Hi, the steel doors in our building have gaps under the doors 8-9mm. Some of the steel doors have drop down seals on them but some don’t. Is a 10mm gap on steel door acceptable or should it be less?

For metal fire doors there is no standard for this so it must be as required by the particular door manufacturer. However, generally I would work on 10mm max for a fire door and where smoke protection is required seal the gap to the floor with a threshold seal/plate combination. If it is impractical to fit a threshold seal then the gap should be 3mm max.