This chapter explores impact assessments (IAs) used for designating Marine Conservation Zones. It starts with an introduction to IAs, leading to a theoretical discussion of cost-benefit analysis as a tool for rationalising regulation. It follows with a critical review of IAs produced by the regional stakeholder groups using umbrella questions derived from the theoretical analysis and then it asks if and how the formal IAs produced by the government for the designation of the three tranches of Marine Conservation Zones constitute an improvement compared to the regional stakeholder groups’ IAs before offering concluding remarks. Throughout the chapter, connections with the concept of commoning are made arguing that IAs for MCZs, by employing economic language in decision-making and focusing on industry costs over benefits, favour the voices of a few over collective ones, highlight interests at the expenses of values and undermine the potential for ethical consideration to play a role in assessment, thereby failing to encourage a shared ethics of care and responsibility towards the marine environment, hence not favouring commoning practices.