now I'm no expert on horses and stables.. but I'll give some input anyhoot
no floorboards - the horses walk on dirt, save the floorboards for the people area
the corridor is to small to properly get the horses around in there - double that
the water through should be placed close to the corridor so you can fill it without entering the booth
there should also be a through for feed
stuff lying about would be grooming equipment, cast off horse shoes, salves and balms, tack, equipment for mending tack, saddles, sacks or barrels of feed a couple of foot stools to sit on.
also a dirt path would leed to the stables as no grass can survive a steady stream of horses
thats it for now