Velux Windows and Rooflights London 2025: Types, Costs, and Installation Guide

**Velux product range and pitched-roof rooflights — the full picture**:

Velux is the dominant brand in UK pitched-roof rooflights — so dominant that 'Velux window' is used generically to mean any pitched-roof rooflight. The company makes a large range of products, and the model code is important because it determines the glazing unit, the thermal performance, the operation method, and the compliance.

*Velux model codes — what they mean*:

• •**GGL** — the most common residential Velux window. The 'G' indicates a centre-pivot operation (the window tilts outward from a pivot point at the centre of the frame). GGL indicates a pine frame (vs. GGU which is polyurethane — see below). Available in a range of sizes from 47×71cm to 114×118cm. The standard GGL comes as an 'Integra' (electric operation) or 'Solar' variant (solar-powered electric operation with no mains wiring required). The GGL is the most commonly specified window for loft conversion bedrooms, bathrooms, and landing windows in London
• •**GGU** — the same centre-pivot operation as the GGL but with a polyurethane (white lacquered) frame rather than pine. More durable in high-humidity applications — the GGU is the recommended Velux model for bathrooms and ensuite bathrooms in loft conversions because the white polyurethane frame does not absorb condensation as readily as a pine frame
• •**GDL** — a top-hung operation (the window opens from a hinge at the top). Provides a larger opening area than the centre-pivot GGL/GGU. Less common in UK residential — the centre-pivot models dominate in London loft conversions
• •**GHU** — the 'Cabrio' balcony model that converts a Velux window into a small roof balcony (the lower portion of the window swings outward and down to form a balcony floor rail). Requires planning permission in most London cases (the resulting roof balcony creates overlooking potential and is specifically excluded from Permitted Development). GHU is more common in mansard or high-pitch London loft conversions where head height allows balcony access
• •**CVU** — a flat-roof window. A double or triple-glazed unit set into a flat roof deck with an insulated curb (upstand) manufactured to integrate with the flat roof waterproofing. The CVU provides vertical-face (±0°) or low-pitch (5°–15°) rooflight glazing in an extension or flat-roof loft. Fixed (non-opening) variants — FCM, CFP — are most commonly used for kitchen-diner extensions
• •**FCM / CFP** — Velux modular flat-roof windows (FCM = Fixed Curb Mounted; CFP = Fixed Curb Mounted with pre-formed curb). These are the standard flat-roof rooflights for rear extension kitchen-diners in London. Available in a range of standard modular sizes from 60×60cm to 120×120cm, and in multiple-unit configurations

*Non-Velux pitched-roof rooflights*:

• Other rooflight brands used in London projects include:
• •**Keylite** — a comparable product range to Velux; competitive on price; often used where specification requires a British-made product (Keylite is made in County Tyrone, Northern Ireland)
• •**Fakro** — a Polish manufacturer with a large UK presence; competitive pricing; wide range including flat-roof and balcony products
• •**RoofLITE** — budget-range competitor to Velux, used on cost-sensitive projects

For high-specification London projects, the brand difference between Velux GGL/GGU and Keylite/Fakro is relatively minor — all major brands now comply with Part L 2021 thermal requirements in their energy-efficient variants. Velux remains the standard specification on most London projects because of its dominance in the market, contractor familiarity, and the breadth of compatible flashing systems for all London roof tile types.

*Manual vs. Electric vs. Solar — how does the window open?*:

• •**Manual (top bar pole)**: operated by a wooden or metal pole hooked to the top bar of the window. Works at any height; no power required. Standard for low loft windows that can be reached by hand or with a short pole. The cheapest option at point of purchase
• •**Electric (Integra)**: mains-powered electric motor operates the window via a wall-mounted switch, app, or building management system. Required for any rooflight that is inaccessible without a ladder — Part F requires openable windows for ventilation in habitable rooms; for a loft conversion with high-set Velux windows, electric operation is both the practical and compliant choice
• •**Solar (Integra Solar)**: photovoltaic cell in the window frame charges a battery that powers the electric motor — no mains wiring required, which simplifies installation significantly. The solar motor and battery unit is built into the window frame. For a loft conversion where mains first-fix electrical wiring would require routing from the consumer unit, the Solar variant avoids the need for wiring to each rooflight. The solar cell charges adequately in UK conditions to operate the window reliably in most months (Velux specifies a minimum average of 2 hours direct sunlight per week for reliable operation — met in most London installations). Solar Integra windows carry a premium of approximately £150–£300 over the manual equivalent but save the cost of mains wiring at each location

