While urban planning must deal with complexity, considering planning as an activity with uncertain outcomes is challenging. This paper explores how urban planning may both proactively motivate development and adapt to change. We view such planning as co-evolutionary, and conceptualize it further by utilizing Cultural Theory, and building on a case study of infill planning in Oulu, Finland.We conclude that while the three active rationales from Cultural Theory can motivate urban development, the fourth, passive, fatalistic rationale, is also needed in order to acknowledge an uncertain future. Further, we discuss three challenges in using fatalism as a planning rationale.