Bifolds appear on conservatories, orangeries, garden rooms and extensions, and the four are different structures with different constraints. A conservatory frame is light and was designed for what it came with. An orangery has brick piers. A garden room sits on its own base. An extension has a real wall to open up. The doors are the same product and the job is not.
Conservatory with bifold doors cost: the frame was not designed for this
A conservatory's glazing frame and dwarf wall were sized for the original doors. Bifolds are heavier, they concentrate load at the stacking end, and the track has to be dead straight and level along its whole length or the panels will not close. On an older conservatory the honest answer is often that the structure needs strengthening first, and a supplier who measures the opening without looking at what holds it up is selling doors rather than quoting a job.
An orangery with bifold doors, and brick piers
An orangery has masonry piers between glazed bays, which is a considerably better starting point: the piers carry the perimeter roof and the bifold run sits between them in an opening that already exists. The questions become the threshold detail where the doors meet the floor slab, and whether the head of the opening is a lintel sized for the span. Both are ordinary building questions rather than glazing ones.
A garden room with bifold doors and the outdoor problem
A garden room's bifolds are exposed on all sides with no house to shelter them, often on a timber frame sitting on a lightweight base. The base has to be level and stable, because ground movement shows up first as doors that will not latch. Specify hardware for exposure, and remember that a wall of glass in a small insulated building is most of its heat loss, so the glass specification matters more here than on a house.
Extension bifold doors: the beam is the job
Opening a rear wall for bifolds means a beam carrying the floor or roof above, designed for the span, with padstones and temporary support during installation. That is the expensive half and it is the half that is sometimes missing from a door quote. It is designed structural work and belongs in the building control submission. Where a conservatory or extension roof lands on the same wall, the two have to be resolved together.
What the glass should be doing
A large glazed elevation is the biggest single thermal element of the room it serves, so the whole door energy rating matters more than it does on a small window. On a south facing run, solar control glass keeps the room usable in summer; on a north facing one it costs you daylight for nothing. It is worth asking the supplier which way the doors face before accepting the standard specification.
Questions people ask about bifold doors conservatory
Can I fit bifolds to an existing conservatory?
Sometimes, after someone has assessed the frame and the dwarf wall. Bifolds are heavier and less forgiving of an out of level track than the doors they replace.
Do extensions with bifold doors need a steel beam?
If you are opening up an existing wall that carries anything above it, yes, and it is designed rather than estimated. That is normally the largest line in the job.
Are garden room bifold doors a bad idea in a small building?
They are the biggest heat path in a small building, so specify the glass properly and expect the room to need heating in winter and shading in summer. Many people fit them anyway because the opening is the point of the building.