Bifold Doors for House Extensions: What to Specify, What They Cost, and Common Mistakes

**The two main frame material options**:

Almost all residential bifold doors installed in London are aluminium — either thermally broken aluminium (the standard) or, for the premium end, aluminium-clad timber (aluminium externally, timber internally). Timber-only bifold doors exist but are uncommon in the current market.

**Thermally broken aluminium**:

*What 'thermally broken' means*: A standard aluminium frame conducts heat efficiently between the inside and outside — resulting in cold frames in winter and condensation. A thermally broken frame has a polyamide (plastic) strip inserted between the inner and outer aluminium extrusions, breaking the direct thermal path and significantly improving the U-value of the frame section.

*Why thermally broken is required*: Bifold doors in a post-2021 extension must meet Part L energy performance requirements. Typically, a thermally broken aluminium bifold door achieves a whole-unit U-value of 1.2–1.6 W/m²K depending on the glazing specification — meeting the 1.4 W/m²K maximum required for new openings.

• *Brands and quality tiers*: Aluminium bifold door quality varies significantly. The market divides broadly into:
• •Budget tier (£800–£1,200 per leaf): typically Far East manufactured, thinner extrusions, less precise tolerances, seals that degrade faster, less smooth operation. Examples: some Alibaba-sourced products, some trade-only budget brands.
• •Mid-tier (£1,200–£2,000 per leaf): UK or European manufactured, adequate tolerance, good hardware. Examples: Smart Alspec, Origin (budget line), AluK.
• •Premium tier (£2,000–£3,500 per leaf): Best tolerances, best hardware (Roto, Siegenia, Winkhaus), longest warranty, smoothest long-term operation. Examples: Origin O30, Schüco, Kawneer.

*For most London domestic extensions, a mid-tier system from a reputable UK manufacturer is appropriate and cost-effective.*

**Timber-aluminium composite**:

A timber internal face with aluminium external capping — gives the warmth and appearance of timber internally (particularly relevant where the client wants timber to match interior joinery) with the low-maintenance aluminium external performance. Significantly more expensive than aluminium-only.

**Key specification decisions**:

*Number of leaves*: Most domestic bifold door installations use 3–6 leaves. An even number of leaves allows the doors to fold evenly to both sides or to one side; an odd number typically stacks to one side. The most common domestic arrangement is 3-leaf bifold or 4-leaf bifold.

*Traffic door (pass door)*: In a bifold door system wider than 2.4m, it is good practice to include a 'traffic door' — one leaf that operates as a conventional single door for daily use without having to fold the full system back. The traffic door is the leaf at one end of the bifold run, hinged to open independently.

*Top-hung vs. bottom-running track*: Most quality bifold door systems use a top-hung (overhead track) system — the doors hang from the track above and a bottom guide channel aligns the lower edge, but no structural weight is carried by the bottom track. Bottom-running systems (where the weight is carried by a track in the floor) are now rare in quality systems. For level-access thresholds (see below), top-hung systems are far easier to detail correctly.

**Structural lintel above the bifold opening**:

A bifold door opening in a rear wall is a structural opening — the load above the opening (roof structure, floors above, any superimposed loads) must be carried by a beam (typically a steel RSJ or universal beam, or a reinforced concrete lintel) spanning the full width of the opening.

• This is a structural engineering matter — not an assumption the builder or bifold door supplier should make. The structural engineer must specify the lintel size based on:
• •The total span of the opening
• •The loads above (single-storey or double-storey structure above the opening)
• •The bearing at each end of the lintel (the existing wall provides bearing — a minimum bearing length of 100–150mm is typically required at each end)
• •The deflection limit (excessive deflection of the lintel causes the bifold door to jam or become difficult to operate)

*Common mistake*: Installing a bifold door in a rear wall opening and specifying an 'off-the-shelf' concrete lintel rated by span tables rather than structural engineer specification. A concrete lintel specified from span tables is designed for masonry loads above a window — it may not be adequate for a wide bifold opening with roof structure above, particularly where there is a first floor bedroom above.

