Plans provide information about how multiple decisions are structured over time, and what the intentions of a particular actor are. As and when these decisions get made, or not made, some parts of plans become irrelevant, while some other new relationships are discovered and considered. Recognising this provides a useful way to interpret the changes in plans by tracking the decisions and vice versa. In this paper, we illustrate the complexities of an ontology of urban systems which are needed to ensure the currency of plans, so that they could be effectively used in urban decision making. Especially, when actors are numerous, jurisdictions overlap, actions are interdependent and interests are unstable, this framework enables us to think about plans in a complex and changing urban environment and make them so that they remain useful.