Nationally Significant Infrastructure Projects: Advice on Cumulative Effects Assessment
Source: gov.uk (2025-03-25)

This advice summarises the process for undertaking cumulative effects assessments in the context of Nationally Significant Infrastructure Projects (NSIPs) under the Planning Act 2008.

The government has published guidance about national infrastructure planning which applicants, members of the public and other parties should read. See the National Infrastructure Planning Guidance Portal. (https://www.gov.uk/government/collections/national-infrastructure-planning-guidance) The guidance should be read alongside the Planning Act 2008 (https://www.legislation.gov.uk/ukpga/2008/29/contents) (the Planning Act).

This advice is non-statutory. However, the Planning Inspectorate’s advice about running the infrastructure planning regime and matters of process is drawn from good practice and applicants and others should follow our recommendations. It is intended to complement the legislation, regulations and guidance issued by government and is produced under section 51 of the Planning Act.

This advice should be read together with Planning Inspectorate’s advice on the Environmental Impact Assessment Process (https://www.gov.uk/government/collections/national-infrastructure-planning-advice-notes) and government guidance on the Planning Act process (https://www.gov.uk/government/collections/national-infrastructure-planning-guidance-portal).

Legal and policy requirements for Cumulative Effects Assessment

The requirement for cumulative effects assessment (CEA) is set out in the Environmental Impact Assessment (EIA) Directive (EIA Directive 2014/52/EU, which amends EIA Directive 2011/92/EU) on the assessment of the effects of certain public and private projects on the environment.

A range of public sector and industry guidance is available, but there is no single industry standard for CEA. The approach taken will vary between applications. This advice page sets out a staged process that applicants may wish to adopt.

This advice page should be read in conjunction with:
- The Infrastructure Planning (Environmental Impact Assessment) Regulations 2017 (https://www.legislation.gov.uk/uksi/2017/572/contents/made) (the EIA Regulations 2017)
- relevant government planning policy
- guidance from consultation bodies, such as the Marine Management Organisation’s A strategic framework for scoping cumulative effects (MMO 1055) (https://www.gov.uk/government/publications/a-strategic-framework-for-scoping-cumulative-effects-mmo-1055) (2014), Natural England’s Development of a generic framework for informing Cumulative Impact Assessments (CIA) related to Marine Protected Areas through evaluation of best practice (https://publications.naturalengland.org.uk/publication/6341085840277504) (2014), and the Highway Agency’s Design Manual for Roads and Bridges (DMRB) (https://www.standardsforhighways.co.uk/dmrb/)
- European Commission guidance, such as Guidelines for the Assessment of Indirect and Cumulative Impacts, Impact Interactions (1999), and Environmental Impact Assessment of Projects Guidance on the preparation of the Environmental Impact Assessment Report (2017)
Applicants may also want to consult relevant institute guidelines and emerging industry guidance.

It is for applicants to ensure that all relevant policy, legislation and guidance has been applied to CEA presented in the Environmental Statement, within the Development Consent Order (DCO) application.

Schedule 4 paragraph 5(e) of the EIA Regulations 2017 requires the Environmental Statement to include a description of the likely significant effects of the development on the environment resulting from:

“the cumulation of effects with other existing and, or approved projects, taking into account any existing environmental problems relating to areas of particular environmental importance likely to be affected or the use of natural resources”

The remainder of Schedule 4, paragraph 5 states that the description of the likely significant effects on the factors specified in regulation 5(2) of the EIA Regulations 2017 should cover any cumulative effects of the development, “taking into account any existing environmental problems relating to areas of particular environmental importance likely to be affected or the use of natural resources”.

In this advice, ‘other existing and, or approved development’ is taken to include existing developments and existing plans and projects that are ‘reasonably foreseeable’.

The need to consider cumulative effects in planning and decision making is also set out in planning policy, particularly the National Policy Statements. The overarching National Policy Statement for energy (EN-1), for example, specifies a range of aspects for which the applicant’s assessment in the Environmental Statement should consider cumulative impacts, as relevant to the development.

