |
|---|---|---|
| Cost | £18,000–£32,000 | £35,000–£65,000 |
| Planning complexity | Usually PD, no application needed | Often PD, but more rules; some need permission |
| Construction time (on site) | 4–7 weeks | 8–14 weeks |
| Floor area gained | Limited by existing roof shape | Significantly more (6–15 m² extra) |
| Headroom at eaves | Limited — slopes sharply | Full height at dormer walls |
| Character | Cosy, angled ceilings | More conventional room feel |
| Disruption | Lower (minimal scaffolding) | Higher (roof opened for weeks) |
| Value added | 10–15% | 15–25% |
The key limitation of a rooflight conversion is headroom at the edges of the room. The usable standing area is restricted to where the ridge gives enough height — typically 1.8 m or more. For a standard inter-war semi with a 2.4 m ridge height, after a 150–200 mm raised floor and 50 mm ceiling void, you may have standing headroom over only 40–50% of the floor area. This is sufficient for a bedroom but can feel cramped. If the loft is naturally spacious (ridge height above 2.5 m, wide span), a rooflight conversion works well. If headroom is marginal, a dormer or hip-to-gable will make better use of the space.
Suitability: Is Your Loft a Good Candidate?
A rooflight conversion works well when:
- Existing ridge height is ≥ 2.3 m from the existing ceiling joist top to underside of ridge (measure before construction, accounting for new floor build-up of 200–250 mm and ceiling insulation).
- The loft has a symmetrical pitched roof — gable-to-gable is ideal.
- You need one room (bedroom, study, gym) rather than multiple rooms and an en-suite.
- Budget is the primary constraint.
- The property is in a Conservation Area where dormer windows would require planning permission.
It is less suitable when:
- Ridge height is below 2.0 m (post floor-build, standing headroom will be very limited).
- You need two rooms or an en-suite (the floor area is simply too small).
- The roof has complex geometry — hips, valleys, and dormers already present can make insulation and joist installation awkward.
For many homeowners, a rooflight conversion is also a first step: establish the space, use it for a few years, then return to add a dormer later. Building Regulations approval is needed each time, but the structural work is largely independent.
