Barn Owls in San Diego County nest in buildings and among the bases of palm leaves more often than in cavities in native trees or on natural cliff ledges. They also readily use nest boxes designed for them.
The best way to attract Barn Owls to your yard is to build a box and mount it 15 to 20 feet high on a pole or in a large tree. Trees are nice because they provide shade in warm weather and lots of perching branches for the fledglings.
When installing the box in a tree, use metal cables or strong rope or chain covered with hose so that it won't cut in to the tree; and wedge the box close to the main trunk so there is no swaying in wind.
The opening is best facing North-east or South-east, away from prevailing wind and weather. It is also a good idea to apply a thin layer of Ivory bar soap on the inside surface of the roof to prevent bees and wasps from colonizing the box. And avoid using rodent poison baits in the surrounding area, as they kill birds as well as other wildlife, pets and children.
A box configuration with a large doorway opening that extends down to within a few inches of the floor allows for good air circulation and observation of the owlets; and doesn't require cleaning, as the female can easily remove unwanted material through the large opening. If the box has a small opening, you can clean out the box in the fall.
Be sure to wear a good dust-mask and latex gloves when cleaning out the box as a guard against diseases that affect humans but not birds. Barn Owls don't require nesting material in the box, as the female will create a bed of shredded pellets on the floor.
It may take up to two years for Barn Owls to move in, but once they've discovered the box, it will often be used year after year. A Barn Owl box can be made from one sheet of 3/4-inch exterior plywood and some lumber stores will make all the cuts for you. To extend the life of the wood, paint the outside of the nestbox with water-based paint.