Look for dead space under stairs and in deep walls. There is often room under a staircase for a small closet, powder room, or even built-in shelves for cataloging books, files and other knickknacks that create clutter.
Mike and Greg used chunks of urbanite and old bricks, and then gravel on top of that. After some tamping the base is ready for the sub floor mix! The mix they used for the sub floor was 4 parts sand to 1 parts clay slip, made from the local clay subsoil.