Fluid Glass Box: Frameless Glass Box Extensions as One Package

A glass box extension is a single-storey space enclosed on three sides by structural glass, topped with a glass roof, and joined to your house on the fourth side. No corner posts. No frames interrupting your view. It's like sitting outside, but with a roof over your head.

Most glass box roofs rely on steel supports. Our Wimbledon project started that way too. But we re-engineered it to span 2255mm in glass alone, carried by glass upstands. No steel in the roof at all. It can support the weight of a person for cleaning and carry snow loads with minimal deflection, leaving the sky above your room uninterrupted.

Our system triple glazed achieves 0.9 W/m²K for the whole system — frame and glass combined, measured on a 2 × 2m pane at a glass U-value of 0.6. Double glazed, it reaches 1.1 W/m²K. These whole-window numbers support comfortable year-round living with high-performance insulated glazing that reduces heat loss. If a quote shows a lower figure, check whether it excludes frame, spacer, or edge seal losses — these are where heat escapes. With the right specification, a modern glass extension stays warm in winter and cool in summer.

Our insulated units go up to 69mm triple glazed. Between the inner and outer aluminium profiles is a 30mm polyamide thermal break — the key to usability in winter. Many suppliers claim thermal breaks, but few specify thickness. We do.

A glass box faces the open sky and usually the garden, collecting sun from multiple angles on a July afternoon. High-performance solar control glass limits unwanted solar heat gain while preserving natural light. High-level ventilation, overhangs, or trees in the right spots help too. Privacy should be planned from the start if your garden or side boundaries are exposed. Integrated blinds manage glare and privacy. Part O requires overheating assessment for new dwellings. Model it at design stage rather than discovering issues on the first hot day.

Not every span can go steel-free. At Richmond, we enclosed a gym in a side infill extension using bonded glass panels on a finely welded minimal steel frame designed with Freeform Architects. The frame is minimal and set back to let the glass read as the building. This decision is made at design stage based on span, and we show you both options.

Where two glass walls meet, structural silicone replaces corner posts, letting your view flow around instead of stopping. Where wall meets roof, glass upstands carry the roof in one continuous form, or glazing can stop at a slim framed eaves. The first option reads as a single volume, the second is simpler and more cost-effective.

Structural glass walls are fixed. Openings go next to them — sliding doors with a 21mm sightline matching silicone joints, bi-folds that stack an entire side away, or pivot doors for a dramatic entrance. Decide early, as post placement depends on door choice. Wimbledon and Richmond both pair the box with Fluid Sliding Doors.

A conservatory is a lightweight add-on, often outside the thermal envelope and unusable in winter or summer extremes. A glass box is part of the house: insulated to the same standard, heated like any other room, and built to the same regulations. This distinction also explains much of the price difference.

Many single-storey rear extensions fall under permitted development rights, with limits on depth, height, and garden coverage. Conservation areas, listed buildings, and side or front extensions have different rules. Check early with your local authority. Building regulations (Part L) limit glazing on extensions to 25% of floor area, which a glass box exceeds by definition. The shortfall is compensated elsewhere, usually with better U-values — a calculation, not a barrier, underscoring why thermal specification matters.

A glass box has many junctions — wall to wall, wall to roof, glass to existing house, floor threshold, plus ground conditions and foundations, all set by our survey. We survey, supply, and install, so the panels and installation team are the same. See our work at Wimbledon and Richmond or visit our Watford showroom.

Architects — ideal for rear and side infill extensions where light is the priority over floor area. Request section details, U-values, and span data.

Homeowners — perfect for kitchens, dining rooms, or garden rooms where you want to feel outside without being outside.

Developers — when the glazed extension is the feature that sells the house, with one supplier covering walls, roof, and doors.

We install across Watford, London, and the UK.