*Deflection* is the critical consideration for bifold doors specifically: because bifold doors are precision-manufactured to close tolerances, even a small amount of lintel deflection (under load) will cause the door system to jam, bind, or become difficult to operate. The structural engineer should specify the lintel with a deflection limit of L/500 (span divided by 500) or better for a bifold door opening.

**The threshold detail — the most specification-sensitive detail**:

The threshold (the junction between the bifold door frame and the floor) is where most bifold door installations are compromised. There are three threshold options:

*Standard raised threshold*: A 15–25mm step up from external to internal floor level. The bifold door frame sits on a small sill, the weather seal is above the floor level, and there is a clear drainage path outside. Simple to install, very low risk of water ingress. But creates a step that is not accessible and breaks the visual continuity of the floor level.

*Rebated threshold (low-level)*: The threshold frame is recessed into the structural floor so that the finished internal floor level is at or very close to the top of the threshold. Creates a near-level transition with only a small (5–10mm) upstand. Requires careful floor build-up coordination — the structural floor must be at the right level before the bifold frame is installed. Risk of water ingress if the drainage slope of the external patio is inadequate.

*True level-access threshold*: Floor level matching exactly between inside and outside, with the threshold detail flush. The most difficult to achieve correctly — requires a drainage channel (linear drain or perimeter drain) directly outside the door, adequate falls on the external patio, and a high-performance weather seal on the door bottom rail. Any inadequacy in the drainage results in water ingress during heavy rain.

*Recommended for most London domestic extensions*: Rebated threshold or low-level threshold — not true level-access unless the client has an accessibility requirement (DDA/PAS 2016 compliance) and the drainage design is carried out by a specialist.

**Part L thermal performance**:

The bifold door's U-value (whole-unit W/m²K) must meet the Part L maximum of 1.4 W/m²K. This is achievable with a thermally broken aluminium frame and double-glazed sealed unit with low-e coating and argon fill. Triple-glazed units can achieve 0.8–1.0 W/m²K. The larger the glazed area (which for a bifold door can be very large — 6m × 2.1m of glass), the greater the heat loss through the door compared with an insulated wall — this is the thermal penalty of extensive glazing in an extension.

**Typical bifold door costs (supply and installation)**:

| Opening width | Number of leaves | Aluminium thermally broken (mid-tier) | Aluminium thermally broken (premium) | |---|---|---|---| | 2.4m | 3 leaves | £3,500–£6,000 | £6,000–£9,000 | | 3.2m | 4 leaves | £4,500–£8,000 | £8,000–£13,000 | | 4.0m | 4–5 leaves | £5,500–£10,000 | £10,000–£16,000 | | 5.0m | 5–6 leaves | £7,000–£13,000 | £12,000–£20,000 |

*Note*: These costs include supply of the door system and frame installation — they exclude the structural lintel and opening formation (carried out by the main contractor), external patio and drainage, and internal floor finishes.

**Common mistakes in bifold door installations**:

1. *Inadequate structural lintel*: The most serious and the most common. An undersized or over-deflecting lintel causes the bifold door to jam within months of installation — the door becomes impossible to operate correctly and the frame distorts. Fix: Structural engineer specification for the lintel, with deflection limits appropriate for bifold doors.

2. *Threshold level not coordinated with floor build-up*: The internal floor level (finished floor level) must be coordinated with the threshold level before the structural slab or floor is cast. Changing the floor level after the bifold door has been installed is very expensive. Fix: Agree threshold detail and finished floor level before the floor slab is constructed.

3. *Inadequate patio drainage*: Water running from the garden toward the building (reverse fall) will enter through the door threshold even on a well-specified door system. The patio outside a bifold door must fall away from the building at a minimum gradient of 1:80. Fix: Design the external patio drainage before the floor is poured — drainage direction cannot be changed after the patio is laid.

4. *Specifying the door system before confirming the structural opening size*: Bifold doors are manufactured to order at a specific size. If the structural opening (between finished walls) differs from the ordered size, the door will not fit. Fix: Always measure the finished structural opening before placing the order — do not order from a drawing dimension.

5. *No traffic door in wide openings*: A 5m bifold door system without a traffic door requires folding back all 6 leaves every time someone walks between the house and the garden. Fix: Specify a traffic door at one end of the system for any bifold installation wider than 2.4m.