Fossmostua, or the main house, is a large central corridor house with two full floors. It is built of log timber with cross notches, and the walls are unclad both outside and inside. The windows are of the Empire style with a center post and a frame with three panes on either side of the post. The windows have simple framing with Swiss-style brackets supporting the drip board above the window. The main entrance in the middle of the house is a double-leaf entrance door with panels, and glass in the upper field. In the late 19th century, this was a common way to bring light into the hallway. The gable roof is covered with turf.

The house as it stands today is from 1872. At that time, a one-story cabin built by the original settlers in 1824 was taken down, and a larger house was erected approximately on the same plot. In the attic on both gable ends of the house, there are windows that differ from the others. These are small-pane Rococo windows that most probably are reuse from the first cabin on the farm. The building was used as a residence until 1959.