*Fixed vs. opening — when can you use a fixed rooflight?*:

Part F of Building Regulations (Ventilation) requires that habitable rooms and wet rooms (bathrooms; kitchens) have adequate ventilation. For habitable rooms (bedrooms; living rooms), openable windows are the standard means of compliance — the minimum opening area required by Part F is 1/20th of the floor area of the room, or alternatively 8,000mm² of purposed opening (in addition to background ventilation). A fixed-only rooflight cannot provide opening ventilation and therefore cannot by itself satisfy Part F in a habitable room — it must be combined with a mechanical extract ventilation system (MEV or MVHR) or with an additional openable window elsewhere in the room.

• For flat-roof extension kitchen-diners, the most common approach is:
• •Fixed flat-roof rooflights (FCM/CFP) for daylight
• •A separate opening bi-fold or sliding door that provides the opening ventilation requirement
• •A kitchen extract hood and duct as the primary extract ventilation

• For pitched-roof loft conversion bedrooms:
• •Openable Velux (GGL/GGU) for both daylight and opening ventilation
• •Each bedroom requires at least one openable Velux to satisfy Part F

**Flat-roof rooflights vs. roof lanterns — when each applies**:

Flat-roof rooflights (single-unit or multi-unit frameless or slim-frame units: Velux CVU/FCM, Korniche, IQ Glass, Glazing Vision) are the standard choice for single or small-group daylighting of a flat-roof extension. They are lower profile (100–200mm above the roof deck), less visually dominant from outside, and typically lower cost.

• Roof lanterns (a pitched or hip-roofed glazed structure rising from the flat roof deck) are used where:
• •A larger architectural statement is desired (the lantern typically rises 400–800mm from the deck and may have internal coffered detail)
• •Planning aesthetics require a pitched or shaped glazed structure
• •The extension footprint is large enough that a lantern proportionately fills the roof space appropriately (lanterns look wrong on a small 2m × 2m flat roof but excellent on a 4m × 5m kitchen-diner extension)

• Roof lantern vs. rooflight cost comparison (London 2025):
• •Fixed flat rooflight 1.0m × 1.0m (Velux CVU/FCM; mid-range supply): £500–£900 + installation £300–£500
• •Opening electric flat rooflight 1.0m × 1.0m (Velux CVU electric): £900–£1,500 + installation £400–£600
• •Triple-glazed frameless rooflight walk-on (IQ Glass; Glazing Vision): £2,000–£5,000+ per m²
• •Aluminium roof lantern 1.5m × 1.0m (mid-range; Korniche; AluK; Ultraframe): £1,500–£3,000 supply + installation £500–£800
• •Aluminium roof lantern 2.0m × 1.0m: £2,500–£5,000 supply + installation £700–£1,200
• •Aluminium roof lantern 3.0m × 2.0m (large kitchen-diner centrepiece): £5,000–£12,000 supply + installation £1,000–£2,000

**Part L 2021 (Conservation of Fuel and Power) — the thermal performance requirement**:

Part L 2021 of the Building Regulations (England) — which came into full effect in June 2023 — sets maximum U-values for all elements of the building envelope. For windows and rooflights:

• •Maximum U-value for a replacement window or rooflight in an existing building: **1.6 W/m²K** (whole-window/whole-unit U-value)
• •For new-build or an extension: the Total Energy Balance approach or the Notional Building approach applies — in practice, a Building Control officer will accept rooflights complying with a whole-unit U-value of ≤1.6 W/m²K for a domestic extension

*How Velux models compare on thermal performance*:

• •Velux GGL (standard double-glazed with 4/16Ar/4 glazing unit, pine frame): whole-window U-value approximately **1.3–1.5 W/m²K** — compliant with Part L 2021
• •Velux GGL with triple-glazed option: approximately **1.0–1.1 W/m²K** — exceeds Part L 2021 requirements
• •Velux GGU (polyurethane frame; standard double-glazed): approximately **1.3 W/m²K** — compliant
• •Standard flat-roof rooflight (double-glazed; 4/16Ar/4): approximately **1.1–1.4 W/m²K** — compliant
• •Roof lantern (aluminium frame; double-glazed): approximately **1.4–1.8 W/m²K** — most quality roof lanterns achieve ≤1.6 W/m²K; some cheaper options exceed this threshold and are non-compliant

