Can be achieved by placing different Teahouse style furniture pieces, it doesn't matter what type (can be kitchen, chairs, art) as long as it has the Teahouse indicator in the game.
More Teahouse furniture will unlock while researching different things at the decoration shrine and you will also be able to unlock some Teahouse Furniture via friendship levels.
The game will state if you already have 1 of an item and that the stats won't go up, i.e.; have 2 of the same Teahouse furniture of Teahouse: 2 stat, is not 4 stat, it's 2 stat.