Download the file called SmallBuilding03.zip and unzip it to your hard drive, then double-click the HTML file to play the video in your browser.  

The walls of the building have been created the size of the floor and they overlap with the corners resulting in coincident faces which can cause problems when applying materials.  

This video shows how to clean the corners up with the Edit Mesh modifier by creating a miter situation.  

What we are achieving with this process is flexibility where each wall can be edited at sub object levels or in either modifier level.  The walls are both efficient and flexible; two goals that will increase productivity. 

Good luck and have fun
Ted