Major projects, in sectors such as transport, energy, minerals and water, have long life cycles and can have significant local and regional environmental and socio-economic impacts. The impacts of the construction stage can be particularly damaging, if not managed well. Environmental Impact Assessment (EIA) seeks to anticipate such impacts, mitigate adverse and enhance positive impacts through design innovations and associated conditions. However, the approach is only as good as the implementation of such innovations and conditions. The issue of monitoring and auditing of actual, as against predicted, impacts is an Achilles heel in the planning and assessment process. Hinkley Point C (HPC) nuclear power station in the UK is currently one of the largest construction projects in Europe. A recent study of the live project provides a unique insight into the actual local impacts of the early construction years, and appropriate methods of assessment. It identifies KPIs, examines monitoring data, and audits findings against the predictions. The results show varying performance across key impact sectors. Explanations of differences are set out, together with recommendations for improving monitoring and auditing practice.