Adachi's house is where Adachi lives. Shimamura Hougetsu first visits Adachi's house in Chapter 2, after they meet up together to walk home only to realize they live in opposite directions from each other. Shimamura doesn't go inside this trip though. To help Shimamura get home, Adachi gives her a map so she can get back to the bookstore.
The map Adachi made for Shimamura