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SSSI Boundaries & Impact Risk Zones

What it is​

Two related but distinct Natural England datasets:

  • SSSI Boundaries — the mapped extents of Sites of Special Scientific Interest, England's core statutory conservation designation for the best examples of wildlife and geological features, notified under the Wildlife and Countryside Act 1981.
  • SSSI Impact Risk Zones (IRZ) — a GIS screening tool, not a designation. Each IRZ is a zone drawn around an SSSI reflecting the sensitivities of its features, indicating which categories of development near it might cause harm and warrant Natural England consultation.

The distinction matters constantly in practice: the boundary is the protected site; the IRZ is the "you should check" buffer. They are different questions — "is my site in an SSSI?" versus "is my site near enough to affect one?"

How it's produced​

  • Boundaries are defined through the statutory notification process — surveyed, legally described, and digitised. High confidence, authoritative geometry.
  • IRZs are modelled buffers generated by Natural England from each site's features and their sensitivities. They are a decision-support overlay, deliberately cautious, not a precise impact prediction.

Update frequency & currency​

Both are maintained by Natural England and updated as sites are notified, amended, or reviewed. Boundaries change rarely; IRZ logic is revised periodically. Check the data.gov.uk record for the current edition.

Spatial resolution / precision​

SSSI boundaries are precise statutory lines. IRZs are intentionally generalised zones — treat an IRZ edge as an advisory threshold, not a hard line.

Known limitations​

  • IRZ is a trigger, not a verdict. Falling in an IRZ means "consult / assess", not "harm confirmed" — and falling outside one does not guarantee no effect.
  • SSSIs underpin higher designations. Many SSSIs also support an overlying SAC, SPA, or Ramsar site — the same ground, several layers of protection.
  • England only.

How it compares to NNR and LNR​

Every National Nature Reserve is also an SSSI; a Local Nature Reserve is generally not. SSSI is the notification that carries the strongest statutory teeth; NNR and LNR are management/declaration designations that sit alongside or below it. See those profiles for the full comparison.

Role in BNG assessment​

SSSI is a planning constraint and a condition signal, not a habitat-parcel source. In WildStack's stack:

  • An IRZ intersection fires a proximity trigger — flagging likely Natural England consultation — enriched with the underlying SSSI (and any NNR) name.
  • An SSSI boundary intersection sets habitat condition to Moderate (SSSI_designated) where better evidence is absent.

Crucially, BNG does not discharge SSSI or Habitats obligations. A site in or near an SSSI has duties that sit entirely outside the 10% calculation — see Statutory Designations and BNG.

WildStack's take

The IRZ is one of the most useful and most misread datasets in the English planning stack. Misread in two directions: people treat an IRZ hit as if the site itself were designated (it isn't — it's a prompt to assess), and people treat no IRZ hit as an all-clear (it isn't — IRZs cover specific development types and sensitivities, not every possible pathway).

We use the IRZ for exactly what it is: a fast, cautious screen that says "a statutory conversation may be needed here". It is brilliant at that and dangerous as anything more. The boundary tells you what is protected; the IRZ tells you where to be careful.

Official source​

Last reviewed​

5 July 2026. Revisit if the IRZ methodology is revised or if the SSSI notification framework changes.

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