The docks feature allows you to build floors outside of houses and plan floors from an adjacent floor border into open air or sloped tiles. Each planned floor over a sloped tile or open air (outside of a house) will have either a stone or wooden support extend to the ground below, or a wooden brace that will allow the floor to be supported by an adjacent wall or grounded support. Once a support has been built, a floor can be built on top of the support - which then allows you to plan another floor adjacent to that.