Paragraph 4.1.5 of EN-1 states that the Secretary of State should take any long-term and cumulative adverse impacts, along with any measures to mitigate or compensate for adverse impacts, when weighing the adverse effects of a project against its benefits. National Policy Statements for other types of infrastructure also set out sector specific requirements for cumulative assessment.

Interrelationships and combined effects

Cumulative effects with ‘other existing and, or approved development’ are separate from an assessment of interrelationships between aspects for the proposed NSIP (such as between ecology and hydrology). These factors are normally assessed as part of the specialist aspect chapters, as combined effects.

The Environmental Statement should set out a table demonstrating where multiple impacts from the proposed NSIP would combine to affect sensitive receptors. Where they are identified, these in-combination effects should be assessed in the Environmental Statement.

Where mitigation is proposed, the Environmental Statement should explain how it has taken other potentially affected sensitive receptors into account to determine that it is appropriate. For example, where planting is proposed to mitigate landscape and visual effects, this needs to take into account how ecology may be affected by the proposed planting.

Consideration of cumulative effects in screening Schedule 2 development

If EIA screening of Schedule 2 development is undertaken (please see the section on screening in the Planning Inspectorate’s Advice on the EIA Process (https://www.gov.uk/government/collections/national-infrastructure-planning-advice-notes)), the selection criteria set out in Schedule 3 of the EIA Regulations 2017 must be considered in the screening. Schedule 3 requires consideration of the characteristics of the proposed development and its potential impacts, taking into account cumulation with other existing and, or approved development.

Overview of the Cumulative Effects Assessment process

NSIPs often have a broad spatial and temporal zone of influence (ZOI). The scale and complexity of an NSIP may result in a complex cumulative effects assessment process that considers a dynamic baseline environment going beyond a static assessment of the current situation. There may be variation in the approach to identifying and assessing ‘existing and, or approved development’.

This advice suggests 4 stages to CEA:
- establishing the long list (stage 1)
- establishing the short list (stage 2)
- information gathering (stage 3)
- assessment (stage 4)
Each stage is described further below.

This staged process is sequential, but the assessment itself should be iterative and may need to be repeated several times during the preparation of the Environmental Statement.

Stages 1 and 2 should be undertaken early in the pre-application stage and ideally before requesting a scoping opinion. Applicants should make use of the EIA scoping process to provide information on the CEA, to ensure it is focussed and proportionate. This provides an opportunity to request information from the local authorities about developments to include in the assessment. Stages 1 and 2 are set out sequentially in this advice but they may be undertaken together.

Further assessment may be required during the examination stage for any newly identified ‘other existing development and, or approved development’ with potential to give rise to significant effects. This may be requested by the Examining Authority.

Templates at Annex 1 (https://assets.publishing.service.gov.uk/media/66e3f913e47cfc6de429d655/appendix_1.pdf) (PDF, 86.8 KB, 1 page) and Annex 2 (https://assets.publishing.service.gov.uk/media/66e3f921bfc2fdc9641316ee/appendix_2.pdf) (PDF, 84.4 KB, 1 page) can be used by applicants to document outcomes from each stage in a standardised way and to support meaningful consultation. The aim is to assist the Secretary of State in making the decision by presenting the CEA process in a clear and legible format.

Stage 1: Establishing the long list of other existing and, or approved development

To establish which other existing and, or approved developments should be included in the assessment, the applicant should define and document the ZOI for each environmental aspect considered within the Environmental Statement. The example below provides an example of how a summary entry could be presented.

The Planning Inspectorate recommends that the ZOI for each aspect should be mapped, using geographic information system (GIS) software. Once generated, the ZOI will enable a transparent and justifiable search area for locating other development. The applicant can present this information, in plans or figures, as an appendix to the Environmental Statement.

