The default threshold clearance is 10mm which is ideal for an equal finished floor height either side of the door.
However, Local Authority Building Control states that if doors are being installed before a finished floor then there should be a minimum of 20mm underneath any door to allow for a new carpet or boards and still some ventilation underneath the sash. Therefore a threshold clearance of up to 50mm can be selected in the instance of a door without fixed frames.
If a door is coupled to a Fixed Frame and the frames are not raised with add-on’s underneath, the threshold can be adjusted up to 27mm.
If a door is coupled to a Fixed Frame and the frames are raised with add-on’s underneath, then a threshold clearance of up to 42mm can be selected.