Sensitivity and magnitude: how significance is scored

A sensitivity and magnitude assessment works because neither variable means anything alone. A large change to something nobody values is not a serious effect. A small change to something irreplaceable can be. Landscape, ecology, heritage and townscape assessment all resolve that tension the same way: describe what is being affected, describe how much it changes, then reach a reasoned judgement about whether the effect matters.

The judgement is the point. Reviewers who treat the matrix as the answer produce assessments that fall apart under cross-examination.

Why significance needs two variables

Significance is a conclusion about consequence, not a measurement. Two projects can produce an identical physical change and entirely different effects, because the receptors differ.

Take a 3 m high acoustic fence. Placed along the boundary of a nationally designated landscape, it is a permanent, visible intrusion into a receptor of the highest value. Placed along a distributor road in a commercial area, it is barely noticed. The change is the same. The effect is not.

Separating the two variables also makes disagreement productive. A consultee who disputes your conclusion is usually disputing one variable rather than both, and an assessment that keeps them apart lets the argument happen where it belongs.

There is a second reason to keep them apart. Mitigation acts on magnitude and almost never on sensitivity. Planting, screening, colour and setback all reduce the size or the extent of the change, while the value of the receptor stays exactly where it was. An assessment that reports a single blended score cannot then show what the mitigation achieved, which is the question a case officer asks about every landscape scheme.

Value and susceptibility combining into sensitivity

In landscape and visual work, GLVIA3 splits sensitivity into two components. Value is the worth attached to the receptor, evidenced by designation, by local plan policy, by community associations, by condition and by recorded appreciation. Susceptibility is the capacity of that receptor to accommodate the specific development proposed without undue consequences.

The distinction does real work. A National Landscape has high value everywhere within it, and yet a degraded quarry edge inside the designation may have low susceptibility to a particular building because the change is absorbed. Conversely an undesignated valley of ordinary value can be highly susceptible to a tall structure because there is nothing else vertical in it.

Susceptibility is always susceptibility to something. An assessment that scores a receptor as "medium sensitivity" without naming the development it is sensitive to has skipped a step, and it is the step a reviewer will find first.

Ecological work runs a parallel logic with different words. The CIEEM approach assigns importance to ecological features using a geographic frame of reference, from international down to site level, rather than a sensitivity scale, and asks whether an effect is significant at a stated geographic scale. The vocabulary of a Preliminary Ecological Appraisal therefore differs from that of a landscape chapter in the same Environmental Statement, and both are correct.

Describing magnitude

Magnitude is the description of the change itself, and it has four components that should be reported separately rather than collapsed into one adjective.

• Size or scale: how much of the receptor is altered, and how completely.
• Geographical extent: whether the change is felt at the site, in the locality, across the district, or further.
• Duration: temporary, short, medium or long term, with the periods defined in the methodology rather than assumed.
• Reversibility: whether the receptor could return to its former condition, and over what timescale.

Reversibility is where assessments most often flatter the proposal. Construction effects are described as temporary and reversible, which is right for a hoarding and wrong for a hedgerow of mature hawthorn removed to build the access. Soil, veteran trees and archaeological deposits do not come back on any timescale that helps.

Duration needs defining numerically in the method. "Long term" means nothing until the chapter says it means more than fifteen years, and every reader then knows what a five-year effect is being called.

How the matrix produces a judgement

The standard device is a two-axis table: sensitivity on one side, magnitude on the other, and a significance category in each cell. Used properly it is a starting point that makes reasoning visible. Used improperly it becomes a machine that generates conclusions nobody has thought about.

GLVIA3 is explicit that landscape and visual assessment cannot be reduced to a formula and that professional judgement, transparently reasoned, is required. That means the matrix output can be departed from, provided the departure is explained.

Three habits keep a matrix honest.

State the criteria for each level before any receptor is scored, in the methodology, so the reader can test whether they were applied consistently. Report the two variables and the resulting category for every receptor in a table, rather than reporting only the conclusion. And where the judgement differs from the cell, say so in the text and give the reason in a sentence, rather than quietly moving the score.

Where the guidance differs in wording

The differences between the main sources are terminological more often than conceptual, and mixing them produces chapters that appear to contradict each other.

GLVIA3, published by the Landscape Institute with IEMA, uses value and susceptibility feeding sensitivity, and separates landscape effects from visual effects with different receptors for each. CIEEM's ecological impact assessment guidance uses importance at a geographic scale and asks whether an effect matters at that scale, avoiding a matrix. Highway schemes assessed under the Design Manual for Roads and Bridges use their own defined value and magnitude tables. Heritage work follows a different logic again, because the statutory tests turn on harm to significance and the level of that harm.

Within one Environmental Statement, say in the methodology which guidance each chapter follows and do not force them into a house template. A Landscape and Visual Impact Assessment written to GLVIA3 and an ecology chapter written to CIEEM guidance can sit side by side, provided the reader is told.

Making judgement transparent

The test of a good significance section is whether a competent reader who disagrees with you can find exactly where they disagree.

Write each receptor as a short reasoned paragraph rather than as a row of adjectives. Name the receptor, state its value and the evidence for it, state its susceptibility to this development, describe the change in the four magnitude terms, then give the judgement and the reason. Where you have weighed something against the matrix, say what and why. Where the evidence is thin, say that too, because an acknowledged uncertainty is far more credible than a confident category with nothing behind it.

Under the Town and Country Planning (Environmental Impact Assessment) Regulations 2017 the Environmental Statement must be prepared by competent experts. A draft Environmental Impact Assessment from PlanScope is a structured starting point for that team rather than a substitute for their judgement, and the same applies to the specialist chapters that feed it.

Browse the 66 report types to see which assessments a site is likely to need before the method is agreed.

This is general guidance and not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.