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Rooflights in London Residential Projects
Rooflights (commonly referred to as Velux windows, after the dominant brand) are windows set flush with or just above the plane of a roof slope. They are used in loft conversions to provide natural light and ventilation to the new habitable rooms, and in existing roofs as a means of improving daylight to top-floor rooms without the structural complexity and cost of a full dormer extension.
In London, rooflights are frequently chosen as the preferred alternative to dormers in conservation areas where full-width dormers would not receive planning approval, or as the most cost-effective way of adding light to a loft space where the existing headroom is adequate without a dormer.
Permitted Development for Rooflights
Rooflights in England can be installed under permitted development rights without a planning application, subject to conditions under the GPDO:
- The rooflight must not project more than 150 mm above the plane of the roof slope
- No rooflight is permitted on a roof slope fronting a highway (i.e., visible from the road) without planning permission in most conservation areas (see below)
- Rooflights must not increase the height of the roof
Under standard permitted development on a non-conservation area property:
- Front (street-facing) roof slopes: rooflights are permitted provided they do not protrude more than 150 mm and the ridge height is not increased
- Rear roof slopes: rooflights are permitted on the same basis
The 150 mm protrusion limit means that standard centre-pivot Velux-type windows, which open outward and protrude significantly when open, still comply as long as the fixed frame sits within 150 mm of the roof plane. Conservation Velux rooflights (designed specifically for conservation areas) sit completely flush with the roof plane.
Conservation Areas and Rooflights
In conservation areas, permitted development rights for rooflights are more restricted. Article 3(1) of the GPDO removes certain PD rights in conservation areas, and Article 4 Directions in many London conservation areas remove PD rights for roof alterations entirely.
For rooflights on the front slope of a property in a conservation area, the default position in most London boroughs is that planning permission is required. The planning officer will typically assess whether the rooflight:
- Sits flush with or below the roof surface (conservation-style rooflights are generally more acceptable than standard projecting units)
- Uses a material appropriate to the roof (black or grey frames are typically preferred over white)
- Does not significantly alter the appearance of the front elevation
For rear slope rooflights in conservation areas, many London boroughs permit them without planning permission under permitted development, but this depends on the specific conservation area and any Article 4 Directions in force. Always check with the local planning authority before assuming rear rooflights are permitted.
Types of Roof Window
Centre-Pivot (Top-Opening)
The standard Velux window opens by pivoting on a central horizontal axis — you pull the bottom inward and the top swings outward. This type is the most common and cost-effective. The 150 mm protrusion rule relates to the fixed frame, not the open sash position.
Top-Hung (Top-Pivot)
Top-hung windows open by pivoting at the top, swinging outward at the bottom. They are often used in higher positions on a roof slope where centre-pivot access would be awkward.
Conservation Rooflights (Flat Glass, Flush-Fitting)
Conservation rooflights are designed to sit completely flush with the roof plane, with minimal frame projection. They are often specified in conservation areas where the planning authority requires flush-fitting units. Brands include Velux's conservation range, Fakro's conservation products, and bespoke timber-framed conservation rooflights. The glass sits almost level with the surrounding tiles or slates.
Conservation rooflights are less ventilating than standard units (they cannot open as widely) and typically cost 30–50% more than standard rooflights, but they are the appropriate specification for sensitive planning contexts.
Roof Lanterns (Flat Roof)
A roof lantern is a raised glazed structure on a flat roof surface, bringing natural light down into the space below. Common on kitchen extensions with flat roofs, they sit above the flat roof membrane and protrude above the roof level. Unlike rooflights on pitched roofs, a roof lantern is subject to planning permitted development rules for extensions rather than for rooflights. Most single-storey flat roof lanterns are within PD parameters but this should be confirmed for conservation area properties.
Flat Rooflights (Flush with Flat Roof)
Frameless or slim-framed rooflights designed to sit flush with a flat roof surface, allowing natural light to flood into rooms below while maintaining the low-profile flat roof appearance. Often used on kitchen extensions where a fully glazed roof area is desired without the raised profile of a lantern. In conservation areas, front-facing flat rooflights may require planning permission.
Rooflight Sizing and Daylight
The Building Regulations guidance on natural lighting (Approved Document O relates to overheating, while daylight adequacy for habitable rooms is covered by glazed area guidance in Approved Document L and ADL design recommendations) broadly requires glazed area to be at least 1/10th of the floor area of a habitable room.
For a loft conversion room of 15 m² floor area, a minimum glazed area of 1.5 m² is recommended. Two Velux FK06 windows (each approximately 0.7 m² glazed area) would satisfy this. For good quality daylight in a loft room, larger units or more units are beneficial.
Velux (and equivalent brands Fakro and Dakea) size rooflights using a code system based on tile batten spacing and rafter width. Common sizes:
- MK04 (78 cm x 98 cm) — small, often used as a secondary window
- FK06 (66 cm x 118 cm) — the most common size for habitable rooms
- CK06 (55 cm x 118 cm) — narrower, for tight rafter bays
- PK06 (94 cm x 118 cm) — wide unit for maximum daylight
Fire Safety and Escape Rooflights
Where a rooflight provides the means of escape from a loft conversion room (where escape cannot be made through the building's protected staircase), the rooflight must comply with fire escape requirements — a minimum clear opening of 450 mm x 750 mm and sill no higher than 1,100 mm from the floor. Many Velux FK06 and larger units meet this requirement.
Cost of Rooflights
Standard Velux or equivalent rooflight (supply and fit, including flashings): £600–£1,200 per unit Conservation rooflight (supply and fit): £900–£1,800 per unit Roof lantern on flat roof (900 mm x 900 mm): £2,000–£4,000 supply and fit Flat roof walk-on or frameless glazed light: £1,500–£4,000 per m²
rcbGroup and Rooflights
rcbGroup specifies and installs rooflights as part of loft conversions, flat roof extensions, and kitchen extensions. We confirm planning compliance before specification, coordinate conservation rooflight selection with the planning authority where required, and ensure rooflight installation meets fire safety and Building Regulations requirements.
Contact our team to discuss rooflight options for your London project.
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Important Note
This guide is for general information only. Building regulations, planning rules, and legal requirements change regularly and vary by local authority. Always seek professional advice specific to your project and location. To talk through your own project, book a project review.
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