Project Design Workflow
The work on a new project usually begins in the 2D Plan view where you draw walls on the floor plan, and then add doors, windows, etc. The tools for adding walls, floor or other parts of the building can be used only in the 2D Plan view. You can then switch to the 3D view to apply materials, set up lighting and so on.
It is recommended that you revise the project settings at the very beginning of your work to select the proper measurement units, scale and other properties.