Velux roof window spec and cost
Velux roof windows (Danish manufacturer; UK market leader for pitched-roof rooflights since the 1960s) are the default specification for traditional Velux-only loft conversions and for pitched-roof additions on dormers and L-shape lofts. Standard Velux ranges: GGL (centre-pivot, opening, manual) — the most-specified, £400–£800 supply only for the standard CK04 (780×980mm) up to UK10 (1340×1600mm); GGU (white polyurethane sash, easy-clean); GPL (top-hung opening — better view, more headroom under the window); GIL (fixed, non-opening). Glazing options: standard 2-pane laminated low-e (Velux 73 series) U-value 1.3 W/m²K; triple-glazed (Velux 75 series) 0.86 W/m²K; high-spec 'Energy' 78 series 0.74 W/m²K. Install kit: Velux flashing kit (EDW for tile roofs, EDN for slate, ECL for various combinations) £170–£280; full installation labour 4–6 hours per window. Total supply and install per Velux: £800–£1,400 for standard, £1,200–£1,800 for triple-glazed energy spec, £1,400–£2,400 for INTEGRA electric-operated (rain sensor, remote control). Building regulations: Velux loft windows in habitable space must meet 1.4 W/m²K U-value and have egress for fire escape if the room is on the second or third floor — standard Velux 'fire escape' opening (centre-pivot opens to 45° clear opening) compliant.
Flat rooflight spec and London suppliers
Fixed flat rooflights (single fixed glass pane within a flat-roof structure) are the dominant specification for new flat-roof dormer conversions, rear extensions and side returns in London. Leading UK suppliers: Sunsquare (Suffolk; premium thermally-broken aluminium frame, slim sightline), Glazing Vision (Sussex; ultra-premium frameless rooflights and walk-on rooflights), The Rooflight Company (heritage rooflights for listed buildings; conservation-spec aluminium with timber inner reveal), Eurocell Skypod (mid-spec value range), Stella Rooflights (heritage), Cox Whillier (slim-frame). 2026 supply & install pricing (single-pane fixed rooflight, 1.0×1.0m): mid-spec Sunsquare or Eurocell £1,400–£1,900; premium Glazing Vision Lumin or Pivot Skylight £2,800–£3,800; heritage Rooflight Company Conservation £1,800–£3,200. Walk-on rooflights (structural glass to walk over a basement or stair void): Glazing Vision Walk-on £3,500–£8,000 per panel. Glazing options: standard double-glazed laminated low-e argon-filled 28mm; triple-glazed 44mm £400–£900 upgrade per rooflight; solar control glass £200–£400 upgrade. Installation: flat rooflights install onto a builder-formed upstand kerb (typically 150mm tall, with EPDM or single-ply waterproofing returned up the upstand and dressed onto the rooflight frame) — the upstand and waterproofing detail is critical for long-term leak performance.
Velux vs flat rooflight — choosing between them
The choice depends on roof type and aesthetic. Velux is right for: pitched-roof loft conversions where the rooflight sits flush within the existing slate or tile roof slope (Class B PD loft conversions, traditional rear-of-property roof slopes). Aesthetic: Velux is visually discrete; the standard white painted frame is barely visible from below in a small habitable room. Cost: Velux is cheaper per unit (£800–£2,400 vs £1,400–£3,800). Flat rooflight is right for: flat-roof dormer conversions where the dormer roof is flat (not pitched), rear extensions with flat roofs, side returns. Aesthetic: a 1.2m×1.2m flat rooflight in a flat-roof rear extension creates a much more dramatic 'light from above' effect — particularly important in deep-plan extensions where natural daylight 4m+ from the bifold wall would otherwise be poor. Glazing Vision premium frameless rooflights at 12-13mm sightline give an almost-borderless aesthetic that elevates the entire room. Hybrid spec: dormer loft conversions often combine: Velux to the front pitched slope of the dormer (where pitched roof remains) + flat rooflight to the dormer flat roof (over the dormer box). This combination is the dominant Builderr loft conversion spec — Velux to the existing pitched slopes for cost-efficiency; flat rooflights to dormers, mansards and flat-roof additions for visual impact and daylight modelling.
Daylighting calculations and Part L requirements
Modern London loft conversions and rear extensions are required to meet daylight performance criteria under Part L 2025 — total glazing area must be sufficient to provide minimum daylight factor (typically 1.5–2% at 2m from window). Calculation approach: total glazing area should be 20–25% of the floor area of the habitable room for typical bedroom or living spaces. Worked example: a 4m×3m loft bedroom (12m²) needs approximately 2.4–3m² of glazing for compliant daylight. Three Velux GGL CK04 (each 0.92m²): 2.76m² glazing — compliant. Two Velux UK10 (each 2.14m²): 4.28m² — comfortably exceeds requirement. One Glazing Vision Lumin 1.2m×1.2m flat rooflight (1.44m²) + one Velux UK04 (1.05m²): 2.49m² — compliant. Builderr's standard loft conversion glazing brief includes Velux/rooflight specification at the early design stage to ensure Part L compliance and daylight modelling targets. Solar gain consideration: south or west-facing rooflights add significant summer solar load — Velux GGU with solar control glass (recommended) or external Velux awning blinds (electric remote-controlled) prevent overheating. Builderr's London rooflight spec routinely includes solar control glass and external awning blinds on south/west-facing rooflights — cost £200–£500/rooflight upgrade.
