Marked-up drawings that tell your electrical contractor exactly what they can install, and where.
Hazardous area classification is the part of a DSEAR assessment that divides the places where an explosive atmosphere may occur in quantities requiring special precautions into zones, 0, 1 and 2 for gases and vapours and 20, 21 and 22 for dusts, according to how often it is present, and records them on marked-up drawings. The zones then dictate the ATEX category of every piece of electrical and mechanical equipment installed in them.
Hazardous area classification is a judgement about how often a flammable atmosphere can form and how far it can spread. Get it too tight and you fail an inspection. Get it too generous and you spend thousands on ATEX equipment you never needed.
Both mistakes are expensive. The second one is more common, and nobody ever tells you about it.
Plan and elevation where it matters, with zone boundaries and extents dimensioned. Issued as a PDF and as a marked-up copy of your own layout if you have one.
Release sources, grades of release, ventilation assessment and the standard applied. If an inspector asks why a boundary sits where it does, the answer is in the report.
What is currently installed in each zone, whether it is suitable, and what needs replacing. Ranked by urgency.
What your contractor needs to certify, and what records to keep, so the next inspection is a paperwork exercise rather than a shutdown.
If your site is not on that list, ring anyway. Most of the interesting work is not on anyone's list.
No charge for the conversation. Most quotes go out the same day.