The posts cannot simply sit on the ground, I assume you will have some sort of post foundations or pier footings.
Couple of quick thoughts to ask your designer/architect:
Not sure how "professional" you are going with the addition. But if you are not going through a permitting process, I would be worried about getting a contractor and him treating your addition like a deck build-out with some walls and a roof.
Check the lateral stability. I've been on several covered porches that were "shaky". The posts were plenty strong, but not enough lateral support.
Also, you may want to design for wind uplift as a worst case scenario. Not sure where you live, but imagine hurricane or tornado force winds hitting the hill and being driven upwards on the bottom of your addition. Everything from your roof down should be strapped to the foundation. Although depending on your post foundation design, this may not be enough.