Example ZOI for:

Air quality:

construction dust and vehicle emissions – ZOI defined by relevant institute guidelines

operational plant emissions – ZOI identified by air quality modelling

Heritage:
- physical effects on buried archaeology – ZOI defined by relevant institute guidelines
The ZOI for each aspect should support a desk study exercise to identify the long list of other existing and, or approved development in the form of planning applications, relevant development plans and any other available and relevant sources, such as consultation response information from a relevant planning authority. Matrix 1 at Annex 1 (https://assets.publishing.service.gov.uk/media/66e3f913e47cfc6de429d655/appendix_1.pdf) (PDF, 86.8 KB, 1 page) could be used to document this information.

Other existing and, or approved development types that should be established for the assessment are listed in the tiers in next section below.

When identifying other existing and, or approved developments, applicants should not restrict their search to development of the same type as the proposed NSIP. For example, offshore wind farms should not only assess other offshore windfarms but also take account of any offshore licensed and consented activities in the ZOI.

Where relevant, applicants should consult consenting bodies in EU states to assist with the identification of cumulative effects with developments outside of the UK.

The availability of information needed to conduct a CEA will depend on the status of the other existing and, or approved developments. Any assumptions or limitations in the collated data should be clearly stated by the applicant. A level of certainty, based on the available information, should be attributed to each development and recorded.

Table 2 provides criteria that can be used to indicate the level of certainty that can be assigned to each other existing and, or approved development, from Tier 1 (most certain) to Tier 3 (least certain). The templates at Annex 1 (https://assets.publishing.service.gov.uk/media/66e3f913e47cfc6de429d655/appendix_1.pdf) (PDF, 86.8 KB, 1 page) and Annex 2 (https://assets.publishing.service.gov.uk/media/66e3f921bfc2fdc9641316ee/appendix_2.pdf) (PDF, 84.4 KB, 1 page) could be used for these purposes.

Assigning certainty to other existing and, or approved development

Tier 1

Other existing and, or approved development
- under construction
- permitted applications under the Planning Act or other regimes but not yet implemented
- submitted applications under the Planning Acting or other regimes but not yet determined
- all refusals subject to appeal procedures not yet determined
Tier 2

Other existing and, or approved development
- projects on the Planning Inspectorate’s programme of projects
Tier 3

Other existing and, or approved development
- projects on the Planning Inspectorate’s programme of projects where a scoping report has not been submitted
- identified in the relevant Development Plan and emerging Development Plans, with appropriate weight given as they near adoption, recognising that there will be limited information available on the relevant proposals
- identified in other plans and programmes, as appropriate, which set the framework for future development consents or approvals, where such development is reasonably likely to come forward
A decreasing level of detail is likely to be available from Tier 1 to Tier 3.

Where other existing and, or approved developments are expected to be completed before construction of the proposed NSIP and the effects are fully determined, effects arising from them should be considered as part of the baseline and may be considered as part of both the construction and operational assessment.

If the effects of other existing and, or approved development under construction are not yet fully determined, for example the outcome of mitigation is being monitored and is not yet known, it may be appropriate to consider these in the CEA. The approach should be agreed with the relevant consultation bodies. The Environmental Statement should distinguish between projects forming part of the dynamic baseline and those in the CEA.

Development related to the NSIP (including permitted development)

A proposed NSIP may comprise multiple sites in different locations, for example where offsite highways improvements are required. This may include development where consent is sought under a different planning regime such as the Town and Country Planning Act 1990. In these circumstances, the applicant should consider if cumulative effects could arise from the different development components of their NSIP, as well as with other existing and, or approved development.

Where the proposed NSIP comprises elements of work classed as permitted development, the applicant should ensure that these are included within the assessment, if they are not already considered within the individual aspect assessments.

Stage 2: Establishing a shortlist of other existing and, or approved development

After Stage 1, applicants should develop and apply threshold criteria to the long list. These criteria should be used to establish a shortlist of the existing and, or approved development to be included in the CEA. This will ensure that the assessment is proportionate. The criteria should be used to include or exclude development projects which fall with the ZOI for the NSIP being promoted by an applicant.

