Planning & Regulation Mar 17, 2026 15 min read
Heritage Impact Assessment: What You Need to Know (2026 Guide)
If you are planning works to a listed building, developing near a conservation area, or proposing changes that could affect any heritage asset, there is a good chance you will need a Heritage Impact Assessment. This guide explains what an HIA is, when you need one, and how it fits into the wider policy framework.
If you are planning works to a listed building, developing near a conservation area, or proposing changes that could affect any heritage asset, there is a good chance you will need a Heritage Impact Assessment. Yet despite being one of the most commonly requested supporting documents in the English planning system, Heritage Impact Assessments remain widely misunderstood -- by homeowners, developers, and even some planning agents.
This guide explains what a Heritage Impact Assessment is, when you need one, what it should contain, and how it fits into the wider policy framework. Whether you are extending a Grade II cottage or converting a former warehouse in a conservation area, the principles are the same: understand significance first, then assess impact.
What Is a Heritage Impact Assessment?
A Heritage Impact Assessment (HIA) is a structured document that evaluates how a proposed development would affect the significance of one or more heritage assets. It is sometimes referred to as a Heritage Statement, Heritage Assessment, or Statement of Heritage Significance, though these terms are not always interchangeable.
At its core, an HIA does three things:
• Identifies the heritage assets that could be affected by the proposal, including their settings.
• Assesses the significance of those assets -- why they matter historically, architecturally, archaeologically, or culturally.
• Evaluates the impact of the proposed development on that significance, including whether the impact amounts to harm, and if so, to what degree.
• Assesses the significance of those assets -- why they matter historically, architecturally, archaeologically, or culturally.
• Evaluates the impact of the proposed development on that significance, including whether the impact amounts to harm, and if so, to what degree.
The requirement for applicants to describe the significance of affected heritage assets is set out in paragraph 207 of the National Planning Policy Framework (NPPF, December 2024). The level of detail expected should be proportionate to the importance of the asset and sufficient for the local planning authority to understand the potential impact.
When Do You Need a Heritage Impact Assessment?
You are likely to need an HIA when your planning application involves any of the following:
• Works to a listed building -- including internal and external alterations, extensions, demolition of part of the structure, or changes of use.
• Development within a conservation area -- particularly where the proposal would affect the character or appearance of the area.
• Development affecting the setting of a heritage asset -- even if the development site itself is not designated, proposals near listed buildings, scheduled monuments, registered parks and gardens, or World Heritage Sites may require an assessment.
• Development affecting a non-designated heritage asset -- such as a locally listed building or a building identified on the local authority's Historic Environment Record.
• Works within a World Heritage Site or its buffer zone -- where the Outstanding Universal Value (OUV) of the site could be affected.
• Archaeological implications -- where there is potential for below-ground heritage assets to be disturbed.
• Development within a conservation area -- particularly where the proposal would affect the character or appearance of the area.
• Development affecting the setting of a heritage asset -- even if the development site itself is not designated, proposals near listed buildings, scheduled monuments, registered parks and gardens, or World Heritage Sites may require an assessment.
• Development affecting a non-designated heritage asset -- such as a locally listed building or a building identified on the local authority's Historic Environment Record.
• Works within a World Heritage Site or its buffer zone -- where the Outstanding Universal Value (OUV) of the site could be affected.
• Archaeological implications -- where there is potential for below-ground heritage assets to be disturbed.
Many local planning authorities include Heritage Impact Assessments on their local validation checklists, meaning your application may not even be registered without one. It is always worth checking the specific requirements of your council before submitting.
The Policy Framework: NPPF Chapter 16
The principal national policy governing heritage and planning in England is Chapter 16 of the NPPF, titled "Conserving and Enhancing the Historic Environment." In the December 2024 revision of the NPPF, the heritage paragraphs were renumbered (Chapter 16 now runs from paragraph 202 onwards), but the substantive policy wording remains unchanged from previous editions.
Key paragraphs to understand
Paragraph 202 establishes the breadth of heritage assets, "from sites and buildings of local historic value to those of the highest significance, such as World Heritage Sites which are internationally recognised to be of Outstanding Universal Value."
Paragraph 207 places the onus on the applicant: local planning authorities should require an applicant to describe the significance of any heritage assets affected, including any contribution made by their setting. This is the paragraph that effectively mandates the HIA.
