I am getting ready to change out my present HVAC system which is 20 years old. The project will involve relocating the outside compressor, which is presently in a confined space under a deck - when running the heat generated by this component can be intense. I am considering replacing the current gas/electric conventional system with a new dual-fuel system that will result in replacing the compressor with a heat pump. One contractor has suggested locating the heat pump on a small deck (which he will build) on a backyard slope. I have never seen this solution as these external units seem to usually be installed on 4-inch concrete slabs. Is there any problem in installing a heat pump on an elevated timber platform?

All a outside unit needs is a steady platform. It can be concrete,plastic A/C pad, or pressure treated timber etc.