Smoothing only selected areas

Been through lots of tutorials and I’m slowly improving but can’t find anything for this. I want to smooth just certain parts of my mesh, but the subdivide and other tools only seem to smooth an entire object. Is there a way to just select certain faces and have those smoothed? What I’m trying to do right now is to extrude a thin section from a building and make it into a pipe/drainpipe. I can extrude it ok but I don’t know how to smooth it into a rounder section.

With the desired selection in 3d View and in Edit Mode press the W-Key and in the menu that appears select Smooth

You can also do this via the Tool Shelf by clicking Smooth Vertex