Paragraph 208 directs local authorities to identify and assess the particular significance of any heritage asset that may be affected by a proposal, taking account of the available evidence and any necessary expertise, and to minimise any conflict between the asset's conservation and the proposal.
Paragraph 212 requires that great weight be given to the conservation of designated heritage assets, and that the more important the asset, the greater the weight should be. This is a critical principle: importance is not binary, it is a spectrum.
Paragraphs 213-215 set out the harm framework (discussed in detail below).
Paragraph 216 addresses non-designated heritage assets, requiring a balanced judgement that considers the scale of any harm and the significance of the asset.
The legislative backstop
Underpinning the NPPF is the Planning (Listed Buildings and Conservation Areas) Act 1990, which provides the statutory framework for listed building consent and conservation area controls. Section 66 requires that special regard be had to the desirability of preserving a listed building, its setting, or any features of special architectural or historic interest it possesses. Section 72 places a similar duty regarding the character or appearance of conservation areas.
These statutory duties carry considerable legal weight. Case law has established that where harm is found, there is a strong presumption against granting permission that can only be outweighed by material considerations of real substance.
Understanding Significance
Before you can assess impact, you must understand significance. This is the foundation of any credible Heritage Impact Assessment.
What is heritage significance?
The NPPF Glossary defines significance as "the value of a heritage asset to this and future generations because of its heritage interest." That interest may be archaeological, architectural, artistic, or historic.
Historic England's Conservation Principles (2008) breaks significance down into four overlapping values:
• Evidential value -- the potential of a place to yield evidence about past human activity. This includes archaeological deposits, building fabric, and documentary records.
• Historical value -- the ways in which past people, events, and aspects of life can be connected through a place to the present. This can be illustrative (the asset visibly demonstrates something about the past) or associative (it has links to notable people or events).
• Aesthetic value -- the ways in which people draw sensory and intellectual stimulation from a place, whether through deliberate design or the fortuitous beauty of age and weathering.
• Communal value -- the meanings of a place for the people who relate to it, or for whom it figures in their collective experience or memory.
• Historical value -- the ways in which past people, events, and aspects of life can be connected through a place to the present. This can be illustrative (the asset visibly demonstrates something about the past) or associative (it has links to notable people or events).
• Aesthetic value -- the ways in which people draw sensory and intellectual stimulation from a place, whether through deliberate design or the fortuitous beauty of age and weathering.
• Communal value -- the meanings of a place for the people who relate to it, or for whom it figures in their collective experience or memory.
A thorough HIA should consider all four values when articulating why a heritage asset matters. Historic England's Advice Note 12: Statements of Heritage Significance provides detailed guidance on how to structure this analysis as part of a staged approach to decision-making.
Designated Heritage Assets
Designated heritage assets are those formally recognised under legislation. They carry the greatest weight in planning decisions. The main categories are:
Listed buildings
There are approximately 400,000 list entries in England covering around 500,000 buildings (since a single entry can cover more than one structure). They are graded according to their relative importance:
• Grade I -- buildings of exceptional interest. Only about 2.5% of all listed buildings fall into this category.
• Grade II* -- particularly important buildings of more than special interest. Around 5.8% of listed buildings.
• Grade II -- buildings of special interest, warranting every effort to preserve them. This is the most common grade, covering approximately 91.7% of all listed buildings.
• Grade II* -- particularly important buildings of more than special interest. Around 5.8% of listed buildings.
• Grade II -- buildings of special interest, warranting every effort to preserve them. This is the most common grade, covering approximately 91.7% of all listed buildings.
Under NPPF paragraph 213, Grade I and Grade II* listed buildings are considered heritage assets of the "highest significance," alongside scheduled monuments, protected wreck sites, registered battlefields, and World Heritage Sites. Any substantial harm to or loss of these assets requires "wholly exceptional" justification. For Grade II listed buildings, the threshold is "exceptional" -- still a very high bar.
It is worth noting that listing covers the entire building, including the interior, regardless of grade. A common misconception is that Grade II listing only protects the exterior; this is not the case.
Conservation areas
Conservation areas are "areas of special architectural or historic interest, the character or appearance of which it is desirable to preserve or enhance" (Section 69, Planning (Listed Buildings and Conservation Areas) Act 1990). There are over 10,000 conservation areas in England.
