The article explores the hidden costs and challenges associated with multi-use marine spaces, particularly in the context of the EU-funded UNITED project. Multi-use, which involves co-locating multiple activities such as renewable energy production, aquaculture, and tourism, aims to optimize resource use and space efficiency. However, it faces significant transaction costs, including legal, insurance, health and safety, and governance issues. These costs can deter investment and hinder the successful implementation of multi-use projects. The study analyzes five pilot projects across Europe to understand these costs and proposes solutions to reduce them. Key findings include the importance of comprehensive risk management, improved governance frameworks, and better insurance policies. The article emphasizes the need for a supportive regulatory environment, stakeholder engagement, and internal measures to mitigate transaction costs, ultimately promoting the sustainable development of multi-use marine spaces.The article explores the hidden costs and challenges associated with multi-use marine spaces, particularly in the context of the EU-funded UNITED project. Multi-use, which involves co-locating multiple activities such as renewable energy production, aquaculture, and tourism, aims to optimize resource use and space efficiency. However, it faces significant transaction costs, including legal, insurance, health and safety, and governance issues. These costs can deter investment and hinder the successful implementation of multi-use projects. The study analyzes five pilot projects across Europe to understand these costs and proposes solutions to reduce them. Key findings include the importance of comprehensive risk management, improved governance frameworks, and better insurance policies. The article emphasizes the need for a supportive regulatory environment, stakeholder engagement, and internal measures to mitigate transaction costs, ultimately promoting the sustainable development of multi-use marine spaces.