Flat roof extensions are one of the most common project types in London, and one of the most important design decisions in a flat roof extension is how to bring natural light into the space from above. Two main solutions exist: rooflights (skylights) set flush within the flat roof plane, and roof lanterns that project above the roof plane as a glazed structure.
Rooflights are flat glazed units mounted within the plane of the flat roof, finished level with or just slightly above the roof surface. They range from standard domestic sizes of six hundred by nine hundred millimetres up to very large structural glass panels spanning several metres. They can be fixed or opening. Modern high-performance rooflights have triple glazed units, thermally broken frames, and very low U-values, making them appropriate for a well-insulated extension roof. They provide good natural light without creating a strong visual feature — the light enters the room evenly from above without a dominant architectural element. Brands commonly used in London residential work include Velux, RoofLITE, and several specialist structural glazing suppliers for larger units. Supply cost ranges from five hundred to two thousand five hundred pounds for standard domestic sizes, rising to three thousand to eight thousand pounds or more for large structural flat glass panels.
Roof lanterns are glazed structures that project above the flat roof plane — typically a four-sided or multi-sided pitched glazed frame on a low upstand. They create a distinctive architectural feature visible from both inside and outside the extension, and they provide a greater sense of volume and height in the room below than a flat rooflight. The light they bring is more directional and more dramatic. A well-designed roof lantern becomes the centrepiece of a kitchen-dining extension. High-quality roof lanterns have slim aluminium or powder-coated steel frames with low profile sightlines and high-performance glazing. Supply cost ranges from two thousand five hundred to five thousand pounds for a standard size of one by two metres, rising to eight thousand to twenty thousand pounds for larger or higher-specification units, and significantly more for bespoke structural glass designs.
The choice depends on the design intent of the extension. Where light is the primary goal and the architectural approach is clean and understated, a rooflight or multiple rooflights spread across the roof plane is often the better choice. Where the roof lantern becomes a design statement and the kitchen-dining space is designed around it, a roof lantern adds genuine spatial drama that rooflights cannot match.
Both options require the flat roof structure to accommodate the opening. A structural engineer must confirm that the roof joists or frame can be modified to form the required opening without compromising the structural integrity of the roof. 07359 872594 | contact@rcbgroup.co.uk | www.rcbgroup.co.uk