In the Ocean Cave dungeon there is an NPC on B4F that tells you they know of a secret room, but they "don't want to tell a lot of people". Return here with only one Denpa Man in your party to be warped to a Secret Room with multiple treasure boxes. One of them contains "Room Parts". When you return to Antenna Island, your Denpa House will be upgraded (typically from 2 to 4 rooms)!