Barns are indeed a nice thing to have when raising alpacas, especially in extreme weather conditions.  You can set up heat for the winter and air conditioning in the summer.  However, you do not necessarily need a barn if you are in an area that is not too extreme in weather conditions.  Three-sided shelters work great for alpacas too.  If using three-sided shelters, you will want them big enough to fit several alpacas in at a time.  We find a twelve by twelve size can fit four or five alpacas comfortably.  The alpacas like to stay together, so setting up several twelve by twelve shelters next to each other works great for larger herds.  We like the shelters for the alpacas because we can divide our land up easier, with adult males, junior males, females, and a quarantine area, so that all the alpacas have shelters in their areas.  The opening should be wide with no obstructions.  Alpacas can spook easily and occasionally a fight breaks out between the herd members.  A wide opening gives them a chance to flee out of the shelter easily without getting hurt.

An ideal place would be to have both barn and many shelters.