question about expandiing the hull

So I want to expand my hull.  I made some sort of "outline" of where I wanted it built, then used the building menu and managed to turn the design into yellow squares.  Does this mean it's queued?  When will the colonists start building it?  Also, how do I access the new area once built?  I saw several door designs but am not sure how to replace existing walls/hull with a door.  Thanks!