I'm raising the coop where the floor will be 24 inches of the ground so that will give the full 16' by 20' run. I thought that the 100 sq ft under the coop would add some shade. plus I plan to add some removable wind breaks around the north and west sides to give them some protection from the wind
My coop is smaller 5 feet by just under 5 feet and a little over 24 inches elevated - it can be a back breaker to rake/clean under that. And if you have to catch a chicken under there ,you get to lay in the area
If possible you may want to give it a little more height to ease those chores?
But they love the under the coop part of the run - a safe place from sun/rain/winds. And often a favorite place to dust bathe.