Bi-fold Doors Headroom

Hi,
We are in the middle of a renovation project, and just realised that the builders left an opening of just 2000mm (brick to lintel) to install an outward opening bi-fold door. My research on bi-fold doors suggests that we will loose some of this height to the door's threshold and hanging rails, and knowing that the bottom end of the opening sits 100mm lower than the level at which finished floors will be, does anyone have any suggestions on how to maximise the headroom? As by my calculation, we would have 1822mm of headroom only (accounting for 100mm threshold for the door to be flush against the finished floors and 78mm for overhang rails), which I believe to be way too low.

Many thanks in advance

2100mm is a standard head height for door and window openings. It can be less but things become a bit intrusive especially if you are tall.

Why was the opening set a 2m? Struggling for fall on the roof pitch?

Thank you for your reply. The low ceiling is a result of a combination of many "small" errors: the calculations were initially off by 10cm, the builders started the roof lower than they should, and used 2 inches thicker rafters than recommended by the drawings to satisfy building control; all these little errors accumulated to 20cm less than what we should have had in height. One of the options suggested was to lift the roof, however, this will be very costly. Can you think of an alternative to this radical remedial action?

Many thanks again.