Biodiversity compensation policies have emerged around the world to address the ecological harms of infrastructure expansion, but they have historically experienced weak compliance. The English government is introducing a requirement that all new infrastructure developments demonstrate they achieve a Biodiversity Net Gain (BNG). Previous research has highlighted governance gaps that risk undermining the policy’s ecological outcomes, as well as exploring the risks caused by fundamental capacity constraints in regulators, but the magnitude of their effects on the policy’s potential impacts on biodiversity remains unexplored. We collated BNG information from all new major developments across six early adopter councils from 2020-2022. We quantified the proportion of the biodiversity outcomes promised under BNG which are at risk of non-compliance, explored the variation in strategies that developments use to meet their biodiversity liabilities, and quantified the occurrence of simple errors in the biodiversity metric calculations submitted by project proponents.