IGE hover height given in the FM for civil helicopters is based on a takeoff technique (constant collective), and done over a smooth flat surface.
Asking the effect of other surfaces opens a can of worms similar to light fixed wing aircraft using wet grass runways, or soft muddy ones and trying to determine takeoff distance from the FM chart. No one has the answer, and the list of variables is quite long.
Anything that waves, like long grass or trees can soak up downwash and kill performance - but by how much? Wish I knew.