What, if anything, defines how far off grade a house keeping pad may or should be? How about how far that pad extends beyond the equipment. I'm thinking of concrete pads on grade - outdoors. Not inside.
Your question is very general. If the pad is for POCO owned xfrmrs ahead of the meter, it's under their construction standards and not subject to the NEC. Of course you'll have to take into account flood, seismic, etc.
If the pad is on the load side of the meter in an industrial facility for example, you're under the NEC. You'll still need to comply with flood, seismic, etc. and IMO you'll need to provide the same clearances required for equipment whether it's inside or outside. Can someone stand on grade and work on the equipment or is there going to be landscaping all around it? If it's going to be hidden by landscaping, I'd design it to have the required clear working spaces on the concrete.