While the conference will be held on the premises of the ZiF, its campus does not include a guest house. Bielefeld offers many possible accommodations, but at the beginning of December hotels easily tend to be full. I strongly suggest that you book your hotel in advance.
The ZiF has negotiated reduced rates with agreements with a few hotels, but one has to book early and the booking cannot be subsequently cancelled. Partner hotels have rates of approx. 65, 75, or 80 €/night in single room.
Additionally, the ZiF offers on its campus some "studios". For these studios the ZiF charges no money at all. These studios will be reserved for junior researchers with limited resources.