Detnome, you furnish the requirements quite accurately and then come to an odd conclusion.
There is a requirement for sufficient space for the continued take-off following an engine failure after TDP but that space does not have to be on the surface (or even within the bounds of the heliport).
It has to be above the level of the obstructions in the departure direction regardless if the heliport is on the surface or elevated.
That might be an issue for the AW189 with a TDP at 110ft and a dropdown just less than that, but not for other helicopter types which have variable TDPs.
Jim
Last edited by JimL; 15th Dec 2019 at 07:43.