Carpentry 60 · The stairs’ own materials: 12 × Planks, 6 × Wooden block. A second storey is a Carpentry 60 achievement.
The whole price adds the normal recipe of every floor, wall, window and door of the new storey (an outside door becomes a window above); the build menu shows the whole price before you build.
Stand on an empty floor tile inside your enclosed, floored house on your own plot, open its action menu and choose Build, then Home, then House stairs. Turn the stairs until the floor tile behind them, their landing, is clear.
Choose Go upstairs and Go downstairs on the stairs. Furnish the upper floor as any room, with its own walls, windows, doors, beds, storage and lights; gardens, waystones, boats and other outdoor pieces stay on the ground floor, and there is no third floor.
The stairs and the walls and floors holding the upper floor stay fixed while it stands. When the plot moves, the upper storey goes with the house.