Building a house on stilts.

I'm not an expert but have you tried placing your pillars and then placing a foundation atop it? It should create a flooring where you can then use the wall tool.

I'm currently building a house with large floating sections on pillars. At first I had to build a room below so I could build something on the right level, but after that anything below can be deleted or changed however you like. There appears to be no structural constraints regarding the walls/rooms below, I've just added pillars where I think they look right.

Thank you two for the responses. I never thought of those solutions. <3

Easy solution for anyone googling for answers: it's a built in foundation option. Build a room with normal walls, go back and enter the foundation options. There are options for stilt foundations, and the ability to grab and raise/lower the height of the foundation. This method allows you to build a house on stilts over water or on dry land.