The terms “skylight”, “roof lantern”, and “roof window” are often used interchangeably in brochures and by salespeople, but they describe quite different products with different price points, planning implications, and structural requirements. Getting the terminology right before you speak to a contractor can save you thousands of pounds and prevent costly mistakes.
The Key Differences at a Glance
| Feature | Roof Window | Flat-Roof Skylight | Roof Lantern |
|---|---|---|---|
| Roof type | Pitched (15°–90°) | Flat or very low pitch (0°–15°) | Flat or very low pitch |
| Profile | Flush-fitting, opens | Low-profile kerb or frameless | Raised, structural ridge frame |
| Typical size | Up to ~1.6 m² | 0.5 m² to 4 m² | 1 m² to 15 m²+ |
| Openable? | Yes (most models) | Fixed or electric only | Fixed (small ones) / electric |
| Installed cost | £800–£1,500 | £900–£2,500 | £2,500–£10,000+ |
| Statement feature? | No | Subtle | Yes — centrepiece |
| Planning risk | Low | Low–medium | Medium–high |
Roof Windows: For Pitched Roofs
A roof window is what most people mean when they say “Velux” — a glazed sash installed flush in a pitched roof surface, replacing a section of tiles or slates. It opens (centre-pivot or top-hung) to provide ventilation and is typically fitted with a factory-made flashing kit that seals against the roof covering.
Best for: loft conversions, attic bedrooms, roof terrace access.
Pitch range: most manufacturers specify a minimum pitch of 15°, though some models go down to 10° with a special flashing.
Cost drivers: size (MK06 at 78×118 cm is a common mid-size), glazing specification (double vs triple), and whether structural rafter modifications are needed.
Flat-Roof Skylights: For Modern Extensions
A flat-roof skylight (also called a rooflight) is a fixed or electric-opening unit set into a flat or shallow-pitched roof on a raised kerb — typically 150 mm above the deck — or frameless at deck level in a “walk-on” configuration.
Fixed vs opening: fixed units are cheaper and simpler; electric opening units add natural ventilation but add £300–£600 to the unit cost. Under Part F of the Building Regulations, habitable rooms above a certain area require either trickle vents or openable lights.
Walk-on rooflights use toughened laminated glass rated to carry foot traffic and must meet BS EN 1991-1-1 for imposed loads (minimum 2.0 kN/m²). Expect to pay a 30–50% premium over a standard fixed unit of the same size.
Cost table (supply only, ex-VAT):
| Size | Fixed Standard | Fixed Premium | Electric Opening |
|---|---|---|---|
| 600×600 mm | £350–£600 | £600–£900 | £700–£1,100 |
| 900×900 mm | £450–£750 | £750–£1,200 | £900–£1,400 |
| 1200×1200 mm | £600–£950 | £950–£1,600 | £1,200–£2,000 |
| 1500×1500 mm | £800–£1,300 | £1,300–£2,200 | £1,600–£2,800 |
Add £400–£900 for installation including kerb preparation and weatherproofing.
Roof Lanterns: The Statement Option
A roof lantern is a structural glazed upstand — essentially a small glazed room-within-a-roof — typically sitting 300–900 mm above the flat roof deck on a perimeter frame. The classic form has a central ridge and two or more pitched glass faces meeting at an apex, giving the impression of a Victorian-era glasshouse in miniature.
Modern roof lanterns use thermally broken aluminium frames and triple-glazed units, giving U-values of 1.2–1.6 W/m²K. Timber-framed lanterns look more traditional but require more maintenance and rarely achieve the same thermal performance.
Planning note: a roof lantern on a single-storey rear extension is usually permitted development, but anything that raises the ridge of a dwelling, or any work in a Conservation Area or on a Listed Building, requires a planning application.
Cost breakdown (rear extension, 3×2 m lantern):
| Item | Cost |
|---|---|
| Aluminium roof lantern (supply) | £2,500–£5,500 |
| Structural upstand / kerb frame | £400–£800 |
| Delivery and crane hire (large units) | £200–£600 |
| Installation labour | £800–£1,500 |
| Making good (plasterwork, decoration) | £300–£700 |
| Total | £4,200–£9,100 |
Bespoke or heritage-style timber lanterns start at £8,000 and can exceed £25,000 for large custom commissions.
Planning Permission: What You Need to Know
Roof windows on rear- or side-facing pitches are almost always permitted development as long as the window does not protrude more than 150 mm above the roof plane. Front-facing windows in Conservation Areas typically need planning permission.
Flat-roof skylights on extensions within permitted development limits (single-storey rear extensions up to 4 m for detached houses, 3 m for others) are generally permitted development. The extension itself may have required prior approval under the larger homes extension scheme.
Roof lanterns are more likely to trigger planning scrutiny because:
- They raise the apparent height of the roof at the rear
- Their visual prominence is greater than a flat rooflight
- Conservation Area designations often specifically restrict them
When in doubt, a Lawful Development Certificate from your local planning authority (fee: £103 in England as of 2026) gives you written confirmation that no permission is needed and protects your position on resale.
Building Regulations
All three types must comply with Part L (Conservation of Fuel and Power):
- Replacement rooflights in existing dwellings: maximum U-value 1.6 W/m²K (the whole unit, not just the glass)
- New extensions: the total glazing area must not exceed 25% of the new floor area, unless the overall carbon compliance of the dwelling is maintained
- Part B (Fire): escape requirements apply if the rooflight is in a habitable room above certain storey heights
Which Should You Choose?
Choose a roof window if:
- Your roof is pitched at 15° or more
- You are converting a loft or attic
- Budget is £800–£2,000 per opening
Choose a flat-roof skylight if:
- You have a flat or very shallow roof
- You want a clean, low-profile look
- You do not need a statement architectural feature
Choose a roof lantern if:
- You want a dramatic centrepiece over a kitchen extension or dining room
- You have a flat-roofed single-storey extension
- Budget is £5,000–£10,000+ and you want maximum visual impact
[!warning] Thermal Bridge Risk All three types are potential thermal bridges. Ensure the frame is thermally broken and the installation detail matches the insulation layer in your roof build-up. A poorly detailed rooflight can cause condensation on the frame and cold spots that reduce the effective U-value of the whole roof.
