Dark areas appear after extruding Novice in Blender, sorry:)

Why do I have this view of chimney on the house? 

I've extruded the hollow region. On the video tutorial that I followed, everything is ok with chimney view

What may be the problem?
That looks to me like the normals are inside-out.  The first thing I'd try is to select the face then ALT-N.  But then I'm a beginner, so after that, I'd be lost.