Bifold and Sliding Doors for London Extensions: Costs, Options, and What to Consider

**Bifold doors** fold back on themselves in accordion sections. A set of bifold doors can open to create a clear aperture of up to 90% of the total width. They are available in 2-panel sets up to very wide multi-panel configurations (8+ panels).

Bifold advantages: - Wide clear opening — panels can be stacked to one or both sides, leaving almost the full width open - Can incorporate a traffic door (a single-width panel that opens independently without unfolding the full set) - Very wide selection of manufacturers and products at all price points - Can span from 1.8m to 8m+ wide apertures

Bifold disadvantages: - Panels project into the interior when opening — you lose approximately 200–400mm of floor space in the opening zone depending on panel count - More hardware (hinges, pivots, top track) than sliding doors — more potential maintenance points - Wider threshold profile than minimal-frame sliders

**Sliding doors** (also called 'slimline sliders' or 'inline sliders') run on a track, with panels sliding parallel to the face of the building rather than folding into the room. Multi-panel sets can achieve very large openings with panels stacking behind each other.

Sliding advantages: - No projection into the room when opening — the panel stays in the plane of the wall - Very slimline frames available (20–40mm face widths) for a more minimal look - Less hardware and simpler operation than bifolds - Better suited to very wide openings (6m+) where bifold panel count becomes unwieldy

Sliding disadvantages: - The opening is limited to 50–66% of the total width (one or two panels always remain fixed or parked behind open panels) - Typically more expensive than bifolds for equivalent sizes - Track at floor level requires a low threshold (flush with or slightly above the floor)

**For most standard London rear extension widths (3–5m), bifold doors are the most popular choice.** For wider openings (5m+) or for a more minimal, contemporary aesthetic, slimline sliding doors are increasingly preferred.

**Aluminium** is the dominant material for bifold and sliding doors in London extensions, and for good reason: - Narrow section sizes — aluminium's structural strength allows much slimmer frame profiles than uPVC - Available in any RAL colour (powder-coated) — the most popular choices are anthracite grey (RAL 7016), black (RAL 9005), and white (RAL 9010) - Thermally broken aluminium (with a plastic thermal break between inner and outer aluminium sections) achieves good U-values (1.4–1.6 W/m²K typical for double glazed; 1.0–1.2 W/m²K for triple glazed) - Maintenance-free - Long lifespan (25–40 years)

**uPVC** bifold and sliding doors are available at lower price points, but the wider section sizes required for uPVC to achieve equivalent structural performance mean less glass area and a less minimal look. Not the first choice for premium extensions.

**Timber** bifold doors are available from specialist manufacturers. Oak or engineered timber bifolds have a warmer, more natural aesthetic. They are typically more expensive than aluminium, require periodic maintenance (oiling or painting), and are less commonly specified for London extensions.

For 95%+ of London rear extensions, thermally broken aluminium bifold or sliding doors are the right choice.

Bifold and sliding door costs in London (2025) — supply and installation:

**Aluminium bifold doors (thermally broken, double glazed)**: - 2.1m wide (3-panel): £2,800–£4,500 - 3.0m wide (3-panel or 4-panel): £3,500–£6,000 - 3.6m wide (4-panel): £4,200–£7,500 - 4.8m wide (5 or 6-panel): £5,500–£10,000 - 6.0m wide (6 or 7-panel): £7,000–£13,000

**Aluminium slimline sliding doors (thermally broken, double glazed)**: - 2.4m wide (2-panel): £3,000–£5,500 - 3.6m wide (2-panel): £4,000–£7,000 - 5.0m wide (3-panel): £6,000–£11,000 - 7.2m wide (4-panel): £8,000–£15,000

**Upgrade to triple glazing**: add approximately 15–25% to the above costs.

**Important note**: these are supply-and-install costs from a specialist glazing/door supplier and installer. If using a design-and-build contractor for the whole extension, the door supply cost is typically passed through at trade price + margin.

A large bifold or sliding door opening in the rear wall of a London terrace or semi-detached house is almost always a structural alteration.

The rear wall of most London Victorian and Edwardian terraced houses is a load-bearing wall — it carries the floor joists from the rear rooms and often the rear part of the roof. Creating a wide opening in this wall requires:

1. **A structural engineer** to design the beam spanning the opening 2. **A steel beam (RSJ)** installed above the new opening to carry the loads the wall was transferring 3. **Temporary propping** during the structural alteration works 4. **Building Regulations** approval for the structural elements

For a new rear extension, the structural engineer typically designs the full rear elevation as part of the extension structural package — the bifold/sliding door opening is integrated into the structural design from the start rather than treated as a separate alteration.

The maximum practical span for domestic extensions is around 6–7m for a steel beam spanning a typical terraced house depth, though larger spans are achievable with heavier sections or structural posts/columns at the sides of the opening.

If you want bifold doors spanning the FULL width of the extension, make sure the structural engineer is aware of the intended door system before designing the beam — the beam depth affects the headroom above the doors.

For permitted development (PD) rear extensions, the glazed rear opening typically falls within the PD allowance as long as the extension itself is within PD rules.

In conservation areas, special care is needed around: - The choice of door material and colour - Whether the extension requires planning permission (because conservation areas have more restricted PD rights) - Whether the doors are visible from the street (generally unlikely for rear extensions)

For properties in conservation areas or with planning-restricted extensions, discuss the door design with your architect or planning consultant before committing to a product. Some conservation area planning authorities are cautious about large expanses of glazing and narrow aluminium framing on rear extensions — though this is rare.