What is the difference?
Bifold doors fold back in sections so almost the whole width opens; sliding doors move large panes behind fixed ones, so typically half to two-thirds of the width opens but the frames are slimmer and the views wider when closed.
Which suits a rear extension?
If you want to throw the room open to the garden in summer, bifolds. If the doors will be closed most of the year and you want the clearest view, sliders. Large sliders also suit wider openings with fewer frames.
Cost and performance
For a 3 to 4 metre opening, allow £4,000 to £12,000 supplied and fitted depending on system and glass. Both can meet Building Regulations; check the whole-door U-value and choose a low or level threshold with a drainage channel.
About this guide
Written by the Rear Extensions design team and checked against GOV.UK, Welsh and Scottish Government and Northern Ireland guidance in November 2026. It is general guidance, not legal advice; we confirm the rules for your house at the survey.