Let the rain drain away!
The drainage system must include gutters, downpipes and drains for the removal of water from the roof.
What lies beneath?
The subfloor. That’s the bit under the house if there is space above the ground. If your rental has one of these and it is enclosed then a polythene ground moisture barrier must be installed if it is reasonably practicable to do so.
Deny the damp
A ground moisture barrier sounds more technical than it is. Basically, it’s just there to block the moisture in the ground from rising into the property. It also helps to prevent moisture damage to the underfloor insulation.