The short answer
Sliding doors give the better view, because the panes are larger and the frames slimmer, and they are shut for most of the British year. Bifold doors open up almost the whole gap, which matters if you want house and patio to become one space in summer. Pick sliding doors for the view and for small rooms; pick bifolds if the fully open wall is the point.
On this page
The doors are the largest single item of glazing in most extensions and the one you will touch every day. Here is how the two main types compare in use.
How far they open
A bifold is a row of hinged panels that concertina to one or both sides. Folded back, it clears about nine-tenths of the opening. On a warm day the wall is simply gone.
A sliding door has two, three or four large panes on parallel tracks. A two-pane slider opens half the gap; a three-pane version on three tracks opens two-thirds. Only a pocket slider, where the panes slide into a cavity in the wall, clears the whole opening, and that needs wall to slide into.
Be honest about how often the whole wall will be open. For most households it is a few dozen days a year.
The view when they are shut
This is the other three hundred days. Bifold panels are usually 700 mm to 1.2 m wide, so a 4 m opening has four or five panels and three or four vertical frames in the view. Sliding panes can be 2 m or 3 m wide each. The same 4 m opening has one meeting stile in the middle, and on slim systems it is about as wide as two fingers.
If the garden is what you are building the room for, sliders show more of it.
Space inside and out
Folded bifolds stack at right angles to the opening and stand 600 mm to a metre into the room or onto the patio, depending on which way they are hung. Furniture and planters have to keep clear. Sliding doors take no floor at all, which is why they suit a small garden room extension.
Everyday use
Most bifold sets include a traffic door: one panel that opens like an ordinary door so you can step out without folding the lot. Check the set you are quoted has one. An even number of panels sometimes does not.
A slider opens a hand’s width for air and stays put. It is lighter to use than its size suggests if it is a lift-and-slide type, where turning the handle raises the pane off its seals.
Thresholds
Both can be fitted with a low or flush threshold so the floor runs level to the patio. Flush tracks need careful drainage beneath them and are a little less weathertight on an exposed site. A low upstand of 15 mm or so is a sensible middle. Ask what threshold is in the quote and see one in a showroom if you can.
Warmth and weather
New doors in an extension must meet a maximum U-value of 1.4 W/m²K, and both types do. Bifolds have more frame and more seals for the same opening, so on paper they lose slightly more heat and give wind more joints to find. On a sheltered suburban garden you will not notice. On a hilltop or the coast, a lift-and-slide door seals harder.
Glass this large must be safety glass. That is standard, and the glazing certificate should say so.
Cost
Prices overlap and depend more on the system and the pane sizes than on the type. Very large sliding panes cost more to make, deliver and lift into place. A plain three-panel bifold is often the cheaper way to fill a 3 m opening; beyond 5 m, sliders tend to compare better. We do not print a per-metre figure because we have not found an independent source for one. Get both priced by the same firm.
Quick guide
| What matters most | Better choice |
|---|---|
| The view on a wet Tuesday | Sliding |
| Opening the whole wall for a party | Bifold |
| A small room | Sliding |
| An opening under 3 m | Bifold or French doors |
| An opening over 5 m | Sliding |
| An exposed, windy site | Lift-and-slide |
| Tightest budget | French doors with side panes |
What to ask for on the quote
Ask for the frame material and system name, the pane sizes, the U-value of the whole door, the threshold type, the colour inside and out, and who measures the opening. The last matters: doors are made to the millimetre, and the firm that measures should be the firm that carries the cost if they do not fit.
Doors are one line of a larger price. To see what the whole room comes to, a garden room extension of 20 m² is a useful yardstick, and the cost calculator will size your own.