Evaluating the Multi-dimensional Impacts of the 10% Biodiversity Net Gain Requirement: A Methodological Proposal for Public Administrations

Biodiversity Net Gain (BNG) has been introduced in England as a mandatory planning requirement to reverse biodiversity loss and mobilize private investment in nature recovery. From February 2024, most new developments are required to deliver a measurable improvement of at least 10% in biodiversity value. With the start of BNG implementation, positive and negative externalities are emerging and since it is still in its early implementation phase, no widely validated academic framework currently exists that comprehensively evaluates its multi-dimensional impacts. In this context, we propose using the COSIMA (Composite SIgnificance and Multicriteria Assessment) framework as a tool for a holistic ex-ante evaluation of BNG projects. COSIMA integrates traditional cost-benefit analysis (CBA) with multicriteria decision analysis (MCDA) to generate a composite index called the Total Rate of Return (TRR). In this context, this framework ensures that the priorities of developers, government agencies, and local communities are reflected in the final evaluation by including a variety of criteria, both monetizable and non-monetizable. The framework was developed through a structured three-phase methodology combining an exploratory and a systematic scoping review, a stakeholder analysis, and a structured expert validation procedure involving multi-domain specialists to finalize a set of impact indicators spanning environmental, economic, social, and governance dimensions. COSIMA enables transparent, evidence-based comparison of alternative scenarios and provides public administrators and policymakers with a flexible tool to support stakeholder dialogue and optimize the BNG policy.