*When are rooflights assessed separately vs. the Total Energy Balance approach?*

For a simple loft conversion or extension, Building Control typically accepts individual element compliance — each rooflight meeting ≤1.6 W/m²K whole-unit U-value. For a larger project or where the rooflight area is a significant proportion of the total external surface, the Building Control inspector or SAP energy assessor may require a Total Energy Balance calculation showing that the additional heat loss through rooflights is offset by energy gains (from solar gain) or by compensating measures elsewhere (improved insulation in the floor or walls). In practice, for most London loft conversions and extensions with standard Velux or flat-roof rooflights, individual element compliance at ≤1.6 W/m²K is accepted without further energy analysis.

**Maximum rooflight area — the 25% rule**:

• Part L 2021 contains a provision that limits the aggregate area of new rooflights (and skylights) in a new extension or loft conversion to **25% of the floor area** of the extension/loft. This is the 'limiting value' for glazing — it is not an absolute prohibition on more glazing, but if the aggregate rooflight area exceeds 25% of the floor area, the design must demonstrate through a Total Energy Balance calculation that the overall energy performance is still compliant. In practice:
• •A standard loft conversion bedroom (20–30 m² floor area) with 2×Velux GGL 78×118cm (each glazed area approximately 0.7m²) has approximately 1.4m² total glazing against 20m² floor area = 7% — well within the 25% limit
• •A large kitchen-diner extension (25m² floor area) with a 2.5m × 1.5m roof lantern (3.75m² glazed area) has approximately 15% glazing ratio — within the 25% limit
• •A large rear extension (30m² floor area) with a full-width 4.5m × 2.0m rooflight (9m² glazed area) has 30% glazing ratio — exceeds 25% and requires a Total Energy Balance assessment

**Condensation risk and thermal bridging at rooflight frame**:

• One of the most common defect issues in London loft conversions is condensation running off the Velux frame onto the ceiling plasterboard below. This occurs when:
• •The rooflight frame is not adequately thermally broken — the aluminium or steel frame components create a cold-bridge between the external (cold) glass surface and the internal warm surface of the room
• •The interior humidity is high (bathroom or bedroom with inadequate ventilation)
• •The warm room air hits the cold frame and condensates

• For bathroom Velux windows (GGU is specified rather than GGL precisely because the polyurethane frame is more thermally stable), ensure:
• •Trickle vent or extract fan to manage humidity
• •Insulation installed tightly against the Velux reveal all the way around the window — no gap between the underside of the rooflight upstand and the warm side of the insulation
• •The Velux insulation collar (BDX — the Velux thermal insulation kit) is fitted around the rooflight curb if the window is in a flat roof or in a high-humidity location

**Planning considerations for rooflights in London**:

• *Permitted Development for rooflights on pitched roofs*:
• •Rooflights on a pitched roof fall under Permitted Development Class A of the Householder PD Order (subject to conditions)
• •The rooflight must not protrude more than **150mm** above the existing roof slope
• •On a principal elevation (the front or side elevation facing a highway), the rooflight must not be higher than the highest part of the roof — meaning flat-roof-style solar panels or 'conservation rooflights' that sit within the roof plane (Conservation Velux, e.g. GGL-CK04 Conservation) are needed
• •On a rear-facing pitch: standard PD rules apply — the 150mm projection limit governs

• *Conservation areas and rooflights*:
• •In a conservation area, a rooflight on the front (principal) elevation of the property is NOT Permitted Development — it requires planning permission
• •On rear slopes not visible from a highway, conservation area rooflights are Permitted Development subject to the 150mm projection limit
• •Many London conservation area supplementary planning documents (SPDs) specify 'conservation rooflights' (flush-fitting; same slope as the roof; no visible projection) for any visible rooflight. Velux makes a Conservation range (also called 'In-Roof' or 'Conservation') that is flush with the roof surface — UK-only version designed to comply with conservation area requirements
• •RBKC (Royal Borough of Kensington and Chelsea); Camden; Islington; Hackney and Southwark all have conservation areas where rooflight specification on the front slope routinely requires planning consent with a condition to use conservation/flush rooflights