Within a conservation area, planning controls are tighter. Demolition of unlisted buildings requires planning permission, and permitted development rights are often restricted through Article 4 Directions. Trees are also afforded greater protection.
NPPF paragraph 219 encourages development that enhances or better reveals the significance of conservation areas. Paragraph 220 clarifies that the loss of a building or element that makes a positive contribution to a conservation area should be treated as either substantial or less than substantial harm to the conservation area itself, depending on how much it contributes to the area's significance.
World Heritage Sites
The United Kingdom has 34 World Heritage Sites inscribed by UNESCO. These are designated heritage assets of the highest significance under the NPPF. The key concept for World Heritage Sites is Outstanding Universal Value (OUV) -- the cultural or natural significance that is so exceptional as to transcend national boundaries.
Many World Heritage Sites also have designated buffer zones, which provide an additional layer of planning protection. Development within a World Heritage Site or its buffer zone will typically require a Heritage Impact Assessment that specifically addresses the potential effect on OUV. UNESCO's 2022 Guidance and Toolkit for Impact Assessments in a World Heritage Context provides the internationally recognised methodology.
Other designated heritage assets
The full list of designated heritage assets also includes:
• Scheduled monuments -- nationally important archaeological sites protected under the Ancient Monuments and Archaeological Areas Act 1979.
• Registered parks and gardens -- landscapes of special historic interest, maintained on the Register of Historic Parks and Gardens by Historic England.
• Registered battlefields -- sites of nationally important battles.
• Protected wreck sites -- underwater heritage protected under the Protection of Wrecks Act 1973.
• Registered parks and gardens -- landscapes of special historic interest, maintained on the Register of Historic Parks and Gardens by Historic England.
• Registered battlefields -- sites of nationally important battles.
• Protected wreck sites -- underwater heritage protected under the Protection of Wrecks Act 1973.
Non-Designated Heritage Assets
Not all heritage assets carry a formal designation. Non-designated heritage assets include buildings, structures, sites, and landscapes identified as having a degree of heritage significance that merits consideration in planning decisions but which do not meet the criteria for statutory designation.
Examples include:
• Buildings on a local authority's local list (sometimes called locally listed buildings).
• Sites recorded on the Historic Environment Record (HER).
• Buildings or structures identified during the planning process as having heritage interest.
• Sites recorded on the Historic Environment Record (HER).
• Buildings or structures identified during the planning process as having heritage interest.
NPPF paragraph 216 states that the effect of an application on the significance of a non-designated heritage asset should be taken into account in determining the application. The policy does not use the "substantial" or "less than substantial" harm framework for non-designated assets; instead, it calls for a balanced judgement having regard to the scale of any harm or loss and the significance of the heritage asset.
The Setting of Heritage Assets
Setting is one of the most important -- and frequently contested -- concepts in heritage planning. The NPPF defines setting as "the surroundings in which a heritage asset is experienced." Setting is not itself a heritage asset, but it can contribute to the significance of an asset, and development within the setting of a heritage asset can affect the ability to appreciate that significance.
Historic England's Good Practice Advice Note 3 (GPA3): The Setting of Heritage Assets (Second Edition, 2017) sets out a structured, five-step approach to assessing setting:
• Identify which heritage assets and their settings are affected.
• Assess whether, how, and to what degree these settings make a contribution to the significance of the heritage asset(s) or allow significance to be appreciated.
• Assess the effects of the proposed development, whether beneficial or harmful, on that significance or on the ability to appreciate it.
• Explore ways to maximise enhancement and avoid or minimise harm.
• Make and document the decision and monitor outcomes.
• Assess whether, how, and to what degree these settings make a contribution to the significance of the heritage asset(s) or allow significance to be appreciated.
• Assess the effects of the proposed development, whether beneficial or harmful, on that significance or on the ability to appreciate it.
• Explore ways to maximise enhancement and avoid or minimise harm.
• Make and document the decision and monitor outcomes.
An HIA that addresses setting should follow this methodology. It is not enough simply to say that a proposal is "nearby" a heritage asset; the assessment must explain the functional, visual, and historical relationship between the asset and its setting, and how the proposal would change that relationship.
Levels of Harm: Substantial vs Less Than Substantial
The NPPF establishes a binary distinction for designated heritage assets: harm is either substantial or less than substantial. There is no middle ground, and no harm at all is obviously the third possibility. The level of harm determines which policy test applies.
