Skylights, Rooflights, and Roof Lanterns for Extensions: What to Choose and What It Costs

**Flat roof rooflights (fixed flat or walk-on)**:

The most common rooflight for a London flat-roof extension. A flat rooflight sits in or on the flat roof plane and admits light from directly above.

*Triple-skin polycarbonate dome*: The traditional and lowest-cost flat rooflight option. A polycarbonate dome (usually acrylic or twin or triple-wall polycarbonate) sitting in a kerb above the roof. U-value typically 1.6–2.0 W/m²K (above the Part L requirement for new rooflights); poor appearance from below. Not suitable for quality domestic extensions. Should be replaced with glass-based systems wherever budget allows.

*Double-glazed sealed glass flat rooflight*: The current standard for quality domestic extensions. Aluminium frame with double or triple-glazed sealed unit (typically 6/16/6mm configuration with low-e coating and argon fill). U-value achievable: 1.1–1.4 W/m²K (double); 0.6–0.9 W/m²K (triple). Suppliers: Velux (Flat Roof Window), FAKRO FTT, Korniche, Korniche XL, Glazing Vision.

*Fixed flat vs. opening*: Fixed flat rooflights (non-opening) are cheaper than opening systems but provide no ventilation. In an extension used as a kitchen or dining space, ventilation via the rooflight is valuable — specify an electric or manually opening flat rooflight if natural ventilation from below is not available from another source.

*Walk-on rooflights*: Structural glass units designed to be walked on (typically in a below-grade position — a basement lightwell or a terrace above a room below). Much heavier and more expensive than standard flat rooflights — not commonly used in standard domestic single-storey extensions.

**Roof lanterns**:

A roof lantern is a raised glazed structure sitting on a flat roof or a flat extension roof — essentially a small pitched glass house sitting above the roof. The raised ridge creates a visual feature internally as well as externally, and the sloped glazing panels admit light at multiple angles.

*Construction*: Aluminium frame (thermally broken) supporting multiple glazing panels arranged around a central ridge. Most quality roof lanterns are aluminium; there are some uPVC roof lanterns at the budget end of the market.

*Typical sizes for domestic extensions*: 1.2m × 1.2m (very small — feature only); 1.5m × 2.0m (small dining space); 2.0m × 3.0m (common for kitchen-dining extension); 2.5m × 4.0m (large open-plan extension). Bespoke sizes are available from most manufacturers.

*Part L performance*: Quality roof lanterns achieve whole-unit U-values of 1.1–1.5 W/m²K with double-glazed units and 0.7–1.1 W/m²K with triple glazing. The ridge beam is the weakest thermal element — specify a thermally broken ridge.

• *Cost range (supply and installation)*:
• •1.5m × 2.0m roof lantern: £2,500–£5,000
• •2.0m × 3.0m roof lantern: £4,000–£8,000
• •2.5m × 4.0m roof lantern: £6,000–£12,000
• •Bespoke large-format: £10,000–£25,000+

*Suppliers*: Origin, Atlas (Cornelius Roofing), Korniche, Ultrasky, Sunseeker.

**Pitched roof windows (Velux and equivalents)**:

The Velux centre-pivot roof window is the most widely installed rooflight in the UK — and the most appropriate for a pitched roof. Available in manual, solar-powered opening (GGL Solar), and electrically operated versions.

*Planning note*: On a pitched roof, roof windows within the Class C permitted development size limits (each up to 0.26m² above the plane of the roof; not higher than the existing ridge; not on the front roof slope in a Conservation Area) do not require planning permission — they are permitted development. See loft-conversion-planning-guide.

*Cost*: A standard 780×980mm Velux GGL centre-pivot roof window: £350–£500 (supply only); installed £700–£1,200 including lead flashing, redressing tiles, and plasterboard reveal lining.

**Structural glass roof (frameless or minimal-frame)**:

For the highest design intent — a fully glazed or near-fully glazed extension roof. These systems (Pilkington Planar, Schüco, structural glass roofs by specialist glazing contractors) use structural silicone joints, point fixings, or very slender aluminium sub-frames to minimise visible structure. Very expensive (typically £800–£2,500/m² installed including structural elements) and require specialist design. Not commonly used in standard domestic extensions.

**Permitted development rules for rooflights on pitched roofs**:

• Class C of Part 1 of the GPDO 2015 permits roof windows on a dwelling house subject to conditions:
• •The roof window must not exceed the plane of the roof slope
• •In Area of Outstanding Natural Beauty, World Heritage Site, or Conservation Area: no roof window permitted on a roof slope facing the highway
• •Maximum projection above the roof plane: the window must not project more than 150mm above the existing roof slope at any point
• •On the principal elevation (facing the highway): only in a Conservation Area restriction applies specifically

*The 'front slope' restriction in Conservation Areas*: In a Conservation Area, Class C does not permit roof windows on any roof slope visible from the highway. Roof windows on the rear slope in a Conservation Area are permitted development within the other Class C limits.

