This decision gets made too early on most jobs, often from a showroom photo rather than from how the door will actually be used day to day.
Both are glazed aluminium or timber-aluminium systems capable of a good thermal performance. The differences that matter are in how they open, how they look shut, and how they hold up over years of use.
Opening width and use
Bifold doors can open the entire width of an aperture, with the panels concertinaing to one or both sides. This suits households that genuinely want the wall to disappear for entertaining or hot weather, but every pane has to be folded and unfolded to get any opening at all — there's no quick option for just letting a bit of air in.
Sliding doors typically leave one panel fixed and let others run behind it, so a single slide gives partial ventilation in seconds. For a door used dozens of times a day rather than occasionally thrown fully open, that convenience matters more than most people expect before they live with it.
Sightlines and glass area
A sliding door has fewer, larger panes of glass and slimmer visible framing when closed, which gives a cleaner view out and lets more light in per metre of opening. Bifold doors have a frame member at every panel junction, which reads as a series of vertical lines across the view even when closed.
If the extension's whole design intent is a frameless glass wall onto the garden, sliding usually delivers that better than bifold.
Weather performance and running gear
Sliding doors generally perform better against wind-driven rain because the sealing runs along fewer, longer joints. Bifold doors have a seal at every panel-to-panel junction, and with cheaper hardware those seals are the first thing to start letting water and draughts through after a few years.
Running gear matters more than most buyers realise. A bifold door's rollers and hinges take repeated mechanical wear at every panel, so hardware quality has a bigger effect on long-term performance in bifold systems than in sliding ones.
Cost and structural opening
Bifold systems generally cost more per metre of aperture because of the additional panels, hinges and running gear, and the header beam above a very wide bifold opening still needs the same structural calculation as any other large opening.
Sliding doors with very large single panes can also carry a cost premium if the glass has to be oversized to avoid a central frame, so the cost comparison depends heavily on the exact configuration rather than the door type alone.
Common questions
Which is better for a narrow side-return extension?+
Sliding doors usually suit narrow openings better because they don't need a stacking space at the side for folded panels. A bifold in a tight opening can end up with a stack of panels blocking part of a narrow patio or path.
Do bifold doors need more maintenance?+
Yes, generally. More moving parts and more seals mean more to check — rollers, hinges and weatherstrips should be inspected periodically, and cheaper hardware wears faster under daily folding than a slider's running gear does.
Can either door type be used with underfloor heating?+
Both work fine over underfloor heating provided the threshold detail and insulation below the door track are specified correctly. The door type itself doesn't change the underfloor heating design.
Get advice on doors for your extension
Tell us how you'll actually use the opening and we'll recommend bifold or sliding, with a written quote covering the structural header too.