The options you will have available to you will depend on your climate, landscape and budget. If you are building a brand new barn, your first order of business will be choosing the site. Choosing a site poorly can cause a great deal of headache and expense, so don’t make assumptions – research the matter or consult professionals in your area about the kind of problems that you might encounter on your land that you might not have thought about. There are, however, a few basic principles that you can begin with.

The basics are simple. The barn should be chosen on the section of land that is of the highest elevation possible. This will reduce the likelihood of water problems, and the expense of a dedicated drainage system. Also, the barn should have basic utilities, such as water and electricity. Make sure that your chosen site doesn’t hinder accessibility. The sheep should have easy access to the pasture and you should be able to easily access the barn itself with equipment for the delivery of feed or for cleaning of manure.
If your climate is mild, then you should consider the option of building sheep housing that has three walls instead of four. Three-sided shelter can provide sufficient protection from precipitation and wind while eliminating all concerns regarding ventilation. The most common three-side barn structure is a “hoop house”, which looks like a half-cylinder, resembling military hangars of the olden days. The structure is nearly identical to those, with a metal frame arching from one end to another. The frame can be covered with a variety of materials, ranging from a thick tarp-like fabric, which is the least expensive option, to fiberglass or metal. Choice of roofing will largely depend on your budget, but keep in mind that the fabric option will last about 13-15 years before it needs replacing. An important thing to note when building a hoop house, is that it is vitally important to orient it away from the prevailing wind to maximise the comfort inside the enclosure and minimise stress on the structure itself.

If a full enclosure is necessary then
you may consider a standard barn structure that can be wood or metal. Those
represent a large investment but will likely outlast you, so come with some
peace of mind. But because they are expensive to build, they need to be planned
appropriately to make sure you have sufficient space, maximal accessibility and
optimal ventilation.
Ventilation is key here. When we say that the barn is warmer and drier, that is
always relative to the outside. The barn doesn’t need to be heated. Sheep are
very resilient to cold, including newborn lambs, so “warm” to them is not a
terribly high standard. So ventilating the barn doesn’t come with a concern
about energy efficiency, as long as you don’t install additional ventilation
equipment. Such equipment is rarely needed, usually only becoming necessary in
structures that are being repurposed into barns. Purpose-built barns can be designed from the beginning to be sufficiently
ventilated naturally.