A three-sided shed or lean-to can provide adequate shelter for a horse. A 12′ by 12′ lean-to can accommodate one to two horses. As horse numbers increase, run-in shed dimensions should increase by 12 feet per horse.
For example, if you have three to four horses, a 12′ x 24′ lean-to would be sufficient. With five to six horses, a 12′ x 48′ lean-to would be needed (or two 12’ x 24’ shelters)
Again, these recommendations assume all horses utilizing the shelter get along with each other.
When planning a horse facility and/or pasture, it is important to consider dry lot or sacrifice paddock size and shelter needs. Dry lots provide an opportunity to move horses off the pasture during winter months, wet or dry times, and/or time of needed pasture rest. Dry lots are necessary in a rotational grazing system.
Dry lots can vary in size, but they should provide a minimum of 400 square feet per horse. You also need to account for hay and grain, feeders, water sources, and shelter space. Also, 400 square feet assumes the horses in the dry lot get along with each other.
You can sign up for the University of Minnesota Horse newsletter online.