All the previous posts list great reasons for raising a coop but it doesn't have to be. It happened that I constructed my coop on an area formerly used as a gravel dump. The ground was so hard from settled gravel mixed with sand (think cement) that my husband had to use a pick axe to dig the footings. I am completely convinced no animal is going to burrow into that ground to dig under the walls!
So, if you cannot raise the coop then you could think about footings of tamped down gravel. Gravel that filled trenches at least 18 inches deep and several inches wider than your coop walls would be nearly impossible to dig through (use damp gravel slightly mixed with sand and tamp it down well).
We have plenty of predators (raccoon, skunk, possum, weasel, coyote) and nothing has gotten into our coop...not even a mouse, that I can tell, anyway!
Good luck--enjoy your chickens when you get them!