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Individual bats occasionally enter buildings accidentally, particularly during the spring and fall as they move between roosts. However, groups of bats may also establish colonies in houses or other buildings. Bats are nocturnal animals. They may be found in dark areas such as attics, basements, closets or unlit shelters. Seeing a bat outdoors during the day is unusual and could be a sign that it is ill.
A simple way to tell if your house has bats is, at dusk, to watch for bats flying out of your house. If you see bats, note where they are leaving the house. They will return to the same openings at dawn. Bats can be kept out of buildings by closing openings that allow them entry. The only permanent way to get rid of a bat colony is to exclude them from the building by plugging their entrance holes (bat proofing). After you have established where the bats are exiting and entering the building, seal all openings (as small as 1/4" wide) the bats are using.
Bats play an important role in our ecosystem. For example the average bat is estimated to eat its weight in insects every night. It is neither practical nor desirable to eliminate bats from the environment. In Los Angeles County, bats are the most common source of rabies and avoiding bats is essential to reducing the risk of rabies in humans. Bats should not be picked up or otherwise handled.
There are various pest control businesses that are able to batproof your house and several "how to" booklets.
See websites:
Batproofing - Rabies Prevention in Washington State: A Guide for Practitioners
