As chair of the working group that produced the update, I’d like to highlight the hard work of all who contributed, the ISEP staff, and also acknowledge the advice received from Pinsent Masons.

The first edition of ISEP’s guide to Assessing Greenhouse Gas Emissions and Evaluating their Significance came into being following the 2017 change to the Environmental Impact Assessment (EIA) regulations in the UK, which brought climate change impact assessment into the EIA regime. Since then it has become firmly embedded in practice.

Edition 2 of the guidance, published in 2022, introduced a new approach developed by the working group, which required both stricter and more nuanced assessment – asking how developments aligned with the UK’s trajectory towards net zero by 2050 and its Paris Agreement international commitments. 

Significant evolution in policy and legal landscape

Edition 2 is widely referenced in Environmental Statements, planning decisions and, increasingly, in legal proceedings. This widespread adoption is a testament to its value – but it also highlighted areas where further clarity and consistency were needed.

Over the past four years, practitioners, decision-makers and stakeholders have applied the guidance across a wide range of projects and sectors. In doing so, certain aspects have been subject to varying interpretations. At the same time, the policy and legal landscape has evolved significantly, including the influence of landmark legal cases such as the Finch ruling (R (on the application of Finch on behalf of the Weald Action Group) (Appellant) v Surrey County Council and others (Respondents), which have reinforced the importance of robust and comprehensive assessment of both direct and indirect emissions.

The new supplement responds directly to that experience. The launch of the Supplementary Guidance (Edition 2.1) marks a further important milestone in the evolution of good practice for assessing greenhouse gas (GHG) emissions within EIA.

Reasons for producing the Supplementary Guidance

The Supplementary Guidance has been developed to provide targeted clarification and reinforce key principles. It does not replace the 2022 guidance but is intended to be read together with it. The redesigned version of the 2022 guidance, released together with the Supplementary Guidance, improves accessibility and usability, ensuring that users can more easily navigate and apply the guidance in practice.

A central aim of the supplement is to address areas of potential misunderstanding. For example, it clarifies that there are no simple percentage thresholds that determine whether GHG effects are significant, and that professional judgement, grounded in evidence and aligned with net zero trajectories, remains essential. It also reinforces that conclusions of non-significant effects represent a high bar: requiring clear, evidence-based mitigation that goes beyond minimum regulatory compliance.

The supplement further strengthens expectations around competence, emphasising that effective GHG assessment requires not only technical expertise in quantification, but also a broader understanding of climate policy, carbon budgets and decarbonisation pathways. It highlights the importance of evidence-led assessment, robust baseline scenarios, and meaningful, secured mitigation – particularly in relation to embodied carbon and whole-life impacts.

The guidance also reflects developments in legal interpretation, including the treatment of indirect (Scope 3) emissions and the need to consider causality and likelihood when scoping and assessing impacts. It incorporates updates to best practice, such as ISEP’s recently refreshed GHG management hierarchy and evolving approaches to carbon accounting.

Ultimately, this supplement is about supporting greater consistency, transparency and rigour in GHG assessment. As the urgency of climate action continues to grow, so too does the need for clear, credible and defensible assessment of emissions in the planning process. Together, the updated guidance and supplement provide practitioners and decision-makers with the tools needed to meet that challenge with confidence.

To download the Supplementary Guidance, please click here. 

Further information

Contact Tom Dearing