Less than substantial harm: what it means in planning
Harm to a listed building or conservation area falls into one of two categories in national policy. The lower one still attracts great weight, and it is not a neutral finding.
Less than substantial harm is the lower of the two categories national policy uses to describe harm to the significance of a designated heritage asset. Where a proposal causes less than substantial harm, heritage policy requires that harm to be weighed against the public benefits of the proposal, including, where appropriate, securing the optimum viable use of the asset. The label sounds reassuring and is routinely misread as meaning minor or acceptable. It means neither.
Anything short of substantial harm, which national policy treats as harm that would vitiate or drain away a large part of the significance of the asset, falls into this single wide category. A scheme that removes a fifth of a listed building's historic fabric and a scheme that adds a slightly overbearing extension in a conservation area can both sit in it, at opposite ends.
The statutory duties that come first
Two sections of the Planning (Listed Buildings and Conservation Areas) Act 1990 apply before any policy balance is reached.
Section 66(1) requires the decision maker, in considering whether to grant permission for development which affects a listed building or its setting, to have special regard to the desirability of preserving the building, its setting and any features of special architectural or historic interest it possesses.
Section 72(1) requires special attention to be paid to the desirability of preserving or enhancing the character or appearance of a conservation area.
The Court of Appeal in East Northamptonshire District Council v Secretary of State (the Barnwell Manor case, 2014) held that section 66(1) requires considerable importance and weight to be given to any harm, so that a finding of harm creates a strong presumption against permission which the decision maker must then address. Decisions since have followed that approach. It is indicative of how the courts have treated the duty rather than a formula to be recited, and the underlying point is simple: a heritage harm finding is not a neutral input to an ordinary balance.
Where the category comes from
The harm categories sit in the historic environment chapter of the National Planning Policy Framework. The Framework was re-issued on 17 August 2026 and its chapters now carry policy codes rather than the paragraph numbers used in the December 2023 and December 2024 editions, so check the current text on gov.uk and quote the edition you rely on.
The structure has been stable across editions. Great weight is given to the conservation of designated heritage assets, and the more important the asset, the greater that weight. Substantial harm to or total loss of significance of a grade II listed building should be exceptional, and for assets of the highest significance, including grade I and II* buildings and scheduled monuments, the bar is higher still. Less than substantial harm goes to the public benefits balance instead.
Undesignated heritage assets, including locally listed buildings and unrecorded archaeology, are dealt with separately through a balanced judgement that has regard to the scale of harm and the significance of the asset.
How the balance is actually run
Four steps, in order, and skipping one is the most common failing in both officer reports and applicant statements.
- Establish the significance of the asset, including any contribution made by setting. Significance is described by its archaeological, architectural, artistic and historic interest, not by listing grade alone.
- Identify the effect of the proposal on that significance, element by element, and say which parts of the significance are affected and how.
- Place the harm in a category, and where it is less than substantial, say where in the range it falls. Inspectors routinely use language such as low, moderate or towards the upper end, and an assessment that stops at the category alone gives the decision maker nothing to weigh.
- Identify the public benefits and weigh them against the harm, with great weight on the side of conservation.
Public benefits must be public. Housing delivery, employment, heritage repairs secured by condition, removal of a harmful later addition, improved access and biodiversity gain all count. The applicant's profit does not, and neither does a private benefit dressed up as regeneration. The detailed mechanics are covered in our article on the heritage balance explained.
An illustrative worked case
Illustrative example, for method only. A grade II farmhouse of about 1780 has a single-storey lean-to added in the 1960s on its rear elevation. The applicant proposes to remove the lean-to and build a two-storey glazed link and wing, taking out a section of rear rubble stone wall and one historic window opening.
Significance lies in the surviving plan form, the stonework, the joinery and the relationship to the former yard. The loss of the wall section and the opening is real and irreversible, so there is harm. It affects a secondary elevation rather than the principal front and leaves the plan form legible, so the harm sits in the lower part of the less than substantial range. Against it, the removal of the unsympathetic lean-to restores part of the rear elevation, the scheme funds re-roofing in matching slate secured by condition, and the building returns to full residential use after five years empty.
On those facts a decision maker could reasonably find the public benefits outweigh the harm, having given the harm great weight. Change the affected elevation to the principal front, or take out an original staircase, and the same benefits would not carry the case.
What people get wrong
The first mistake is treating less than substantial as less than important. It is a category, not a measure of acceptability, and permission can be refused on less than substantial harm alone where the benefits are thin.
The second is quantifying harm as a percentage of the asset. The courts have discouraged arithmetic of that kind. Harm is assessed against significance, which is qualitative.
The third is confusing this balance with the tilted balance under the presumption in favour of sustainable development. They are separate exercises, run in sequence, and the heritage weighing is not itself tilted.
The fourth is assuming that a well-designed scheme causes no harm. Design quality affects the degree of harm and can neutralise it, but a high-quality extension that still removes historic fabric causes harm, and saying so plainly in the heritage assessment makes the document more credible rather than less.
Terms that sit alongside it
Substantial harm is the upper category, reserved for harm that drains away a very large part of significance. It is rare and, on appeal, rarely found.
Setting is the surroundings in which an asset is experienced, and harm to setting is assessed in the same way as harm to fabric.
Optimum viable use is the use that causes least harm to significance while remaining economically viable, and it appears in the balance where an asset is redundant or at risk.
Total loss covers demolition, and sits with substantial harm.
Where to take it next on a live application
Commission or write the significance assessment before the design is fixed, not after. A Heritage Statement written to justify a completed design tends to read as advocacy, and conservation officers notice.
Where fabric will be lost, a recording exercise to an agreed level is usually expected before work starts, and the recording brief should be agreed with the conservation officer rather than assumed. How the weighing itself is set out on paper is covered in our note on the public benefits balance, and where the asset is a registered park or garden, a different set of considerations applies, covered in what the designation means.
PlanScope drafts a significance-led Heritage Statement from your site details and flags where evidence is missing, so the conservation officer receives an assessment rather than a sales pitch.
This is general guidance, 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.
Frequently asked questions
Does less than substantial harm mean my application will be approved?
No. It means the harm goes into a balance against public benefits, with great weight given to conserving the asset. Where the benefits are limited, permission can be and often is refused on less than substantial harm alone. The category describes the scale of harm, not its acceptability.
What counts as a public benefit?
Benefits that flow to the public rather than to the applicant. Housing and employment delivery, securing the future of an asset at risk, funded repairs tied to a condition, removal of harmful later additions, improved public access and biodiversity gain are all capable of counting. The applicant's financial return is not a public benefit.
Who decides which harm category applies?
The local planning authority, advised by its conservation officer and, for higher-grade assets and some conservation area cases, by the statutory consultees. On appeal the inspector reaches their own view. The applicant's heritage assessment informs that judgement but does not bind it.
Is harm to setting treated differently from harm to fabric?
No. Setting is part of how significance is experienced, and harm to setting is assessed and categorised in the same way. A new building some distance from a listed farmhouse can cause less than substantial harm without touching the building, where it changes the way the asset is understood in its landscape.
How should the level of harm be described in a report?
Say where in the range the harm falls and why, by reference to the elements of significance affected. Language such as low, moderate or towards the upper end of less than substantial is what inspectors use. An assessment that names the category without locating the harm within it leaves the decision maker with nothing to weigh.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.