If you want to add a house foundation to your project, you can convert your first floor into it.
Just lower the height of the first floor, apply different colour/textures, and put additional floors on top of it.
TIP: When editing first-floor measurements, it is also recommended to hide its ceiling.
Check out how that can be done by watching a short video. We're using the Web version on that video. However, main controllers are the same for all apps, so you won't face any issues reproducing the same steps on your device.