A lot of fantasy RPG supplements include stable layouts. How realistic are they?
As an example, the layout of the barn I ride at is fairly typical for a modern barn. It's designed with the indoor arena in the center, a common design in climates where you may not want to go outside. On either side is a stable "aisle" with stalls on the out side against the wall. At each end are rooms on the inside next to the arena. At the far end, they're stalls. At the end closest to the entrance you find a tack room. The barn has doors at either end of the aisle and also doors in the middle. Barns tend to have a lot of doors so you can evacuate quickly in case of fire. Next to each aisle door is a small room that can be used for storage - one of these rooms is used to keep tools for maintaining the barn. On the left hand side of the entrance is a studio apartment with a bathroom, shower and kitchen (there's no hot water, however - the barn used to have a hot water boiler but it failed years ago and was never replaced because of fire concerned). The kitchen is used to store spare tack and to keep rugs in the winter, whilst the apartment is now a private office - I assume somebody lived there once, but no more. On the other side is the public office, the feed room, and men's and women's rest rooms. A private barn might only have one bathroom. Hay is kept outside in an old semi trailer, but the barn is equipped with hay lofts.
It's worth noting that hay lofts, often above the stalls, were normal in barn layouts until the last thirty years go, when we finally grasped that storing hay (which can spontaneously combust) in the same building you keep the horses in is a rather stupid idea.
Another common design is stalls around a courtyard with an outdoor arena to one side. In America, aisle barns of various sizes with stalls and a tack and feed room are usual for small private stables. In Britain, it's more common to have a row of stalls that open onto the yard, with an overhang.
So, what about Medieval stables?
Many don't survive - because they were converted to garages or housing when horses ceased to be a common means of transport. We do know that both the aisle and courtyard model were used. Taverns would probably have used the courtyard style. Also, many modern stables hold only riding horses - older stables would also have had a carriage house, which was often an open structure along one side of the courtyard, so that horses could be backed straight between the shafts, hooked up, and then pull out. Others were closed structures more like a modern garage.
If doing a scene in a stable bear in mind, then, that there are elements that would be present in a convenient configuration:
1. Stalls for horses, which might be box stalls (loose boxes in the UK) or standing stalls (tie stalls in the UK), positioned so that horses can easily be moved in and out, either by facing the outside of the building or by being along a broad aisle.
2. Some place to store carriages, vehicles or farm equipment, often open on one side to make it easier to maneuver things in and out.
3. A tack or harness room. In larger barns expect to find more than one. Saddles are heavy and nobody wants to carry them further than they need to. (As demonstrated many times in barns by people parking a horse right outside the tack room to unsaddle).
4. Some place to keep hay, traditionally a loft above the stalls. Some stables were designed with trap doors above a manger in each stall so a stable hand could just open the trapdoor and toss the hay down to the horse. Modern hay lofts have generally been removed from use and hay storage is generally removed to a separate building.
5. A feed room, which would contain solid feed "bins" - metal if the culture has it - designed to exclude rodents. Modern feed rooms often have a fridge to hold veterinary supplies and equine first aid kit.
6. In large barns, some kind of office, lounge, rec room, etc - some place for grooms to go take their breaks. Caring for horses often involves starting early, finishing late, but having long gaps with not much to do in the middle. Grooms might be found playing cards or shooting bull in some kind of room, and they might also have food there, although it would be carefully kept because of those rodents. (Oh, and expect a cat or two).
A horse in a box stall in an aisle barn. Note that the walls of the stall stop well before the barn's high ceiling and that they turn into bars at about four feet. This is a normal design for aisle barns - horses like a lot of air circulation.