It's debatable. And the 20% you quote is only in a few selected and specific places; the vast majority of the country is flat when it comes to house inflation. And Berlin was always going to rise given it's only 20 years old in it's current guise and it is receiving heavy investment in...
Where did you get these figures from? Your figure of 23% seems to be well over the top of my understanding of prices. I could not see reference to it in the link you provided.
http://www.reuters.com/article/2012/05/08/germany-property-bubble-idUSL6E8EM3KI20120508