The objective of this paper is to set a complete ex-ante evaluation to support Decision Makers in designing more efficient and effective agri-environmental contracts, through an integrated modelling of elements of private and public decision making. Ex-ante comparison of policy design options in terms of overall effectiveness requires both simulations of farmers' behaviour and evaluation of the farms simulations outcomes. An intermediate step is the aggregation of single farms impacts at territorial level, in order to identify the aggregate impact of each alternative. Alternatives are several contract design, based on different levels of payments. Farm level analysis is based on a real options approach including in the simulations the timing of choice and the uncertainty in the future about price and decoupled payments. Aggregate policy impact is identified through the quantification of economic, social and environmental impacts at territorial level and the weights are elicited with Multiple-Criteria Robust Interactive Decision Analysis (MCRID). Simulations in the case study show that relevant opportunities to improve policy design are available. Multicriteria Analysis is then used to aggregate impacts of many criteria, including not only effects on the environment, but also economic and social impacts.