What a run-in shelter actually is
A run-in shelter — called a field shelter by most horse people in Britain — is a roofed structure with three closed sides and a fourth side open to the field, set in the pasture so horses can walk in and out whenever they choose. There is no door to fasten and nobody has to fetch them in from the field. The horse itself decides when driving rain, a bitter wind or a cloud of summer flies is worth stepping inside for, and that freedom is the entire point. It gives a grass-kept horse the protection of a building without the routine, bedding costs and confinement of stabling, and owners are often surprised how much time their horses spend in one once it is sited well — not only in the worst winter weather, but standing in the shade on still, hot afternoons when the flies are at their worst.
How much room each horse needs
For one average riding horse, a footprint of about ten feet by twelve is a fair minimum, and twelve feet square is more comfortable: enough room to turn around without scraping a hip on the wall, to lie down if it wants to, and to stand well clear of the back panel. For every additional horse expected to share, add roughly another ten feet of width rather than making the shelter deeper, because width is what the herd actually uses. Depth invites one horse to park itself across the back and own the whole building, while a long, shallow shelter with a wide open front lets a lower-ranking horse enter at one end and leave at the other without ever having to push past anyone. Headroom matters as much as floor area: allow generous clearance under the front beam, so a horse that throws its head up on the way in never meets timber, and check that smaller ponies and tall horses sharing the space can both stand without ducking.
Siting it so horses actually use it
The best-sized shelter fails if it faces the wrong way. Point the open front away from your prevailing wet-weather wind — the direction your storms actually arrive from, not the direction that looks tidy from the gate — so rain is not driven straight inside. Ground matters just as much: a shelter planted in the field's natural hollow becomes a pond with a roof, and horses will refuse it. Choose or build up a slightly raised, free-draining spot, and consider a base of hardcore or gravel under and just in front of the opening, because the entrance takes the heaviest traffic and turns to soup first. Horses also vote with their feet for shelter near where they already gather, so a position they pass daily, with a clear view out across the field, gets used far more than one hidden in a far corner.
Room for the horse at the bottom of the herd
Watch the shelter in the first spell of bad weather, not on the day it is built. If one horse is always inside and the rest stand miserably at the fence, the building is being guarded, not shared — and the answer is usually more width, a second smaller shelter in another part of the field, or a partition that creates two bays a single horse cannot hold at once. A shelter that only the boss uses leaves the horses who needed it most still standing in the rain. Once the herd settles into it willingly, note which shelter each horse uses and how the group arranges itself, because that pattern is part of the field's routine. Those details are worth recording properly: KinsLe's Family Sharing lets everyone who helps with your horses see the same shelter and turnout information on each horse's profile, so a helper arriving in a storm knows exactly where each horse will be sheltering.