**Planning rules for flat roof rooflights and roof lanterns**:

• Flat rooflights (on a flat or near-flat roof) and roof lanterns are generally not covered by specific Class C limits — they are part of the flat roof construction and are considered within the PD allowance for the extension itself, provided:
• •The rooflight does not raise the overall roof height above the PD height limit for the extension (3.0m single-storey, 4.0m if within 2m of a boundary)
• •The roof lantern does not raise the ridge height above the eaves of the existing house

In practice, roof lanterns on single-storey rear extensions rarely cause planning difficulties — they are below the existing eaves line and not visible from the street. A pre-application check with the LPA is recommended for any roof lantern in a Conservation Area or where the height of the lantern plus the extension is close to the PD height limit.

**Conservation Area considerations**:

In Conservation Areas, any rooflight visible from the highway requires planning permission rather than being permitted development. Front slope rooflights in Conservation Areas are frequently refused. Rear slope rooflights in Conservation Areas (not visible from the highway) can typically be installed as PD within the Class C limits. Roof lanterns on rear single-storey extensions in Conservation Areas are usually not a planning concern if they are not visible from the street.

**Part L requirements for new rooflights**:

• All rooflights in a new extension must meet Part L (Conservation of Fuel and Power):
• •Maximum rooflight U-value: 1.4 W/m²K for a non-opening rooflight; 1.6 W/m²K for an opening rooflight (slightly higher allowance for opening rooflights because of the ventilation benefit)
• •Maximum rooflight area: the total area of rooflights in a new extension should not exceed 25% of the floor area of the extension (this is the Part L 2021 'notional dwelling' limit for rooflights)

*Practical implication*: For a 25m² extension, maximum 6.25m² of rooflight — a 2.0m × 3.0m roof lantern (6m²) is right at the limit. A very large roof lantern (3.0m × 4.0m = 12m²) on a 25m² extension would be above the Part L notional limit and would need to be offset by better-than-required wall insulation under a heat loss trade-off calculation.

**Key specification decisions for flat rooflights**:

*Frame colour*: Most aluminium rooflights are available in a range of RAL colours — anthracite (RAL 7016), black (RAL 9005), white (RAL 9010), and powder-coat RAL. Black or anthracite is the most popular contemporary choice for domestic extensions.

*Glazing specification*: Double-glazed vs. triple-glazed. For a rooflight (horizontal or near-horizontal glazing), triple glazing is more commonly specified than for vertical windows because: (1) the heat gain benefit of solar energy through the rooflight is less than through a south-facing vertical window (less 'passive solar' benefit); (2) the heat loss through a large horizontal glazed area is significant — triple glazing reduces this meaningfully; (3) the additional weight of triple glazing is less problematic for a fixed flat rooflight than for a large bifold door.

*Kerb height*: Flat rooflights installed on a flat roof require a kerb (upstand) above the roof surface — minimum 75–100mm above the finished roof level to ensure water cannot pool and enter the rooflight. A 150mm kerb is better practice in London's rainfall climate.

*Solar control glazing*: For rooflights in south or west-facing orientations, solar control glass (low solar heat gain coefficient, typically Tg ≤ 0.3) prevents overheating of the space below in summer. Standard low-e glass has a solar heat gain coefficient (SHGC) of approximately 0.5–0.6 — adequate for a small north-facing rooflight; too high for a large south-facing lantern.

**Typical costs (supply and installation)**:

| Type and size | Approximate installed cost | |---|---| | Flat rooflight 600×600mm (double-glazed, aluminium frame) | £600–£1,200 | | Flat rooflight 1200×600mm (double-glazed) | £900–£1,800 | | Flat rooflight 2400×600mm (linear, double-glazed) | £1,800–£3,500 | | Roof lantern 1.5×2.0m (double-glazed) | £2,500–£5,000 | | Roof lantern 2.0×3.0m (double-glazed) | £4,000–£8,000 | | Roof lantern 2.0×3.0m (triple-glazed) | £5,500–£11,000 | | Velux GGL 780×980mm (centre pivot, supply only) | £350–£500 | | Velux GGL installed (with lead flashing and reveals) | £700–£1,200 | | Linear skylight full-length side return (bespoke) | £3,500–£8,000 |

**Installation quality considerations**:

• •The junction between the rooflight/lantern kerb and the flat roof waterproofing membrane is the most leak-prone detail. The membrane (GRP, EPDM, or torch-on felt) must turn up onto the kerb to a minimum height of 75–100mm above the finished roof level, with bonded upstand. A gap or inadequate upstand at this junction is the primary cause of rooflight leaks.
• •Flat rooflights must be installed level (zero tolerance for any fall toward the glass — water must drain away from all edges). An out-of-level kerb causes pooling at the glass perimeter, which degrades the seal.
• •Roof lanterns have thermal bridging risk at the ridge — specify a thermally broken ridge beam and check for condensation on the inner frame in the first winter after installation.