Buildings form the core of Spaces and are quick and simple to create.
Firstly sketch the outline of the proposed building
Once you have the outline in place, make sure you are on the Building Tool:
And then use your finger to tap inside the enclosed shape.
A Building will be created based on the current Default Settings.
With the Usage Palette open you can also drag a specific Usage Type, and drop it within the enclosed shape. Again, a Building is created based on the current Default Settings along with setting the Building Usage based on the Usage Type that you dragged.