Bi-folds are often sold as a solution to the small, dark living room and a lot of the time they are. The measurements matter more in a small extension. With less room either side to absorb a mistake, there’s less room for error.
People are often caught out by the stack. When Bi-folds open, the panels fold and park at one end. That stack has to go somewhere that isn’t blocking a walkway or piece of furniture. On a small extension a 2m stack at the end of a 3m wall is most of that wall. Make sure you measure where the panels will be parked, not just the width of the opening.
When it comes to number of panels, 3 are usually a better fit for small extensions over 4 or 5. A 3-panel set gives you a better “wall of glass” look and less of a stack when closed, which is more important when they’re closed than when they’re open.
It’s worth checking threshold height before you make a final decision, particularly if you’re on a raised ground floor or there is a step down to the garden. A low threshold bi-fold door almost fits flush. In the open position, a standard threshold disrupts the flow of a small space, and is less noticeable in a large space, where your eye has more distance to wander.
Slimmer aluminum frames cost more than uPVC frames, but for a small space, every centimeter of frame equals less glass. uPVC, bi-fold doors, three panel, fitted, start around £2,000, and £4,500 will get you a low threshold aluminum bi-fold with better glazing.
It’s also worth looking at what is structurally possible before falling in love with a photo. A three-meter opening, for example, usually requires a structural lintel, and for some houses, especially the old ones with a load-bearing wall in the wrong place, that may be a bigger job than the doors. Get a builder to look at the wall before you get quotes for the doors.
See also: Natural Light & Rooflights