Structural glazing vs framed extensions.

A framed extension is the conventional way to build a glazed addition. Aluminium or steel mullions and transoms divide the glazing into panels; a structural frame above carries the roof load; the corners are formed by an aluminium post that the two perpendicular walls fix into. The system is buildable from a catalogue, the engineering case is standard, and the lead times are short.

Structural glazing rebuilds the same geometry without the visible framing. Each glass panel is engineered against the load case; the panels are joined to each other and to the surrounding structure with structural silicone bonds; the corners are formed by silicone-bonded glass-to-glass joints rather than aluminium posts. The result reads as pure glass volume against the host building.

On engineering, framed systems are calculated against a manufacturer's standard system catalogue — fast, reliable, predictable. Structural glazing is calculated per project against the specific load case. The engineering work is non-trivial and cannot be skipped; it is the cost driver as much as the glass itself.

On manufacturing tolerance, framed systems work to ±5–10 mm on panel edges because the frame absorbs dimensional variation. Structural glazing works to ±1 mm because there is no frame to absorb the variation. The tolerance discipline cascades through fabrication, transport, and installation.

On cost, framed extensions are typically 30 to 40% cheaper per m² than equivalent structural glazing. The gap pays for the engineering work, the tighter tolerance, and the slower install. Structural glazing programmes also run longer because the engineering happens up front.

When structural glazing makes sense over a framed extension. The architectural priority is the absence of the frame at the meeting points; the project budget supports the per-project engineering, the controlled-conditions fabrication and the dedicated install crew that structural glazing requires; the structural opening can absorb the load case without visible columns. Structural glazing is selected for design reasons cost cannot answer.

When a framed extension makes sense over structural glazing. The brief prioritises floor area, daylight and U-value over the absence of visible aluminium; the budget makes the structural-glazing uplift unjustifiable; the lead time on per-project engineering is incompatible with the contractor programme. Framed aluminium glass extensions deliver most of the same architectural benefit at a fraction of the lead time and cost.

Detail considerations. Framed extensions carry the load through visible aluminium uprights and beams; structural glazing carries the load through the glass itself, with silicone-bonded corners and engineered edge clamps. The thermal performance gap is smaller than the visual difference suggests — both routes meet Approved Document L 2025 limiting U-values with appropriate IGU build-ups, and the BFRC whole-window U-value rating accommodates either approach.

Regulatory context. Framed aluminium extensions are certified under BS EN 14351-1 product standard for the door and window components plus the relevant structural code for the framing assembly; structural glazing is engineered against BS EN 16612 (overhead glazing), CWCT technical notes (cladding and structural glazing), and BS EN 12150/12337 for the toughened/laminated glass build-up. The same regulatory regime, applied through different specification routes.

Five factors. The choice is rarely about cost — framed systems are nearly always cheaper, and projects that pick structural glazing do so for design reasons that the cost cannot answer.