Under Build Mode, hover your mouse over the flooring of the house pictured. It should read "Floors and Ceilings". Now, if you notice, the floor is up on a platform of sorts, hover your mouse on the edge of the platform (solid with dots along it) and it should read "Platforms, Foundations, and Decks". Click on this is and it will take you into the foundation tools. Hovering your mouse over anything in Build or Buy Modes with give you a little pop up with information. To use the foundation, deck, or platform tools, simply select them by clicking on the and then click-and-drag the size you need on the ground of your lot.
Now you can add floor covering to the top of your foundation, deck, or platform, build walls on top of it, and add stairs to the ground. Hope this helps.
