Glass Roof Lanterns for London Extensions — A Complete Guide

A roof lantern is a glazed structure that sits above the flat roof plane, allowing natural light to enter from above. Unlike a standard flat rooflight (flush or slightly raised), a lantern rises above the roof on a frame — typically aluminium or timber — giving the room below a greater sense of height and more diffused, attractive light.

Roof lanterns are usually square or rectangular. Sizes range from 1m x 1m (relatively modest) to 4m x 2m or larger for a generous kitchen-diner.

Natural light: London extensions often suffer from poor natural light, particularly in side return extensions where wall glazing is limited. A roof lantern floods the centre of the space with overhead daylight — the most flattering and even light source available.

Height and volume: Even on a flat-roofed extension, a lantern adds the visual perception of height. A 400mm-high lantern frame makes a 2.4m ceiling room feel significantly more spacious.

Visual appeal: A well-specified lantern is a design feature in its own right. Slim aluminium profiles with large glass panes look striking from inside and from the garden.

Connection to sky: In a single-storey extension, a roof lantern provides a view of the sky — changing with weather and light — that makes the space feel less enclosed.

Standard sizes:

• 1000 x 1000mm: Suitable for corridors or small infill extensions
• 1500 x 1500mm: Compact kitchen or utility extension
• 2000 x 1000mm: Popular for side returns
• 3000 x 1500mm: Generous kitchen-diner
• 4000 x 2000mm and above: Feature lantern for a large open-plan space

Custom sizes are available from most manufacturers.

Pitch: Most lanterns have a low-pitch frame (typically 15–25 degrees). A steeper pitch looks more traditional; a lower pitch appears more contemporary. The minimum pitch for self-cleaning glass to function effectively is around 5 degrees.

Frame colour: Aluminium lanterns are available in any RAL colour. The most popular choices in London homes are anthracite grey (RAL 7016), black, and white. Timber lanterns are typically natural oak or painted.

In most cases, adding a roof lantern to an existing flat-roof extension does not require planning permission — it falls within permitted development as an alteration to the roof that does not increase the volume of the dwelling.

However:

• In conservation areas, any alteration visible from a highway may require planning consent
• If the lantern would exceed the height of the existing roof ridge, planning may be required
• For listed buildings, listed building consent is always required for any alterations
• If the extension itself required planning permission, the lantern should have been included in that application

As always, check with your local planning authority if in any doubt, or apply for a Lawful Development Certificate.

A roof lantern requires a structurally adequate opening in the flat roof structure to sit on. For a new extension, this is designed into the structural package. For an existing flat roof, the opening must be assessed.

Key structural questions:

• Is the existing flat roof structure adequate to support the lantern frame and glazing?
• What is the roof construction — timber joists, structural concrete, or steel frame?
• Does the opening need trimmer joists or steel trimmers?
• Is the junction between lantern and roof properly waterproofed?

For any lantern over approximately 1.5m x 1.5m, involve a structural engineer if the opening is being cut into an existing roof. This is a relatively low-cost check (£300–£600 for a structural assessment) that ensures the installation is safe.

Waterproofing: The junction between the lantern curb and the flat roof membrane is a critical waterproofing detail. Poor detailing here is a common source of leaks. Reputable lantern manufacturers provide detailed installation instructions and compatible upstand heights. The roofer and lantern installer must coordinate closely.

A minimum upstand height of 150mm above the finished roof level is standard. The roof membrane should overlap the lantern's base frame and be sealed with appropriate tape or mastic.

Thermal performance: Modern roof lanterns use triple-glazed or high-performance double-glazed units. A roof lantern that does not meet Part L (thermal) requirements of Building Regulations will be flagged on a Building Control inspection.

Specify a centre-pane U-value of 1.0 W/m2K or better for a compliant lantern in a new extension. Many premium lanterns now achieve 0.7 W/m2K centre-pane.

Solar control glass: South-facing lanterns can cause significant heat gain in summer. Solar control glass (tinted or with a coating) reduces this — but also slightly reduces light transmission. Discuss the orientation of the lantern with your contractor when specifying glass type.

Self-cleaning glass: Roof lanterns are hard to clean manually. Self-cleaning glass (photocatalytic coating) breaks down organic debris in sunlight and allows rain to wash it away more efficiently. Worth specifying for lanterns above 1.5m from the ground.

• Small lantern (1.0 x 1.0m, aluminium): £800–£2,000
• Medium lantern (2.0 x 1.0m): £1,800–£4,000
• Large lantern (3.0 x 1.5m): £3,500–£7,000
• Extra-large lantern (4.0 x 2.0m): £5,000–£12,000+

Installation (including structural opening, roofing junction, and making good): £800–£2,500 depending on complexity.

Premium lanterns from manufacturers such as Korniche, Roof Maker, and Origin cost more than budget suppliers — but the quality of frame, glazing, and thermal performance is significantly better. For a kitchen extension intended to last 20+ years, mid-to-premium specification is strongly recommended.

Lanterns are typically installed after the main roof structure and waterproofing are complete:

1. Structural opening formed in flat roof
2. Upstand detail prepared and roof membrane installed
3. Lantern frame installed and secured
4. Glazing units installed
5. Junction sealed and waterproofed
6. Internal making-good (plastered reveals, painted)

For a new extension, the lantern should be on order before the roof structure is built — lead times of 4–8 weeks are typical for made-to-measure aluminium lanterns.

When specifying a roof lantern for a London extension:

• Confirm the U-value is compliant with Building Regulations Part L
• Specify self-cleaning glass for lanterns not at ground level
• Choose a frame profile in keeping with the architectural style of the extension
• Confirm the installer has experience with flat-roof lantern junctions
• Get a waterproofing guarantee from the roofer for the junction detail
• Check lead time and order early — lanterns are often on the critical path

rcbGroup includes roof lantern specification and installation within our design-and-build extension packages, ensuring the structural, waterproofing, and glazing elements are all coordinated by one team.