Citi Homes Windows

Sliding vs bifold doors. Which works better for a London rear extension?

Bifold and sliding doors are the two systems most commonly fitted to rear extensions in London. Both connect the kitchen or living room to the garden, and both come in slim aluminium profiles that work equally well on contemporary builds and older period properties. Choosing between them is not always straightforward – the right answer depends on the opening, the building and how the space will actually be used.

How the two systems operate

Bifold doors consist of a series of hinged panels that fold and stack to one or both sides of the opening. When fully open, the doorway is almost entirely clear – typically 90% or more of the total width. That is the key characteristic of a bifold: the ability to fully remove the barrier between inside and outside, which makes it a natural fit for wide rear elevations, for homeowners who entertain regularly outdoors, or for families who want a direct, unobstructed connection between the kitchen and the garden.

Sliding doors work on a different principle. Panels travel horizontally along a track, and on a standard two-panel system, one panel is fixed while the other slides, giving roughly 50% of the opening width as usable access. With three or four panels, where all can travel and stack behind one another, that rises to around 65–70%. The full-width clearance of a bifold is not achievable with a sliding door – but what sliding doors offer in return is a noticeably different quality when closed: less frame, more glass, and a cleaner view through to the garden.

Sightlines: the detail that shapes the room

When architects choose sliding doors over bifolds for a London extension, sightlines are usually the reason. The meeting stile, the vertical bar where two sliding panels meet, can be as narrow as 20–25mm on a good slim-profile system. A bifold’s equivalent section needs to be 80–120mm wide to accommodate hinges and the structural demands of the folding mechanism. On a modest opening or a room with a compact garden behind it, that difference is clearly visible.

It matters most when the doors are closed, which is the case the majority of the time even in a decent summer. Multiple bifold panels stacked to one side can look heavy when shut; a slim sliding door reads almost as a glass wall. For homeowners who care as much about how the room looks on a grey Tuesday as on a warm Saturday afternoon with the doors fully open, this is worth factoring into the decision.

Cost: what to expect in 2026

Bifold doors in aluminium for a standard 2.4m–3.6m opening typically cost between £3,000 and £5,000 supplied and installed, depending on the number of panels and the system chosen. Sliding doors at the same width are in a similar range, starting from around £2,800–£4,500. The gap widens at the premium end: slim-profile sliding systems with 20mm sightlines from manufacturers such as Schueco or Cortizo carry a significant premium – often £6,000–£12,000 for a wider opening, compared to a standard bifold at the same span.

Above 4m, the economics shift. Bifold panels become progressively heavier per panel as the span increases, and the installed cost per metre rises accordingly. Sliding systems are engineered for wide spans, generally carry less panel weight, and can be more cost-effective at that scale. For openings of 5m or more, it is worth getting prices for both systems before deciding.

Structural and threshold details

Both systems require a structural opening with a lintel spanning the full door width. Where the opening is wider than the existing aperture, a structural engineer’s assessment is required before any work begins. In the Victorian and Edwardian terraces that make up most of the rear extension work in London, lintels are sometimes undersized for wider spans and need replacing – this is not unusual, but it needs to be established at the outset rather than discovered once production is underway.

Threshold design is worth confirming early in the project. Flush thresholds, where the door track sits level with the finished floor, are now standard on quality installations and are included in most architect drawings as a given. Raised tracks that create a step at the doorway have largely fallen out of use, and most homeowners and architects avoid them. For bifold doors specifically, it is worth confirming at survey stage whether the panels fold inward or outward, and that there is enough clear wall space to accommodate the folded stack – in a small London terrace garden this is easy to overlook and awkward to resolve after the frame is in.

Planning in London

Replacing an existing rear door with a bifold or sliding system generally falls within permitted development and does not require a planning application. There are, however, several exceptions relevant to London. Properties in conservation areas require prior approval or planning permission for changes that affect the character of the area, and the threshold for what triggers that requirement varies between boroughs. Listed buildings always require listed building consent for any change, regardless of scale. Where the project involves widening an existing opening as part of a rear extension, the extension itself requires planning permission, with the door specification forming part of that application.

Some London boroughs operate Article 4 directions that remove permitted development rights for window and door replacements in residential areas. This is not always widely known, and it is worth checking with the local planning authority before ordering to confirm the position – discovering a planning issue after installation is considerably more disruptive than checking before.

Choosing between the two systems

Bifolds are the stronger choice where full-width opening is the priority – a wide rear elevation, regular outdoor entertaining, or a household that wants the maximum connection between inside and outside when the weather allows. Sliding doors suit a narrower opening, a project where clean architectural sightlines take precedence, or a homeowner for whom the quality of the closed room matters as much as the experience of the fully open one. For most London rear extensions in the 2.4m–4m range, both systems are viable and the decision comes down to use and preference. We carry out a full site survey before recommending a system, and we are happy to talk through the options in detail before anything is specified or ordered.

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