PS - reading some of your other replies, I have two additional thoughts:
1- If this is going to double the space your birds have, then maybe it is worthwhile - this isn't the case for mine as they have a larger covered area to roam in and the run is within that space.
2 - I forgot about the...
I'm not sure if this is the answer you're looking for, but I'll put my two cents in - we have an under coop run about 18 inches high and it is a nightmare. Our hens and ducks often go under there and won't come out when it's time to lock them up at night. This is a headache and, as a result, I...