Instructions
Attract a Rashberry pinata by having three gooseberries and two chilies present in your garden. Make the Rashberry a resident by having it eat two rotten gooseberries and two rotten chilies.
Attract a Swanana pinata by having 200 square pinometers of water and a garden worth 20,000 chocolate coins. Make the Swanana a resident by having 250 square pinometers of water and feeding the pinata two sandwiches.
Fulfill the romance requirements of both the Rashberry and Swanana. The Rashberry's requirements include: eating one rotten hazelnut, eating one rotten chili, eating one rotten gooseberry, and having a Rashberry house in the garden. The Swanana's requirements include: 300 square pinometers of water, a swan fountain, a diamond choker accessory, eating three water lillies, and having a Swanana house in the garden.
Access Willy Builder from the menu, and purchase the Mystery House from his stock of buildings.
Place the Mystery House in your garden, pay for it, then allow Willy to complete its construction. You now have a Mystery House in your garden.