Belgravia House

Glass box extension in central London

Belgravia House is a glass box extension in central London added to a traditional terraced townhouse to increase usable floor area with a fully glazed rear volume. IQ Glass designed and installed a package of thermally broken structural glazing, an oversized structural glass rooflight and a slim framed aluminium casement door, allowing the new extension to project beyond the original rear wall and wrap around the side of the building. The glazing strategy keeps the addition visually light while drawing daylight deep into the plan and maintaining direct garden access from the open-plan kitchen and dining area.

Products Used

Project Partners

Architect: Paul Archer Design

Photography: Richard Gill Photography

Glass box extension in central London
Glass box extension in central London

Glass box extension in central London on a constrained townhouse plot

The design challenge for the glass box extension in central London was to add meaningful floor area to a dense terraced plot without introducing a visually heavy rear addition. The new glazed volume projects beyond the original rear wall and continues around the side of the building, increasing the footprint both outward and sideways while keeping the garden edge as transparent as possible. To achieve that, the glazing had to do more than fill openings; it had to form the architecture of the extension itself. Structural glazing was therefore used as the primary solution, creating a minimal envelope that preserves sightlines, increases daylight penetration and allows the new work to read as a precise contemporary insertion against the traditional house.

Up-and-over structural glazing to remove a framed roof edge

One elevation of the extension uses an up-and-over structural glazing detail, where a vertical structural glass pane connects directly to the roof glazing above. This glass-to-glass junction solves the roof-edge transition without introducing a heavier framed corner or roof support, using structural silicone to keep the connection visually light. The result is a continuous glazed surface that maintains uninterrupted views out from the dining area and reinforces the minimal architectural intent of the extension.

Oversized rooflight and flush-threshold door for light and access

A large fixed rooflight stretches the length of the dining area to solve a second challenge: bringing daylight deep into the rear extension rather than relying only on the garden-facing elevation. At ground level, an oversized aluminium casement door provides direct garden access and was specified to align visually with the adjacent structural glazing, avoiding a more visibly framed opening within the glazed composition. A flush threshold detail completes the door design, supporting a cleaner connection between inside and out while maintaining the disciplined appearance of the rear elevation.

Glass box extension in central London
Glass box extension in central London

Performance considerations

The glass box extension in central London needed to balance a minimal appearance with year-round comfort and durable detailing. Both the structural glazing and the casement door are fully thermally broken, helping the new living space achieve modern thermal performance within a highly glazed envelope. The up-and-over junction concentrates support, sealing and weathering into one precise glass-to-glass detail, while the oversized rooflight increases daylight penetration through the dining space. At the threshold, flush detailing supports a cleaner transition to the garden without compromising everyday usability.

Technical details

  • Frameless structural glazing: Thermally broken fixing profile depth 63mm; max glass thickness 41.5mm; expected Uw 1.1 W/m²K; minimum fixing setback 55mm; structure deflection allowance 5mm

  • Structural glass roof: Glass thickness up to 37.5mm DGU; typical Ug 1.1 W/m²K; example Uw 1.2 W/m²K; recommended single rooflight up to 1.5m × 3m; minimum upstand 150mm; fall 5° to 45°

  • Aluminium casement door: 101mm profile sightline; glass thickness 24–40mm; max sash size 1200mm × 2400mm; max leaf weight 90kg; Uw from 1.4 W/m²K; air permeability Class 4; water tightness Class 9A; wind resistance Class A3

Belgravia House shows how a glass box extension in central London can add usable space to a terraced home without relying on visually heavy construction at the rear elevation. This approach is particularly well suited to contemporary rear extensions where architects and specifiers need structural glazing, roof glazing and flush-threshold access to work together as one disciplined architectural glazing package. To discuss a similar glass box extension in central London or another architectural glazing project, contact IQ Glass for technical advice on design detailing, specification and installation.

Associated Products

Aluminium Casement Doors

Aluminium casement doors are sturdy and low-maintenance, offering excellent energy efficiency and security. They come in a variety of designs and colors, and can enhance the appearance and value of any home or building.
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