No, no. There was no slab poured in error. The barn's shape was changed.
The barn is carved into a hillside. The animals were supposed to be in a portion that jutted out over soil. Instead, nothing juts out and the entire second floor is sitting on the first floor.
If what you're suggesting is to put the animals where I now have the store room, it won't work. They'd be on the wrong side of the barn, as far from the pasture as they could be. Moreover, the foundation has already been poured, and the foundation includes cuts where doors will be. I can't have cows entering through doors designed for humans. And plumbing has already been started. Ripping out the slab would destroy it.
It's a significant problem. I think the best solution is to enclose the lean-to that was going to be on the side of the barn opposite of the store room, stemming off of the barn's roof. I've seen barn extensions like that many times. It would look a little like THIS - if you could imagine the lean-to being ten feet higher, built upon a hillside, with its roof extending out from the barn's upper roof:
| 
