Bats can get inside the house through a number of different ways. Most people that discover a bat inside the house actually have a bat colony living inside the attic space. The bat get into the living quarters by accident. They can gain entry by crawling down the walls into an unfinished section of the basement. Additionally, they can get into the house by squeezing through gaps in the attic hatch, recessed lighting, or cold air returns.
During the summer months, in particular August, there are a large number of bats flying outside. In fact juvenile bats are just learning to fly at this time. This is the period of time when a bat may accidentally fly through the front door of your home.