Buy nesting boxes made specifically for barn owls. If you can't find any in stores, you'll be sure to find them online.
Build barn owl nesting boxes if you're handy and have the proper tools, such as wood, a saw, a drill and screws. Nesting boxes should be at least 16 inches deep and 12 inches wide, with an entrance hole 4 to 5 inches in diameter; a hole any larger than 6 inches in diameter makes barn owls vulnerable to great horned owls.
Buy or build more boxes than you think you'll need, as on average half the boxes you set up will be used.
Fill the boxes with 2 inches of soft wood shavings to create a warm, secure lining for the eggs.
Place the boxes at least 10 feet off the ground in safe, quiet places where the barn owls will not be disturbed. Make sure the boxes are sheltered from the wind and in the shade. Open barns and stands of trees near fields and pastures are best.
Open the boxes by removing the screws and clean the boxes out with warm water and a very mild soap after the chicks have left the nesting box. Let the box dry in the sun before putting it back up. Barn owls will not want to nest in a dirty box.