If you have 12 stalls and only want to use 4 for horses, I would make two of the stalls double 12x24 stalls. Both my horses live in double stalls and they are much happier being able to walk around more inside. What about using a stall for one month's worth of hay storage?
My farrier told me to design my place so a non-horsey person could come in and be able to feed my horses without having to handle them, in case of an emergency. It was great advice, and I've had to enlist inexperienced help just once, but what a relief it was when I needed it. My stalls open to 40 foot runs, which open to the pasture. Gives me all kinds of flexibility.
Automatic wateres are fantastic, too. (I have Nelsons.)
And lots of gravel around the barn and from my back yard fence to the barn, so I don't have to deal with mud in the spring. Try to grade the area around your barn for good drainage. It's worth the money.
I can store a year's worth of hay in a separate structure that's next to the barn, and it's great to only have to get hay once a year.
I built my barn with skylights and huge doors on either end for great ventilation, so I never get an ammonia smell. It helps that I leave the back stall doors open all the time, too.
I second designing the place so non horse people can feed. My best friend boards her horse at our place and she and her fiance stay here when we go out of town. Occasionally her ever so non horsey fiance feeds if she'll be home late. "The one with the line on his face won't go in his cage" was texted to her one time...
We have out place set up so that if the barn door is opened, the horses can just walk into their stalls and then the process reversed when they're done eating. We have a lean to off the back of the barn with a hay feeder under it. The main water trough is just outside the lean to. There is a second one that is in the left pasture (the only one the hose will reach...) so we can lock them in there if we need to move equipment around.
All of our pastures (3) open off of the sacrifice lot that surrounds the barn. They always have access to the sacrifice lot and our riding lot. The riding lot is actually what they walk out into to the right of the barn and then the sacrifice lot is behind the barn. They can be locked on either lot if we need to.
I am married and my husband is horsey and does have a horse, but the vast majority of the horse care falls on me just due to the nature of our jobs. I needed to be able to take care of them efficiently and easily by myself for days at a time.
My most important requirement is, if you have stalls, that they have a back door or opening to runs outside.
You can keep a horse shut in the stall for any imaginable reason to do so, but in general, most horses are immensely more happy and relaxed if they can come and go, even if outside is only a little bitty run.
That also helps many horses stay cleaner, they find a designate bathroom and use it, or you can even housebreak them to whatever spot is the most handy for you, as we do here.
Cleaning after stalled horses any more than necessary is a real waste of time.
Saving labor where you can will give you more of your barn time to interact with horses.
A second requirement is, if building a barn, that it be if possible a center aisle, so we have a good place to work out of the rain.
If that is not possible, try to make some spot as weather proof as possible, out of the weather, rain and wind and snow.
Have a horse friendly three sided, roofed spot, oriented in the best direction for that.
Definitely climate makes a difference. Everyone here raves about runs off of the stalls but that is a no-go in our climate. Those outside doors invariably create draft or freeze open or closed, definitely something I would avoid in a cold climate, you never see those here. What people do instead is have fan systems to exchange air rather than open doors and windows.
-wash stall with hot and cold water
-stalls that can have horses heads out or closed in
-foaling stall (or ability to make one stall into a larger stall)
-Separate tack and feed rooms
-washer/dryer in tack room
-room to park tractor in barn if necessary
-Hay storage next to or in barn
-option to completely close in barn, or have large open doors in nice weather
-running hot and cold water somewhere in barn
The thing I would spend my money on is heated auto waterers in the stalls and in the paddocks. Saves so much time and effort, unless of course you live someplace warm. Also a good manure storage area and a plan to dispose of it, such as spreading, getting it picked up.
Finally a few nice sized sacrifice paddocks with level screen footing, that open into your fields.
Lots of electrical outlets. My current 6 stall barn has a great tack room, feed room, nice stalls and two outlets - that's right two TOTAL plugs and they are right in the center of the barn, which really turns into a pain in the bum when I put up fans, clip, plug-in tank heaters, etc. I'm in the process of adding outlets, but it's certainly something that should have been addressed at the beginning. Also, carefully plan where your water is - it can also be a pain if not strategically located. Best of luck with your new barn!
For a small number of horses, the tack and feed room can be the same room. Add a utility room because the broom, muck rakes, ect. smell and don't need the AC like the tack and food.
In our tack/food room, we have a small window AC, small fridge, hot water heater, deep sink, stereo, and other outlets to charge clippers. So I'd agree with having good electrical. We dump our feed in plastic storage bins and keep the feed buckets in plastic storage bins, so you really can't smell the food in there.
The hay is stored separately, and we only keep a bale or two in the barn at a time. The 1-2 bales of hay is/are not in the tack room, they are kept in the utility which is actually an extra stall.
We also have skylights in barn and I think this cuts down on electricity use.
Back to electricity since you haven't wired yet; we have both indoor and outdoor speakers connected to the stereo in the tack room; it's really nice and not expensive. Good luck with your project!
Depending on where you live, one of my first calculations would be:
a) cost of land per acre
b) value of my time per hour
c) amount of time it takes to remove manue on the acreage you're considering
d) cost of hay, delivery, storage and time spent feeding it
e) number of horses
f) annual rainfall in your area and quality of soil for grass growth
You may find it worth the money to buy more land which requires less poop picking and hay throwing.
For example, keeping 2 horses on 1 acre means daily poop picking and probably 2x daily hay feeds at least, year round.
Keeping those same 2 horses on 10 acres crossfenced into 2-3 pastures likely only means rotating the pastures every month, and hay fed at a lower rate/for fewer months of the year.
Keeping 12 horses on your 10 acres = lots of maintenance work. Don't fill up that barn unless you're ready for that!
Apparenly people in my area have done the math and can't afford more space, because horses are commonly stuffed in small pens with tiny turnouts. But that's CA land prices for you.