A bifold is a run of hinged panels that fold and stack to one side, and the price follows the number of panels, the material, the size of the opening and what is holding the building up above it. That last item is the one that turns a door purchase into a building job, and it is the reason two quotes for the same opening can differ by more than the doors are worth.
Bifold doors installation cost: what you are paying for
The panels themselves, the track and the running gear, the threshold, the glass specification, and the installation. Running gear is where quality shows: a bifold is a moving part you will operate thousands of times, and cheap rollers bind and drop within a few years. Aluminium dominates because it carries large panes on slim frames; timber and composite exist and are heavier. More panels means more hinges, more hardware and more to go out of alignment.
The cost to fit bifold doors: the opening above is the real cost
Bifolds usually replace a wall, so something has to carry the floor or roof above. That is a lintel or a steel beam designed for the span and the load, with padstones and sometimes temporary support while it goes in, and on a semi or terrace it may engage the party wall. It is designed work under Approved Document A and it belongs in the building control submission. Any quote that prices doors for an opening that does not yet exist and says nothing about the beam is incomplete.
Thresholds, drainage and the flat you can walk over
Everyone wants a flush threshold and it is the detail that leaks. A low threshold still needs a way to shed and drain water on the outside, usually a drainage channel immediately outside the door and a fall away from the building. Where accessibility matters, level access is a requirement worth designing in rather than retrofitting. Ask what the threshold detail is and where the water goes, because on a north facing elevation in a British winter this is not theoretical.
Which way should they fold
Panels can stack left, right or split, and they can fold inward or outward. Outward folding keeps the internal floor clear and puts the stack on the patio; inward does the reverse. Most runs include a traffic door, a single leaf that opens on its own so you are not folding the whole set to let the cat out. Decide the traffic door position before ordering, because it cannot be moved afterwards.
Corner bifolds, and where they get difficult
A corner bifold removes the structural corner of the building, which is a significantly larger engineering problem than a straight run and often needs a goalpost frame or a corner post that can be removed only because a beam above is carrying it. They are impressive and they are the most expensive way to open a corner. Get the structural answer before falling for the drawing.
Questions people ask about bifold door prices
Do bifold doors need building regulations approval?
Forming or widening a structural opening does, and the new doors have to meet thermal and safety glazing requirements. Replacing like for like in an existing opening is usually certified by the installer under a competent person scheme.
Can a single broken bifold panel be replaced?
Usually the sealed unit within the panel can be replaced rather than the whole leaf, provided the frame and hardware are sound. Matching an older system's glass and beading is the practical difficulty.
Are bifolds or sliding doors better?
Bifolds open almost the whole aperture and have more frame when closed. Sliders give larger uninterrupted glass and open half the opening. It is a trade between the view when shut and the opening when open.
How much maintenance do they need?
Keeping the bottom track clear of grit and the hinges adjusted is most of it, and it matters more than on any other door because misalignment compounds. Ask what the servicing expectation is under the guarantee.