“Habitat: The Barn Owl’s favorite habitat consists of an open low-lying area, preferably below 750-ft. elevation, with an abundant vole population. Although the owl has an extensive range it is uncommon higher in the mountains. Grasslands in areas of milder climates with old buildings or hollow trees for perching and nesting may support large concentrations. The owl also avoids areas with low mean January temperatures, densely forested and intensively cultivated areas.”
http://www.owling.com/Barn_nh.htm

If you get the barn owl box, here is some good info:
“Recommended Site Characteristics for Barn Owl Nest Box Placement:

At least 12 ha of contiguous, good quality hunting habitat (pastures, hay fields, fields that have been fallow for at least 3 years, grassy marshlands) available within 1 km radius of the nest box site (about 5% of the land area within this radius). Within a 10 km radius an additional 100 ha of good hunting habitat for dispersing birds is preferable.
Several natural nesting and roosting opportunities (e.g. hollow trees, old barns, old bridges) within 3 km radius.
Dense forest should not exceed 50% of the habitat in a 3 km radius, to decrease risk of Great Horned Owl predation.
Within 30 km of the Lake Erie shoreline, to benefit from moderating lake effect.
A low density of well-traveled paved roads (e.g. provincial highways, busy regional roads) within 1 km radius.
Rodenticides are not used in and around buildings and nearby orchards and fields.
A reasonable expectation that the amount of foraging habitat will not decline over the next 5 years.
Presence of corn crib or hay lofts on the farm, which may sustain rodent populations and provide foraging opportunities during heavy snowfall or prolonged periods of significant snow cover on ground.
Notes on Barn Owl Nest Box Placement:

Boxes should be located in or on farm structures (i.e. large wooden barn, unused concrete silo, abandoned tobacco kilns) so that birds have unrestricted, year-round access. Avoid erecting nesting boxes on trees or telephone posts due to high predation risks (unless providing a predator baffle).
Preferably there should be little human disturbance around the nest box, although barn owls will nest in barns being used for regular and routine farm activities.
Boxes should be fastened on the outside or inside of farm structures at least 6 metres (18 feet) off the ground, preferably with the entrance facing directly to the outside and towards hunting habitat.
If affixed to the inside of a barn, a 6″ by 6″ hole should be cut in the side of the barn and the nest box attached so that birds fly directly into the box from outside.
If a box is placed on the exterior face of an existing wooden barn, it should be fastened about 1 metre (3 feet) below the eave. This will give some protection from the weather but not interfere with the birds coming and going. Place the box so that the nest exit faces towards hunting habitat.”
http://www.bsc-eoc.org/regional/barnowlbox.html