Consideration of effects that are deemed individually not to be significant should still be included in the assessment, as the cumulative effect of several non-significant effects could be significant.

The criteria used should be presented at an early stage, such as within the applicant’s scoping report. It should be clearly presented and prepared having regard to relevant policy and guidance, and in consultation with the relevant statutory bodies, especially the local planning authority.

The criteria should address:

Temporal scope

The relative construction, operation and decommissioning programmes of the other existing and, or approved developments identified in the ZOI together with the NSIP programme, to establish whether there is overlap and any potential for interaction.

Scale and nature of development

The scale and nature of the other existing and, or approved developments identified in the ZOI that are likely to interact with the proposed NSIP. Statutory definitions of major development and EIA screening thresholds may be of assistance when considering issues of scale.

Other factors

For example, the nature and, or capacity of the receiving environment, which could make a significant cumulative effect with the other existing and, or approved developments more or less likely. Consider using a source-pathway receptor approach to inform the assessment.

Documentation

Matrix 1 in Annex 1 (https://assets.publishing.service.gov.uk/media/66e3f913e47cfc6de429d655/appendix_1.pdf) (PDF, 86.8 KB, 1 page) may be used for the assessment shortlisting process and to provide a clear record of the applicant’s decision-making for decision makers, consultation bodies and members of the public.

Professional judgement may be used to supplement the threshold criteria and to avoid excluding other existing and, or approved development that is:
- below the threshold criteria limits but has characteristics likely to give rise to a significant effect, or
- below the threshold criteria limits but could give rise to a cumulative effect by virtue of its proximity to the proposed NSIP
Professional judgement could be applied to support the exclusion of other existing and, or approved development that exceeds the thresholds but may not give rise to evident effects. All the other existing and, or approved development considered should be documented and the reasons for inclusion or exclusion clearly stated.

Where other existing and, or approved developments have potential to produce significant cumulative effects, the applicant should proceed to information gathering at Stage 3.

The applicant should consult the relevant consultation bodies, including local planning authorities, to ensure the shortlist produced is comprehensive and accurate. Applicants should ideally use completed matrices to set out and discuss the issues. This should also help identify unresolved problems with any mitigation measures proposed that might otherwise require exploration during an examination.

This process may need to be repeated during the pre-application stage and should be based on the most up to date list of developments available. The CEA should include a summary of the consultations undertaken and evidence of any agreements reached.

Stage 3: Information gathering

At this stage, the applicant should gather information on each of the other existing and, or approved developments shortlisted at Stage 2. The applicant is expected to compile detailed information to inform the Stage 4 assessment. The information should include but not be limited to:
- proposed design and location information
- proposed programme of construction, operation and decommissioning
- environmental assessments that set out baseline data and effects arising from the other existing and, or approved development
Relevant data is likely to be available from different sources including the Planning Inspectorate, planning authority and statutory bodies’ websites, and direct liaison with stakeholders including applicants’ developers. Information gathered should be summarised and presented in an accessible format, for example in line with Matrix 2 in Annex 2 (https://assets.publishing.service.gov.uk/media/66e3f921bfc2fdc9641316ee/appendix_2.pdf) (PDF, 84.4 KB, 1 page).

Stage 4: Assessment

The applicant should assess the cumulative effects of the proposed NSIP with the other existing and, or approved development identified in Stages 1 to 3.

Assessments should:
- be undertaken at a level of detail proportionate to the information available
- explain and record any limits or gaps in the information
- consider all Tier 1 and Tier 2 other existing and, or approved developments where possible
- consider all Tier 3 other existing and, or approved development where possible, although this may be qualitative and high level
- be documented in the Environmental Statement
The assessment will move from a qualitative to a more quantitative assessment as the availability of information increases.

