Arranging the distribution of data, objects or components is a critical task that can ultimately affect the performance, integrity and reliability of distributed system. This paper suggests to write down what must (not) be available where in order to reveal conflicting distribution requirements and to detect problems early on. Distribution requirements are expressed via a new notion of constraints: a context-based constraint (CoCon) can indirectly select its constrained elements according to their context. The context of an element characterizes the situation in which this element resides and is annotated via metadata. CoCons facilitate checking the compliance of a system or a model with distribution requirements during (re-)design, during (re-)configuration or at runtime. This paper focuses on validating UML models for compliance with distribution CoCons in order to take distribution requirements into account right from start of the development process.