In 40 seconds
A bifold door set in the UK typically costs around £2,000–£3,500 per leaf installed, so a 3-door aluminium set usually lands in the £3,500–£7,000 range and a 4- or 5-door set runs higher again. uPVC bifolds are lower-priced than aluminium but carry chunkier frames and a shorter life, while aluminium gives slimmer sightlines and a 40–45 year lifespan. Compared with sliding doors, bifolds open up most of the aperture but show more vertical frame when closed. Every bifold install must meet building regulations for thermal performance (Part L glazing), structural support and safety glazing — and if the wall opening is widened, a structural lintel and possibly planning permission come into play. The honest answer is always a range, because it depends on your opening width, material and whether structural work is needed.
Most bifold guidance is published by companies fitting the doors, so the numbers tend to be optimistic and the building-regs side glossed over. The pages below give honest cost ranges, compare bifolds with sliding doors fairly, weigh aluminium against uPVC, set out the 3-door vs 4-door gap, and explain the building regs and planning rules — before you take a single quote.