The Case for Large Glazed Openings
In a London rear extension, a large glazed opening at the rear is almost universal. It provides:
- Direct visual and physical connection to the garden
- Natural light deep into the extension and original kitchen space
- A feeling of space that makes even a modest extension feel generous
- Acoustic and thermal separation from the garden when closed
The choice between bi-fold and sliding door systems affects how that opening functions.
Bi-Fold Doors
Bi-fold doors consist of panels that fold against themselves, concertina-style, and stack at one or both ends of the opening. When open, they create a very wide clear aperture — typically 80–85% of the frame width.
Advantages
- Maximum open aperture: When fully folded, the glass virtually disappears, merging indoor and outdoor space
- Wider design choice: More manufacturers, more finishes, more price points
- Lower cost than comparable sliding doors at standard widths
- Traffic door option: Most bi-fold sets include a single door that opens independently without operating the full set
Disadvantages
- Floor track: Bi-fold doors require a low-profile floor track that can accumulate dirt and can be a trip hazard or obstacle for wheelchairs and buggy access
- Stack width: The folded panels stack at one or both ends, reducing the effective opening width in that area
- Panel width limitation: Individual panels are typically 600–1000mm wide — a 4-panel set for a 3m opening means narrower glass panels than a 2-pane sliding door
- Wind sensitivity: Fully open bi-folds can be caught by wind more easily than a flush sliding door
Typical Costs
| Opening Width | Aluminium Bi-Fold (supply only) |
|---|---|
| 2.4m (3 panels) | £1,800–£3,500 |
| 3.6m (4 panels) | £2,500–£5,000 |
| 4.8m (6 panels) | £3,500–£7,000 |
Installation adds approximately £500–£1,500 depending on scope.
Sliding Doors (Slim-Frame / Lift and Slide)
Sliding doors use one or more panels that slide along a track — either at the same plane (inline sliding) or using a lift-and-slide mechanism for better thermal and acoustic sealing. Premium systems are slim-framed, maximising the glass-to-frame ratio.
Advantages
- No floor obstruction: Flush floor track options or fully frameless systems allow level threshold access (excellent for wheelchair access, stepping out to garden, and cleaning)
- Slim frames: Premium sliding systems offer significantly slimmer sight lines than bi-folds, maximising glass area
- Thermal performance: Modern lift-and-slide systems achieve excellent U-values (often Uw 1.0 W/m²K or better)
- Large single-pane glass: Possible to specify very large individual glass panels, creating dramatic uninterrupted glazed elevations
- Smooth operation: One-handed sliding even for heavy panels
Disadvantages
- Less open aperture: One panel always overlaps the other (or slides behind a fixed panel) — typically 50% of the frame width is open at maximum
- Higher cost: Premium slim-frame sliding systems are more expensive than equivalent bi-folds
- Limited budget options: Very cheap sliding doors sacrifice thermal performance and long-term reliability
Typical Costs
| Opening Width | Aluminium Slim Sliding (supply only) |
|---|---|
| 2.4m (2 panels) | £2,500–£5,000 |
| 3.6m (2–3 panels) | £4,000–£8,000 |
| 4.8m (4 panels) | £6,000–£14,000 |
Thermal Performance
Both systems can achieve good thermal performance when correctly specified:
- Double glazing: Typical centre-pane U-value 1.0–1.2 W/m²K; frame U-value varies significantly
- Triple glazing: Centre-pane U-value 0.5–0.7 W/m²K — premium option with significant weight implications for large panels
For Building Regulations compliance, the overall window U-value (Uw) — including frame and glass — should be 1.4 W/m²K or better (the standard threshold, though individual calculations vary).
Choosing Between the Two
| Priority | Choose |
|---|---|
| Maximum open aperture and blur between in/out | Bi-fold |
| Slim sight lines and architectural drama | Sliding (slim-frame) |
| Level access (accessibility or buggy) | Sliding (flush sill) |
| Budget | Bi-fold |
| Large panes with minimal frames | Sliding |
| Small/medium opening (under 3m) | Either |
RCB Design & Build
RCB specifies bi-fold and sliding door systems as part of extension projects, advising on frame finish, glass specification, opening configuration, and threshold detail to suit each project's design intent and performance requirements.