Cumulative impact assessment at strategic scale

Why this is an SDS question

An SDS apportions growth and identifies broad locations across many local planning authorities. Effects on water, nutrients, transport, air, landscape and habitats do not stop at district boundaries. If each local plan is assessed alone, the combined load can be missed. That is the problem HRA calls "in combination" and IEMA calls inter-project effects.

PM14 requires an SDS to be appropriate, taking into account reasonable alternatives. At plan level, reasonable alternatives are the usual home for comparing spatial options (for example a concentrated pattern against a dispersed one) on cumulative terms.

This page is about methods. It does not argue for a particular settlement form.

The named methods

Use these as the starting set. Say which you used and why.

Source	What it gives you
PINS advice on cumulative effects assessment (formerly Advice Note 17)	A staged process and a zone-of-influence approach. Explicit that there is no single industry standard. Written for NSIPs; the staging is still the most cited public method.
IEMA, Demystifying Cumulative Effects (2020)	Distinguishes inter-project cumulative effects from intra-project inter-relationships.
GLVIA3 (2013), plus later Landscape Institute technical notes	Cumulative landscape and visual effects.
CIEEM EcIA guidelines	Cumulative ecological effects.
Habitats Regulations 2017	Screening must consider effects in combination with other plans or projects.

A workable SDS evidence note will, for each topic:

1. define the receptor and the zone of influence
2. list other plans and projects included, and those excluded with reasons
3. say whether the effect is in-combination (HRA) or inter-project (EIA/SEA language)
4. compare at least two reasonable spatial alternatives on that receptor
5. state residual risk and what is left for local plans and project assessment

Print this as the evidence-base checklist.

What is changing

Part 6 of the Levelling-up and Regeneration Act 2023 provides for Environmental Outcomes Reports to replace the current EIA/SEA/SA stack over time. The March 2026 consultation outcome and roadmap described three phases, with regulations intended by the end of 2027 and a non-regression duty. Until those regulations apply to plan-making, do not treat EOR as the SDS appraisal system.

EOR proposals have been described as retaining reasonable alternatives and cumulative-effects assessment, with project-level reports tiering down from plan-level conclusions. That is a design aim, not yet an operative rule.

Honest limits

• Nutrient and water neutrality show catchment-scale cumulative loading in named areas. They are a case of the method, not a conclusion about any particular SDS geography.
• A concentrated pattern can reduce some cumulative loads and increase others. Location and infrastructure timing change the result. A method that only reports one spatial option is not comparing reasonable alternatives.
• If the evidence base cannot support a cumulative claim, say so. An open gap is more durable than an unsourced number.