You can create a basement or even a garage with the help of a Magic Cube tool.
This tool allows you to make all versions of cubes, with a wide variety of colours and textures. You can find it in the "Rooms" section. Add it to your project, click twice and set the desired dimensions.
Check out the video tutorial on how to build a house with the basement/garage. We're using the Web version on that video. However, main controllers are the same for all apps, so you won't have any difficulties reproducing these steps on your device.