Substantial harm
Substantial harm is a high threshold. Case law (including the High Court decision in Bedford BC v Secretary of State [2013]) has established that substantial harm occurs where the impact would "seriously affect a key element" of a heritage asset's special interest or significance, or "drain away much of the significance" of the asset.
Where substantial harm (or total loss) would result, paragraph 214 requires consent to be refused unless one of two conditions is met:
• The harm is necessary to achieve substantial public benefits that outweigh the harm; or
• All of the following apply: (a) the nature of the asset prevents all reasonable uses; (b) no viable use can be found through appropriate marketing; (c) conservation by grant-funding or charitable ownership is not possible; and (d) the harm is outweighed by the benefit of bringing the site back into use.
• All of the following apply: (a) the nature of the asset prevents all reasonable uses; (b) no viable use can be found through appropriate marketing; (c) conservation by grant-funding or charitable ownership is not possible; and (d) the harm is outweighed by the benefit of bringing the site back into use.
Less than substantial harm
The majority of heritage cases involve less than substantial harm. Under paragraph 215, this harm should be weighed against the public benefits of the proposal, including where appropriate, securing its optimum viable use.
It is critical to understand that "less than substantial harm" does not mean "less than significant" or "acceptable." The courts have repeatedly emphasised that even a small degree of harm to a highly significant asset carries considerable weight. The term is a policy classification, not a value judgement about seriousness.
No harm
Where a proposal would preserve or enhance the significance of a heritage asset, there is no harm, and the heritage policy tests in paragraphs 214-215 do not apply. An HIA should still articulate this conclusion clearly and explain why the proposal is considered to be benign or beneficial.
What Should a Heritage Impact Assessment Contain?
A well-structured HIA will typically include the following sections:
• Introduction -- the site, the proposal, and the purpose of the assessment.
• Planning policy context -- relevant national policy (NPPF Chapter 16), local plan policies, and any supplementary planning documents or conservation area appraisals.
• Historical development -- the history of the site and its surroundings, drawing on primary and secondary sources, historic mapping, and the Historic Environment Record.
• Identification of heritage assets -- a schedule of all heritage assets (designated and non-designated) that could be affected, including those whose setting may be affected.
• Assessment of significance -- for each identified asset, an analysis of its heritage significance across the relevant values (evidential, historical, aesthetic, communal).
• Assessment of impact -- for each asset, an evaluation of how the proposal would affect its significance, including the contribution of setting. The level of harm (if any) should be clearly stated.
• Mitigation and enhancement -- any measures proposed to avoid, reduce, or compensate for harm, or to deliver heritage benefits.
• Conclusions -- a summary of the heritage case, setting out whether the proposal complies with national and local heritage policy.
• Planning policy context -- relevant national policy (NPPF Chapter 16), local plan policies, and any supplementary planning documents or conservation area appraisals.
• Historical development -- the history of the site and its surroundings, drawing on primary and secondary sources, historic mapping, and the Historic Environment Record.
• Identification of heritage assets -- a schedule of all heritage assets (designated and non-designated) that could be affected, including those whose setting may be affected.
• Assessment of significance -- for each identified asset, an analysis of its heritage significance across the relevant values (evidential, historical, aesthetic, communal).
• Assessment of impact -- for each asset, an evaluation of how the proposal would affect its significance, including the contribution of setting. The level of harm (if any) should be clearly stated.
• Mitigation and enhancement -- any measures proposed to avoid, reduce, or compensate for harm, or to deliver heritage benefits.
• Conclusions -- a summary of the heritage case, setting out whether the proposal complies with national and local heritage policy.
The level of detail should be proportionate. A rear extension to a Grade II listed house in a village will not require the same depth of analysis as a major mixed-use scheme adjacent to a World Heritage Site. But the fundamental structure remains the same.
Practical Advice for Homeowners and Developers
Start early
Heritage considerations should inform the design process from the outset, not be retrofitted to justify a scheme that has already been drawn up. Understanding what is significant about a building or place before you start designing means you can often achieve what you need without causing harm.
Talk to your local authority
Most councils have a conservation officer or heritage team. Pre-application discussions are invaluable and can save significant time and cost. Ask about any conservation area appraisals, local list entries, or Article 4 Directions that might affect your site.
