+ How do you usually start to model your buildings?
From scratch ;]
+ Do you have different work flows for different approaches?
Yes. Depends on the project.
+ Do you use spline/loft techniques, polygonal box modelling?
Yep.
+ Has someone ever used nurbs to model building and what where the experiences from that?
Yes. Experiences? Hmm... i experienced a tingling sensation in my big toe... ;] but seriously i dont quite follow you here....
+ Which work flow appears the efficientest in practice?
I think there is no right workflow. Everyone has got a workflow based on experience. The more you model the better your workflow gets. Ones workflow adapts to how fluent and fast he is with the tools at hand.
The ability to produce fast images comes ones fluency of the software if u like and the familiarity with a project he is doing (example: producing a rendering of a stadium takes more time if you havent done renderings in this kind of architectural field in the past, youll need to explore it)
+ Which tutorials on this topic do you advise?
On modeling? Hmm... cant think of any really... havent been looking for modeling tutorials in a long, long time...
Hope that helps a bit
Cheers