From the Super Mario Wiki
9-Volt's House is a house in the WarioWare series of games and the residence of 9-Volt and 5-Volt. 9 and 18-Volt both play their latest games in this house during the events of WarioWare: Touched!. Since 9-Volt loves video games, his house is scattered with video game consoles from the past and present. 9-Volt's house is also in WarioWare: Twisted!. In this game 9-Volt invites 18-Volt to his house to play some NES. In WarioWare: Smooth Moves, 9-Volt again invites 18-Volt to his house, but this time to play a Game & Watch. However 18-Volt breaks it and they get into a fight.