For a flat on the 3rd floor of a 4 storey block of flats. A door inspection found a 9mm gap under one part of the threshold drop down smoke seal. The flat is new and the developers contractor came and said there was no problem as the gap could be up to 10mm. The inspection report says it should be 3mm as the door has smoke seals. The inspection report matches my understanding but am I missing something? 

The contractor is partly correct, the gap can be up to 10mm as long as when the door is closed there is a drop-down seal that touches the floor

BS 8214: 2016 requires that a flexible threshold seal, where fitted, seals the gap.   Alternatively, the maximum gap is 3mm.

Great, thank you for the quick responses.