This tutorial shows how to connect two
rooms by using csg sbtract.

You should allready know how to build a basic room for this
section.  If you don't, go back to the room tutorial.  You'll want
your new room next to your existing one.  Make sure that the
doorway that will eventually exist, isn't going through vital
brushes or enities in your existing room.  Hollow out the new
addition and texture the brushes to your liking.  

  

Now create a brush between the two rooms that will be the size of the door 
way.  Make sure the brush is one unit above the floor in both rooms and
that it passes through both walls on either side.  This brush will be used
to cut a hole between both sides so it should line up just above the floor
level on both sides.  This will keep the floor brushes intact.

  

Now on to busting out some walls.