Some assessments, such as transport and associated assessments of vehicle emissions (including air and noise), may inherently be cumulative as they may incorporate modelled traffic data growth for future traffic flows. Where these assessments are thorough and include a worst-case assessment, no additional cumulative assessment of these aspects is required.

However, separate consideration may be needed for the accumulation or inter-relationship of these effects on an individual set of receptors, for example as part of a socio-economic assessment. Any assumptions should be clearly stated in the aspect chapter and cumulative effects assessment chapter. If any new other existing and, or approved developments are identified that could change the worst-case assumptions based on growth data, updates may be needed to the modelling work.

In preparing the assessment, applicants should remember that a main purpose of the Environmental Statement is to enable the examination necessary to inform decisions on the NSIP application. While applicants should make a genuine attempt to assess the effects arising from multiple, individually non-significant effects, the assessment should be proportionate and no longer than necessary to identify and assess likely significant cumulative effects.

Where significant cumulative effects between the proposed NSIP and other existing and, or approved development are likely to arise in relation to one environmental aspect area, the assessment should focus on that issue only. Some effects may only need brief information to indicate that they have been considered.

Where baseline data about other existing and, or approved development is incomplete, a precautionary but reasonable approach should be taken based on the best available evidence, with an explanation as to how the applicant has attempted to source data.

A co-ordinated approach

Where a Habitats Regulations Assessment is required alongside an Environmental Statement, information should not be duplicated between assessments. The use of shared datasets is recommended. The Planning Inspectorate recommends use of clear cross-referencing between documentation, with specific section and paragraph numbering, to assist the reader.

Significance criteria

Terminology used to determine significance should be explicit and support a clear outcome of the cumulative effects assessment. Criteria should consider capacity of the receiving environment and receptors to accommodate changes likely to occur.

Where bespoke significance criteria are developed, applicants should consider:
- duration of effect (temporary or permanent)
- extent of effect (the geographical area)
- type of effect (whether additive or synergistic)
- frequency of the effect
- value and resilience of the receptor affected
- likely success of mitigation
An example of an additive effect would be the loss of two pieces of woodland of one hectare, resulting in two hectares cumulative loss. An example of a synergistic effect would be two discharges combining to have an effect on a species not affected by discharges in isolation.

Assessment cut-off date

Applicants should state the assessment cut-off date in the Environmental Statement. Where new other existing and, or approved development comes forward following the cut-off date, the Examining Authority may request additional information during the examination in relation to effects arising. The applicant may need to conduct additional assessments to reduce delays and questions during examination.

Mitigation and monitoring

The measures envisaged to avoid, prevent, reduce or, if possible, offset any identified significant cumulative effects and, where appropriate, any proposed monitoring arrangements should be described in the Environmental Statement. The means of securing delivery of these measures should be explained. This should be documented within Matrix 2 ( Annex 2 (https://assets.publishing.service.gov.uk/media/66e3f921bfc2fdc9641316ee/appendix_2.pdf) (PDF, 84.4 KB, 1 page)).

Where mitigation and, or monitoring is proposed to be secured and delivered through a requirement in the draft Development Consent Order, for example within a Construction Environmental Management Plan, it should be clearly referenced in the mitigation column of Matrix 2 and, or as part of an overarching schedule of mitigation.

As a minimum, applicants are expected to include the mitigation necessary to address impacts associated with their proposed NSIP. Apportionment of effect and mitigation between projects may be acceptable subject to justification and agreement with relevant consultation bodies. This would need to be evidenced in the Environmental Statement.

Where possible, applicants should consider opportunities to develop holistic mitigation strategies in collaboration with other relevant bodies identified in the CEA, including developers. For example, the National Policy Statement for renewable energy infrastructure (EN-3) at paragraph 2.8.48 encourages applicants to work collaboratively on shared mitigation for offshore infrastructure and activities and to evidence agreement through statements of common ground. The method to secure such mitigation should be agreed between the applicant, its legal advisors and other relevant bodies.