Plans are all over the internet. What I did was to BUY a bat house from a bird store and copied it many times using reverse engineering (disassembly). The secret is not the house itself, but the placement of it to get very warm during the day so that the thermal mass keeps the critters warm at night.
I have lots of bats but bluebirds and swallows are better for mosquito reduction, and less scary to deal with. The twitter of a bluebird is very recognizable. Same for barn swallows. More friendly, too. My whole back 10 acres of hay fields is now a bluebird and swallow society. Houses are 30 feet apart and alternate (by their choice) bird type.