**Velux installation into a pitched-roof loft conversion**:

Installing a Velux window into an existing pitched roof involves: 1. Stripping tiles in the opening area (allow 3–4 tiles clearance in every direction) 2. Cutting the roof structure (rafters) if needed — for a wide Velux, a trimmer/header is required to reform the structural opening (typically two trimmer rafters at each side, a horizontal trimmer above and below the window, with temporary support during cutting) 3. Installing the Velux frame and adjusting position 4. Fitting the appropriate flashing kit (Velux EDW/EDL for standard interlocking tiles; EDE for flat tiles; EDN for natural slate; EKW for plain clay tiles — using the wrong flashing kit is a common cause of rooflight leaks) 5. Re-tiling around the rooflight including the lead soakers or flashing integration at head, sill, and jambs 6. Internal finishing — insulated reveal (typically 100mm PIR board to the 4 sides of the reveal between the tile batten level and the rafter depth); internal cill board; and plasterboard return to the reveal

Installation time per rooflight: approximately 0.5–1 day for a straightforward installation into an existing roof by an experienced roofer (working with a labourer)

**2025 installation costs per Velux rooflight (London)**:

| Configuration | Supply (Velux GGL/GGU mid-size CK04/MK04) | Installation (include flashing; structure; reveal) | Total per window | |---|---|---|---| | Standard GGL/GGU manual (CK04: 55×98cm) | £350–£550 | £400–£700 | **£750–£1,250** | | Standard GGL/GGU manual (MK04: 78×98cm) | £400–£600 | £450–£750 | **£850–£1,350** | | GGL/GGU Solar Integra (electric; no wiring) | £600–£900 | £450–£750 | **£1,050–£1,650** | | GGL/GGU Integra mains electric (+ wiring) | £500–£800 + £150–£300 wiring | £450–£750 | **£1,100–£1,850** | | GGL Conservation/In-Roof flush-fitting | £700–£1,100 | £500–£900 | **£1,200–£2,000** |

For a typical 2-bedroom London loft conversion with 4 Velux windows (2 per bedroom, standard mid-size, solar Integra): total supply and install approximately **£4,200–£6,600** for all four rooflights as part of a full loft conversion package.

**Flat-roof rooflight installation in a rear extension**:

Flat-roof rooflight installation differs from pitched-roof installation: 1. The flat roof deck must have a pre-formed upstand (curb) prepared during the waterproofing stage — GRP, EPDM, or TPO membrane is turned up onto the rooflight curb before the rooflight unit is installed 2. The rooflight curb must be positioned during the extension construction (before the flat roof is completed) — retroactively cutting an opening in a completed flat roof and installing a curb is more complex and expensive 3. The rooflight unit is fixed to the curb; the membrane laps under the sill of the curb at least 150mm and over the head of the curb by at least 150mm 4. Thermally, the flat-roof rooflight must be installed with continuous insulation up to the curb face with no cold bridge — the built-up insulation of the warm roof deck should meet the underside of the rooflight curb all the way around

**2025 supply and installation costs for flat-roof rooflights (London)**:

| Type | Size | Supply | Installation (as part of new extension) | Total | |---|---|---|---|---| | Velux FCM fixed flat-roof | 0.6m × 0.6m | £350–£550 | £250–£450 (curb pre-formed) | £600–£1,000 | | Velux FCM fixed flat-roof | 1.0m × 1.0m | £550–£800 | £300–£500 | £850–£1,300 | | Velux CVU electric opening flat-roof | 1.0m × 1.0m | £900–£1,400 | £400–£600 | £1,300–£2,000 | | Korniche aluminium roof lantern | 1.5m × 1.0m | £1,200–£2,500 | £500–£800 | £1,700–£3,300 | | Korniche aluminium roof lantern | 2.5m × 1.5m | £2,500–£5,000 | £700–£1,200 | £3,200–£6,200 | | Large aluminium roof lantern | 3.0m × 2.0m | £4,000–£9,000 | £1,000–£2,000 | £5,000–£11,000 |

*Note: installation costs assume the rooflight is being installed as part of a new extension build (curb pre-formed; no retroactive cutting). Retroactive installation into an existing flat roof adds £300–£700 per unit for cutting the deck, forming the curb, and integrating the waterproofing.*

**Second-fix materials rule — applies to rooflights**:

Velux windows themselves (the glazed unit and frame) are typically included in the main contractor's supply. However, as with all RCB projects, **internal finishing materials** — cill boards; reveal plasterboard; decorative finish materials — are **client-supplied** in the RCB standard scope. This should be clarified at scope stage.