I just discovered The Animal Farm Research which is a coding blog containing a lot of information on procedural content generation (and coding in general).
Quote from Gamedev:
The screenshots are part of my research. The buildings models are generated 100% procedurally according to a shape grammar specified by Wonka and Mueller (with som extensions/modifications), which is similar to an L-System but acts on building components instead of performing string rewrites. The grammars are capable of a lot more, but unfortunately my artistic talents and architectural knowledge are somewhat limited.
There's a data mining component in the background which can refine the grammar according to user likes/dislikes, but it's hard to illustrate that here.
My next steps are to extend shape grammars to generate random dungeons, act as string rewrite systems (ala L-Systems), and generate certain classes of textures. I'm also working on generating an entire building in the geometry shader. Some of this is detailed on my research site, but I admittedly do not update that as much as I should.