New ownership is limited to one plot per character. Existing second plots stay with their owners, and each existing plot can have a cellar.
At Carpentry 10, build a Cellar hatch on your own surface plot using 20 planks, 10 wood blocks and 40 stone. Its floor area matches the plot at excavation. Descend through the hatch and use the lamp-lit ladder to return. Keep the landing clear.
The current plot owner and approved builders can build downstairs. Gardens, boats, claim stakes, bridges and hitching posts belong above ground. Fires cannot be lit underground. A cellar transfers with a sold plot, including its built furniture; personal bank contents stay personal.
Empty the room and have everyone leave before dismantling its hatch. A lapsed or released plot retains its cellar and belongings, with the same loss of land protection as the surface house. Removing an empty hatch safely returns an offline visitor home on their next login.