Ex-ante impact assessment of agricultural, environmental, and rural policies has become an integral part of political decision-making processes in the European Union. While a variety of agri-environmental modelling tools exists, ex-ante policy assessment tools capturing the institutional dimension are rare and need to be improved. In this paper, we introduce a standardized procedure for ex-ante modelling institutional aspects for policy implementation: the Procedure for Institutional Compatibility Assessment(PICA). PICA has been designed as an explorative and flexible, yet formalized methodology that enables policy-makers to identify, at an early stage, potential institutional incompatibilities. After relating PICA to relevant approaches for policy assessment, we elaborate on its four distinct steps, use a core element of the EU Nitrate Directive to illustrate its function, and then provide model validation by means of a test case.