The Mountain Cottage is a house that can be obtained by being married for 50 years in the game. On your 50th anniversary, your spouse will present this cottage to you as a gift. It features wood-themed furnishings, including a clock, chimney, bed, diary and calendar. The house is located north of the Kappa Pond, and south of Mother's Hill.
A Jewel of Truth can be found in the calendar if you go investigate it.
Like all the other homes that aren't your primary farmhouse, you cannot upgrade or expand the Mountain Cottage.
Despite being an anniversary gift, your spouse will not spend time with you in the cottage.