Mr. Slave's house is where Mr. Slave and Big Gay Al live.
The house is red and wide with a white door and four windows. The garage, unlike most houses in South Park, is on the left side with a silver door. There is also no upstairs.
Mr. Slave gives his sidequest here to pick up his package. When you give it to him in his house, he tells you that you can now summon him in battles.