Less than substantial harm: the heritage balance explained

Less than substantial harm is the category national heritage policy uses for almost every case that reaches a planning officer. It means harm to the significance of a designated heritage asset that falls short of seriously eroding or destroying that significance, and it triggers a balancing exercise: the harm is weighed against the public benefits of the proposal. Two things about that balance surprise people. Great weight attaches to conservation whatever the level of harm. And the balance is a separate exercise carried out before the ordinary planning balance, not folded into it.

Where the phrase comes from in national policy

The National Planning Policy Framework divides harm to designated heritage assets into two categories, substantial and less than substantial, and gives each its own test. Substantial harm or total loss is resisted except in narrowly defined circumstances that vary with the grade of the asset. Less than substantial harm is weighed against the public benefits of the proposal, including securing the optimum viable use of the asset.

The Framework has been reissued more than once and the numbering has changed each time, most recently when the chapters were recast around policy codes. Always state the edition you are citing and check the current text on GOV.UK rather than copying a paragraph number from an older report.

The categories apply to designated assets: listed buildings, scheduled monuments, conservation areas, registered parks and gardens, registered battlefields, protected wreck sites and World Heritage Sites. Non-designated heritage assets are handled differently, through a balanced judgement having regard to the scale of harm and the significance of the asset.

The threshold between substantial and less than substantial

Substantial harm is a high bar, and the courts have said so. The formulation most often quoted asks whether the harm would have so serious an impact on the significance of the asset that its significance is drained away altogether or very much reduced.

That means demolition, or the loss of the feature that makes the building interesting, or the obliteration of a setting that carries the significance. It does not mean a large extension, an unsympathetic window, or a new building that competes with a church tower in some views. Those are usually less than substantial, and officers who reach for the substantial category on that kind of case are usually corrected on appeal.

The consequence is practical. Almost everything lands in the less than substantial category, which is why arguing about the category is often less productive than arguing about where within it the harm sits.

Why great weight attaches regardless of the level of harm

Policy requires great weight to be given to the conservation of a designated heritage asset, and says that the more important the asset, the greater that weight should be. It also requires clear and convincing justification for any harm.

Behind that sits statute. Section 66 of the Planning (Listed Buildings and Conservation Areas) Act 1990 requires the decision-maker to have special regard to the desirability of preserving a listed building, its setting and any features of special architectural or historic interest. Section 72 requires special attention to the desirability of preserving or enhancing the character or appearance of a conservation area.

The Court of Appeal in Barnwell Manor established that the section 66 duty is not simply one consideration among many. Considerable importance and weight must be given to the desirability of preserving the setting of a listed building, and a decision-maker who treats heritage harm as a neutral factor in a straight balance has misdirected itself. That case is the reason a heritage argument cannot be won by volume of public benefit alone.

Placing harm on a spectrum within the category

Inspectors and officers routinely locate harm at the low, moderate or high end of less than substantial, and the position on that spectrum drives how much benefit is needed to outweigh it.

So do the work. Identify the elements of significance the asset holds, using the four heritage values Historic England describes in its guidance on managing significance in decision-taking. Say which of those elements the proposal affects, how, and to what degree. Then place the harm and justify the placing.

A Heritage Statement that asserts "less than substantial harm" without locating it on the spectrum invites the officer to place it higher than you would. One that says "harm at the lower end of less than substantial, because the affected fabric is a 1970s rear addition of no special interest and the principal elevation is unaffected" gives the officer a position to adopt.

Public benefits capable of outweighing harm

Planning Practice Guidance sets the shape of this. Public benefits should flow from the proposal, should be of a nature or scale to benefit the public at large, and need not be visible or accessible to the public to count.

The ones that carry weight in practice are heritage benefits first: repair of the asset, removal of harmful later accretions, restoration of a lost feature, securing a viable use for a building at risk, improved public understanding through recording or interpretation. Then wider benefits: housing delivery where supply is short, employment, regeneration of a site that blights its surroundings, environmental improvements.

Private benefit to the applicant is not a public benefit. Neither is the avoidance of a loss. And a benefit that would arise anyway, without the harmful element, cannot be used to justify it, which is the point at which enabling development arguments are usually tested and fail.

The statutory duties that sit behind the policy test

Three procedural consequences follow from the statutory duties, and they are easy to get wrong.

The duties apply to the decision, not to the officer report, so a report that never mentions section 66 but plainly applies the correct approach will usually survive. The reverse is also true.

Listed building consent is a separate application from planning permission, with its own tests under section 16 of the 1990 Act, and there is no equivalent of the section 96A non-material amendment power for consents. A change to approved works on a listed building needs a fresh application.

And the development plan still governs. Local heritage policies sit alongside national ones, and the statutory starting point remains determination in accordance with the plan unless material considerations indicate otherwise, as set out in the notes on what counts as the development plan.

An illustrative shopfront alteration example

The following is illustrative only. A grade II listed former bank in a market town, in a conservation area, has a mid-nineteenth century shopfront with pilasters, a fascia and a recessed entrance. A restaurant operator wants a new fascia sign, external seating, and an extract duct running up the rear elevation.

The significance sits in the classical shopfront, the surviving joinery and the building's contribution to the street. The sign, if it uses the existing fascia panel with applied lettering and no internal illumination, causes little or no harm. The duct on the rear elevation causes harm at the lower end of less than substantial, because the rear is plainer, though the fixings go into historic brickwork and the termination is visible from a side lane.

The benefits are the return of a vacant unit to active use, repair of the shopfront joinery as part of the works, and the removal of a 1980s aluminium door. Those are real heritage benefits and, on this framing, they are capable of outweighing harm at that level.

What changes the answer: routing the duct up the principal elevation, or losing the recessed entrance to gain two covers. Either moves the harm up the spectrum, and the same benefits stop being enough.

Browse all 66 PlanScope report types and start a draft if you are assembling a heritage case, or enter your postcode to see what a constrained site is likely to need. 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.