How many rooflights do I need for a kitchen extension?

A rule of thumb is that rooflights should cover at least 15–20% of the floor area to achieve good natural light levels without artificial lighting during the day. For a 20m² kitchen extension, 3–4m² of rooflight area (e.g. three 1200×900mm rooflights or one 2000×1500mm unit) is typically specified. Placement matters as much as area — a single large central rooflight positioned over the kitchen island provides better ambient light than multiple small rooflights at the edges.

Do rooflights add value to a house?

Yes — rooflights are consistently cited by London estate agents as a key value driver in ground-floor extensions and loft conversions. A well-lit, naturally bright extension commands a premium over a dark poorly-lit room. The added value is indirect — rooflights are rarely itemised separately by valuers, but they significantly improve the saleability and first impression of an extension. The cost of rooflights (£1,000–£3,000 per unit installed) is a high-ROI specification choice.

Can I add a rooflight to an existing flat roof?

Yes — adding rooflights to an existing flat roof extension is a popular renovation project. The structural process involves cutting an opening in the existing deck, fitting a structural trimmer frame around the opening, and installing the rooflight on a proprietary curb. Cost for retrospective installation is £1,200–£2,500 per rooflight (higher than during original construction due to the deck work and making-good required). Building Regulations notification is required if the existing roof is not already covered by an open or full plans application.

What is the difference between a rooflight and a skylight?

The terms are largely interchangeable in the UK — both refer to glazed openings in a roof to admit natural light. Technically: 'skylight' often refers to pitched-roof flush-mounted units (Velux-style); 'rooflight' often refers to flat-roof curb-mounted units. 'Roof lantern' refers to a raised structural glazed structure on a flat roof. In practice, building professionals use all three terms loosely. What matters for specification is: flat roof vs pitched roof, opening vs fixed, and the required U-value and safety glazing specification.

Rooflight Skylight Cost London 2025 | Builderr

How Much Does a Rooflight or Skylight Cost to Install in London?

A rooflight or skylight costs £800–£3,500 installed in London, depending on size, type and glazing specification. Fixed flat roof rooflights (the most common choice for single-storey extensions) start at £800 for a 600×900mm unit. Walk-on structural glass rooflights cost £2,500–£8,000. Opening skylights for pitched roofs (Velux or equivalent) cost £600–£1,800 installed.

Rooflight cost by type and size

Fixed curb-mount rooflight (flat roof, non-opening): the standard choice for single-storey rear extensions. Frameless specification with 24mm triple-glazed unit, powder-coated aluminium frame. 600×900mm: £800–£1,200 installed. 1000×1500mm: £1,200–£1,800 installed. 1500×2000mm: £1,800–£2,800 installed. 2000×3000mm (statement light): £2,500–£4,500 installed. Opening rooflight (electric or manual): add £300–£600 to fixed rooflight price. Roof lantern (pitched hip on flat extension): structural glazed pyramid or hip profile. 1000×1500mm: £3,500–£6,000 installed. 1500×2000mm: £5,000–£9,000 installed. See separate guide for full roof lantern pricing. Walk-on structural rooflight (used as floor glazing, corridor roof, or terrace element): 600×900mm: £2,500–£4,000 installed. 1000×1500mm: £4,000–£8,000 installed. Velux pitched roof skylight (replacement or new): 550×780mm (CK02): £600–£900 installed. 780×980mm (F06): £700–£1,100 installed. 940×1600mm (SK10): £900–£1,500 installed.

Fixed rooflight vs roof lantern vs opening skylight

Fixed flat roof rooflight: flush-profile unit sitting on a low-profile curb (150–200mm above roof finish), flush with or minimally projecting above the flat roof plane. Best for: modern extension aesthetic, maximum light without visual clutter, planning-sensitive sites where projection is restricted. Thermal performance: U-value 0.9–1.5 (fixed), up to 1.4 (opening). Roof lantern: raised pitched structure with multiple glazed panes and a structural ridge. Provides architectural presence and allows side ventilation. Best for: kitchen-diners, orangeries, living rooms where height and light are key design goals. Higher profile — 600–900mm above roof level. Often used on high-end extensions where the lantern becomes a feature. Higher cost and requires more structural support. Opening rooflight (electric): provides ventilation in addition to light — useful for kitchens. Electric opening units can integrate with home automation, rain sensors and CO₂ monitoring. Premium: £300–£600 over fixed equivalent. Velux (pitched roof): for pitched roof extensions, dormers and loft conversions, Velux remains the dominant brand. Models include standard, centre-pivot, top-hung, solar-powered and flat-roof variants. Builderr fits Velux as standard on pitched roof applications and Atlas/Korniche for flat roof rooflights.

Planning permission for rooflights

Pitched roof skylights: Velux-type rooflights on pitched roof slopes are permitted development provided they do not project more than 150mm beyond the roof plane. Conservation area exception: skylights on roof slopes visible from the highway in conservation areas require full planning permission. Front elevation skylights are always restricted in conservation areas. Flat roof rooflights: as part of a permitted development extension, rooflights within the extension footprint follow the extension's planning status (no separate application needed). If adding rooflights to an existing flat roof as a standalone project, the general PD tolerances for additions apply. Listed buildings: any rooflight on a listed building (any grade) requires Listed Building Consent — this applies even to internally fitted units or replacement of existing rooflights with identical-profile units. Builderr handles all necessary LDC and LBC applications where rooflights are added to restricted properties.

Building regulations for rooflights

Rooflights must comply with Building Regulations Part L (thermal performance), Part K (safety glazing) and Part B (fire) in some configurations. Thermal performance (Part L): fixed rooflights in new extensions must achieve U-value ≤ 0.35 W/m²K; opening rooflights in new extensions must achieve U-value ≤ 1.4 W/m²K (concession for ventilation function). Replacement of single-glazed to double/triple-glazed skylights in an existing roof requires notification to Building Control unless the area does not exceed 25% of the original floor area. Safety glazing (Part K): rooflights must use toughened or laminated glass compliant with BS EN 12150. Walk-on rooflights: must carry a minimum 3kN/m² point load and use laminated glass (typically 44.2 or 55.4 specification). Ventilation (Part F): extensions over 70m² require mechanical ventilation — rooflights contribute to natural ventilation calculations. Builderr sizes rooflights to meet both daylight and ventilation requirements in the extension design.