Use proportionate evidence
NPPF paragraph 207 is explicit: the level of detail in your assessment should be proportionate to the asset's importance. For a straightforward application involving a Grade II listed building, a concise Heritage Impact Assessment that clearly describes significance and impact may be all that is needed. For more complex cases, you may need input from a heritage consultant, archaeological surveys, or detailed historical research.
Do not ignore setting
Many applications fail because they overlook the impact on the setting of nearby heritage assets. Even if your site is not designated, check whether there are listed buildings, conservation areas, or scheduled monuments nearby whose setting could be affected.
Be honest about harm
If your proposal will cause some degree of harm, say so. Planning officers and inspectors are far more receptive to an HIA that honestly identifies less than substantial harm and presents a clear case for why the public benefits outweigh it, than to one that implausibly claims no harm at all. Credibility matters.
Consider cumulative impact
Individual changes may each cause only minor harm, but their cumulative effect can be significant. If a building has already been altered, or if other developments in the area have affected the character of a conservation area, your HIA should acknowledge this context.
Getting Your Heritage Impact Assessment
Commissioning a Heritage Impact Assessment does not have to be complicated or prohibitively expensive. The key is to ensure that whoever prepares the document has a sound understanding of heritage policy, can clearly articulate significance and impact, and produces something that is proportionate to the nature of the proposal.
Site Intelligence's Heritage Impact Assessment report provides a policy-compliant heritage assessment powered by structured data analysis, delivered within 48 hours. It covers the identification of affected heritage assets, assessment of significance, impact evaluation against the NPPF harm framework, and relevant local policy context -- the essential components that local planning authorities expect to see.
Frequently Asked Questions
Is a Heritage Impact Assessment the same as a Heritage Statement?
The terms are often used interchangeably, and many local authorities treat them as equivalent. Strictly speaking, a Heritage Statement is the broader term used in validation checklists, while a Heritage Impact Assessment implies a more detailed evaluation of impact on significance. In practice, the document needs to satisfy NPPF paragraph 207 regardless of what it is called: it must describe the significance of affected heritage assets and assess the impact of the proposal.
Do I need a Heritage Impact Assessment for internal works to a listed building?
If you need listed building consent for internal works, then yes, you will need to describe the significance of the building and assess the impact of the proposed changes. Listing covers the entire building, inside and out, at all grades. Even if planning permission is not required, listed building consent almost certainly will be for any works that affect the building's character as a building of special architectural or historic interest.
My property is not listed but it is in a conservation area. Do I still need an assessment?
Very likely, yes. Development within a conservation area that could affect its character or appearance will usually require a Heritage Impact Assessment. This includes demolition, new buildings, extensions, and sometimes changes to boundary treatments, hard landscaping, or trees. Your local authority's validation checklist will confirm whether one is required.
What is the difference between "substantial harm" and "less than substantial harm"?
These are policy terms from the NPPF, not colloquial descriptions. Substantial harm is a high threshold -- it means the proposal would seriously affect the significance of the heritage asset or drain away much of its value. Less than substantial harm covers a wide spectrum, from negligible impact to serious but not devastating harm. The distinction matters because different policy tests apply: substantial harm triggers a strong presumption against consent (paragraph 214), while less than substantial harm is weighed against public benefits (paragraph 215).
How much does a Heritage Impact Assessment cost?
Costs vary considerably depending on the complexity of the project. A simple HIA for a domestic extension near a listed building might cost a few hundred pounds. A major development affecting multiple designated heritage assets could run into several thousand pounds for specialist consultant input. Site Intelligence's Heritage Impact Assessment is available from £545, providing a structured, policy-compliant report suitable for a wide range of planning applications.
Can I write the Heritage Impact Assessment myself?
There is no legal requirement for an HIA to be prepared by a qualified heritage consultant. However, the document must demonstrate a competent understanding of heritage significance and the relevant policy framework. For straightforward cases, a well-informed applicant may be able to prepare an adequate assessment. For more complex proposals, particularly those involving assets of the highest significance or contentious levels of harm, professional input is strongly advisable.
Planning works that could affect a heritage asset? A clear, policy-compliant Heritage Impact Assessment can make the difference between a smooth approval and a costly refusal. Find out how Site Intelligence can help at pfandco.co.uk/for-developers.
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