In moderate climates alpacas only require a three sided shelter. Severe cold, especially coupled with wind, requires a barn which is readily available able to be blocked off from the wind. They tend to be a bit claustrophobic when they are completely locked in, so we have shelters with at least one door on two different wall. This way one door can block out the wind and they can still access the outside.
Alpacas are hardy animals, and most prefer to be out in the open instead of being in a small enclosure. As a general rule, 1 or 2 animals may use a 12 x 16 foot (192 sq ft) three sided open faced shelter with a 12 foot roof to “loaf” in. In inclement weather, 12 to 16 alpacas will crowd in. Given a choice, they will not crowd in more than one animal per 3 x 6 (18 sq ft) foot space. They crowd in when the temperature is less than 20 F especially if it is raining or snowing. When sunny, most will remain outdoors at all times. We have found some alpacas will run inside with the first drop of rain while others will stay out in a blizzard. When they stay in a large barn, they stay anywhere from 2 to 10 feet away from each other, utilizing ½ of the barn for lying and the dung pile for the other half.
You do not need a big, fancy barn. 12' x 24' lean-tos worked for years before we built a large barn.
By the way, you can never build a big enough barn...