Building a nestbox for Barn Owls isn't hard. Just make sure there's enough room inside for lots of nestlings. A good plan is at http://fw.ky.gov/pdf/barnowlboxes2010.pdf The Barn Owl (along with the Great-horned Owl) feasts on rodents - up to 5 per night.
A group of owls is called a "parliament". Also, owls have roughly twice as many bones in their necks as humans, so they can rotate their heads 270 degrees, a valuable tool when hunting. A typical adult Barn Owl can eat about 1500 rats a year, in exchange for very inexpensive housing.