Rooflights for Extensions and Loft Conversions: Types and Costs Explained

The term 'Velux' is a brand name that has become generic — like Hoover or Biro — for any centre-pivot or top-hung rooflight installed in a sloping pitched roof. The correct generic term is 'pitched rooflight'.

• **How they work**: The frame is fixed into the roof structure between rafters. The glazed unit sits flush with the roof plane and is flashed at the perimeter with lead or a pre-formed soaker system. Most manufactured pitched rooflights are available as:
• •**Fixed** — no opening, light only
• •**Centre-pivot opening** — handle at centre of frame; sash rotates 180° for cleaning
• •**Top-hung opening** — sash hinged at top; suitable for higher positions where centre-pivot is inaccessible
• •**Electrically operated** — chain actuator; essential for high-pitched or hard-to-reach positions

**Glazing specification**: Modern pitched rooflights are double- or triple-glazed with low-emissivity coatings. Standard specification: double-glazed 28mm unit with Krypton or Argon fill, U-value approximately 1.1–1.3 W/m²K for the unit (centre-pane).

**Sizes**: Standard sizes from 55×78cm (small bedroom) to 114×140cm (large room). Non-standard sizes are available to order with 4–8 week lead time.

**Planning**: Under Permitted Development (PD), pitched rooflights in the principal elevation (front) are not permitted if they project more than 150mm above the roof plane. Side and rear rooflights are generally PD-permitted provided they do not exceed the height of the ridge.

• **Costs (supply and install, London 2025)**:
• •Fixed pitched rooflight 78×98cm: £450–£750
• •Opening centre-pivot 78×98cm: £550–£900
• •Electrically operated 78×118cm: £900–£1,500
• •Triple-glazed equivalent: add £150–£300

Flat rooflights — sometimes called 'flat glass rooflights', 'frameless skylights', or 'overhead glazing' — are installed into flat roof constructions with a kerbed upstand. They are the defining feature of contemporary open-plan rear extensions and have become the design-led alternative to traditional Velux in modern London architecture.

**Construction**: A flat rooflight consists of a structural insulated glazed unit (typically laminated toughened glass, triple-glazed) mounted on an aluminium or thermally broken aluminium frame sitting on a concrete or timber upstand (typically 150mm above the finished roof surface to achieve weather-tightness).

• **Frameless vs framed appearance**:
• •**Slim-frame flat rooflights** (e.g., IQ Glass, Brett Martin, Vision AGI): 20–40mm visible frame edge internally — gives the 'frameless' feel
• •**Standard flat rooflights** (e.g., Fakro, Dakea, VELUX flat roof): 60–80mm internal frame — more traditional appearance, lower cost

• **Fixed vs opening flat rooflights**:
• •Fixed flat rooflights are the standard choice — lower cost and simpler weathering detail
• •Opening flat rooflights (electric only, typically) add ventilation — relevant for rooms without alternative opening windows, but the condensation risk is higher in flat units, so fixed is often preferred and cross-ventilation achieved via doors or side windows

**Part O — overheating**: Part O of the Building Regulations (England, 2022) requires new dwellings and extensions to demonstrate that overheating is controlled. Large flat rooflights in south-facing extensions can be a Part O risk — your building control officer may require solar control glass (low SHGC value) or limiting rooflight area.

• **Costs (supply and install, London 2025)**:
• •Fixed flat rooflight 800×800mm: £600–£1,100
• •Fixed flat rooflight 1200×1200mm: £900–£1,600
• •Fixed flat rooflight 1500×2000mm: £1,800–£3,200
• •Slim-frame/frameless equivalent: add 40–80% to standard cost
• •Electric opening flat rooflight: add £400–£800 to fixed equivalent

Walk-on rooflights (also called 'walk-on glass' or 'trafficable rooflights') are structural glazed units designed to be walked on — usually as part of a rooftop terrace, ground-floor extension roof that forms a first-floor terrace, or a basement light well covered at pavement level.

**Construction**: Walk-on rooflights use laminated structural glass (typically 3-layer laminated toughened glass at 40–60mm total thickness) in a heavy-duty aluminium frame with a non-slip surface treatment. The units must carry the design imposed load (typically 3.0 kN/m² for residential terraces) — this is significantly heavier than standard overhead glazing.

• **Where walk-on is used in London residential projects**:
• •Extension roof converted to usable first-floor terrace — walk-on glass over the ground-floor extension room below brings light in and makes the 'lost' roof space usable as outdoor amenity
• •Basement light wells — walk-on glass at pavement level allows light into a basement below while the area is trafficked above
• •Covered passage — glass floor to light a room below from a passage above

**Permitted Development note**: Adding a first-floor terrace with walk-on glass generally requires planning consent because a first-floor terrace changes the impact on privacy and the character of the roof.

• **Costs (supply and install, London 2025)**:
• •Walk-on rooflight 1000×1000mm: £1,500–£2,800
• •Walk-on rooflight 1500×2000mm: £3,500–£6,500
• •Walk-on glass terrace system (full terrace): £400–£900 per m² (including frame, balustrade interface, drainage)
• •Basement light well cover: £1,200–£2,500 per unit

**Roof lanterns**: A roof lantern is a raised glazed structure — typically 400–600mm high — sitting on a flat roof, allowing light to enter from multiple angles. Lanterns are popular on kitchen-diner extensions as they create a dramatic, light-filled focal point.

• •Standard aluminium roof lantern 1500×1000mm: £1,200–£2,500 supply only
• •Thermally broken, slim-frame lantern (e.g., Korniche, Atlas): £2,000–£4,500 supply only
• •Installation (fitting to prepared upstand): £400–£800
• •Note: roof lanterns require a prepared concrete or timber upstand — costs for the upstand structure are in addition

**Conservation rooflights**: Conservation rooflights are designed to replicate the appearance of traditional cast-iron-framed rooflight panels used in Victorian and Edwardian buildings. They have a distinctive low-profile, slim-framed appearance and sit almost flush with the roof surface.

• •Required by planning in Conservation Areas, on listed buildings, and often in Article 4 Direction areas where the roof is visible
• •Typically single-slope (roof plane following) with slim 20–25mm internal frame
• •Suppliers: Rooflight Company, Clement, Whitesales
• •Costs: Conservation rooflight 500×900mm: £450–£800 supply only; larger units £800–£1,500
• •Note: conservation rooflights achieve lower U-values than modern double-glazed units — often 1.6–2.0 W/m²K — which may require compensating improvements elsewhere in the thermal envelope

• **Planning guidance for rooflights in London**:
• •Conservation Areas: conserve the appearance — conservation rooflights required on front elevation; flat rooflights may be acceptable on rear
• •Listed Buildings: Listed Building Consent required for any rooflight; conservation rooflights almost always required
• •Standard residential PD: pitched rooflights — permitted on rear and side, not projecting more than 150mm above roof plane; flat rooflights on flat-roof extensions — generally PD