Low compliance with mandatory ecological mitigations in English housing developments – an overlooked threat to the integrity of onsite biodiversity net gain

Globally, biodiversity no net loss or net gain (BNG) policies are potential key solutions to urbanisation’s ecological impacts. The UK’s 2012 National Planning Policy Framework (NPPF) required developers to achieve BNG and mitigate losses to protected species. Under the NPFF, and subsequent 2024 metricised BNG policy, ecological enhancement and mitigation features are a condition of planning consents, or mandated by binding legal agreements. We assess delivery of ∼6,000 individual habitat (e.g. grassland creation) and species-based features (e.g. bird boxes) within 42 major housing developments, across five English Local Planning Authorities (LPAs) that obtained planning permission under the 2012 NPFF. We quantify compliance rates and use generalised linear mixed models to assess if delivery varies with year of planning permission, number of houses in a development, and developer type (local, regional, national, other). 54% of conditioned ecological features were delivered, varying from 0% (invertebrate boxes) to 75% (pond creation). The number of ecological features conditioned per house increased slightly over time, but compliance rates were largely invariant over time, development size, and developer type. Low-effort features were less commonly conditioned, especially in larger developments. There is a serious policy-implementation gap between conditioning of ecological improvements and delivery. Stronger governance and enforcement systems are urgently required to ensure delivery of BNG practices, otherwise biodiversity declines associated with increasing UK house building will continue. BNG style policies currently being developed in 100 countries must ensure they pay sufficient attention to the monitoring and enforcement of outcomes when designing governance schemes.