The first thing is to make sure that the house is weather proof.  If the structure cannot keep the weather out bats will not occupy it.  All wood joining surfaces should be properly caulked and its roof should properly treated to withstand the weather conditions for your particular area.  In order for the bat house to retain heat the outer surfaces should be painted a dark color.  The shade of color would depend on the average outside temperature and how exposed the bat house is to sunlight.  Bat houses should have at least a minimum of 6 hours of sunlight per day.

The type of wood used is also important.  Cedar is the most common wood used in bat house construction but any rough surfaced exterior wood will do.  The bats must be able to cling to the interior surfaces. These surfaces should have groves spaced about a 1/4 to a 1/2 inch apart.  Some manufacturers line the interior surfaces with some type of wire mesh.  The structure should be held together with galvanized screws for it to hold up.

The average sized bat house is about 2 feet tall by 1 foot wide.  The depth can vary depending on  the number of chambers it contains.  The reason for having chambers is that bats like cramped spaces and it also helps contain their body heat within the structure.  The chambers should be at least 20 inches tall and as wide as the interior space will allow And they should be no more than an inch apart.

A good bat house should have vents for proper air circulation.  The size and placement of these vents would depend on the size of the house or the plans being used to build the house.  To prevent predoators from entering, the bottom opening should only be no more than an inch wide.  Most designs feature landing areas.  Just like the interior surfaces, landing areas should be roughened or covered with wire mesh.

Whether you decide to buy or build a bat house, they need little maintenance. They should be checked on a yearly basis  for cracks or any gaps caused by weathering.  Bats like to be warm and cozy.   Caulk and  repaint the bat houses as needed during the off season when bats are not present.