Danchi is a Japanese building for public housing. They used to be quite popular in Japan, but nowadays they are artifacts from past, and are either abandoned or used by poor people.
In Shimeji Simulation, few people live in a danchi. The Tsukishima sisters and their Neighbors do. After the events of ss30, both sisters left the danchi: Sis had disappeared, and Shimeji had moved to a new housess31.