Overshadowing Studies – How They Affect Planning

In the UK planning system, the impact of new development on neighbouring properties is a critical consideration. One of the most common concerns is loss of light caused by overshadowing. Local planning authorities frequently require overshadowing studies as part of daylight and sunlight assessments to understand how proposed buildings will affect surrounding homes, gardens, and public spaces.

For planners, architects, and developers, understanding how overshadowing is measured and assessed is essential to avoid objections, reduce the risk of refusal, and support a smooth planning process.

This article explains what overshadowing studies are, how they are carried out, the guidance that governs them, and how they can influence planning decisions across England and Wales.

What Is an Overshadowing Study?

An overshadowing study is a technical assessment that evaluates how a proposed building or extension will cast shadows on nearby properties, gardens, or amenity areas. The study typically:

Uses 3D modelling and sun path analysis.

Simulates shadow lengths and directions at different times of day and year.

Tests compliance against recognised benchmarks, particularly BRE 209 (2022) – Site Layout Planning for Daylight and Sunlight.

Provides evidence to support a planning application or to defend against objections.

Unlike daylight and sunlight assessments, which focus on indoor living conditions, overshadowing studies often focus on gardens, open spaces, and communal amenity areas.

The BRE 209 (2022) Guidance

The most widely used standard for overshadowing analysis in England and Wales is the Building Research Establishment’s BRE 209 (2022) Guidelines. The guidance is not mandatory, but most local planning authorities adopt it as a benchmark when assessing applications.

Key Principles

March 21st Equinox Test – Overshadowing studies use the spring equinox (21 March) as a reference point, since it provides a balance between summer and winter sun angles.

Two Hours of Sunlight – A garden or amenity space should receive at least two hours of direct sunlight on 21 March over 50% of its area.

Comparative Analysis – The study compares conditions before and after development. If the amount of sunlit area falls substantially below the guideline, the development may be deemed to cause unacceptable overshadowing.

Flexibility in Application

The BRE guidelines themselves emphasise flexibility, noting that local context and design quality must be taken into account. In dense urban settings, full compliance may be unrealistic, but the aim is to ensure that development does not result in materially worse conditions for neighbours.

How Overshadowing Studies Are Carried Out

Overshadowing studies combine technical modelling with planning judgement. The process typically involves:

Baseline Modelling – Creating an accurate 3D model of existing site conditions, including surrounding buildings and site levels.

Proposed Development Modelling – Adding the proposed scheme to the model.

Sun Path Simulation – Using software to track shadows on 21 March at hourly intervals between 8am and 6pm.

Comparison of Results – Measuring the extent of overshadowing on gardens, amenity spaces, or windows before and after development.

BRE Benchmark Testing – Determining whether at least 50% of the garden/space continues to receive two hours or more of sunlight.

Reporting – Presenting findings in clear diagrams and written analysis to support the planning application.

Overshadowing and Planning Policy

National Policy Context

While BRE 209 (2022) provides technical benchmarks, planning decisions also rely on national and local policy. The National Planning Policy Framework (NPPF) requires development to create “high standards of amenity” for existing and future users. Overshadowing is therefore a direct consideration in whether a scheme delivers sustainable development.

Local Plan Policies

Many local planning authorities include specific wording in their Local Plans or Supplementary Planning Guidance requiring proposals to avoid “unacceptable loss of sunlight to gardens and amenity spaces.”

London boroughs often apply additional design standards (such as the Mayor of London’s Housing SPG).

Suburban authorities may adopt a stricter residential amenity focus, particularly for rear gardens.

Planning Appeals

At appeal, Inspectors regularly assess whether overshadowing has a material impact on neighbour amenity. Developers who fail to provide robust overshadowing studies risk refusal being upheld.

Case Law and Overshadowing

Although overshadowing is primarily a planning matter rather than a legal right, case law demonstrates how it can influence decisions.

Sabine Guerry v London Borough of Hammersmith & Fulham [2018] – The High Court quashed planning permission for a proposed hotel development after finding that councillors were misled by an inadequate daylight and sunlight assessment. Overshadowing of neighbouring properties was central to the ruling.

Watt v Hackney [2021] – This judicial review challenged the grant of planning permission for a major development in Hackney. The court found that the local authority had failed to properly consider daylight and sunlight impacts, including overshadowing of nearby homes and amenity spaces. The case underscored the importance of robust and transparent assessments in planning decisions.

The takeaway is clear: developers must ensure overshadowing studies are accurate, transparent, and compliant with BRE methodology, or risk both planning refusal and potential judicial review.

Overshadowing in Urban vs Suburban Contexts

Dense Urban Areas

High-rise and mixed-use schemes often create unavoidable overshadowing.

The key issue is whether overshadowing is “material” in planning terms, not whether any overshadowing occurs.

Urban authorities may accept partial guideline breaches if outweighed by regeneration benefits.

Suburban and Rural Areas

Overshadowing of gardens is often a more sensitive issue, with objections from residents carrying significant weight.

Even modest extensions may fail if they substantially reduce garden sunlight.

Practical Implications for Developers and Architects

Overshadowing studies should not be treated as a box-ticking exercise. They play a strategic role in scheme design and planning risk management.

Best Practices:

Engage early – Commission overshadowing studies at concept stage to inform design decisions.

Test massing options – Use the analysis to shape building height, orientation, and layout.

Provide clear graphics – Planning committees and neighbours respond better to visual shadow diagrams than to technical data tables alone.

Address objections proactively – Use the study to demonstrate that overshadowing has been minimised and is within acceptable limits.

Common Pitfalls in Overshadowing Assessments

Incomplete site context – Failing to include all relevant neighbouring properties in the model.

Misapplication of BRE methodology – For example, using incorrect dates or thresholds.

Over-reliance on averages – Masking significant impacts on individual gardens by focusing only on overall percentages.

Poor presentation – Submitting technical diagrams that are difficult for non-specialists to interpret.

Each of these can lead to planning objections, refusals, or costly delays.

The Future of Overshadowing Studies

As urban density increases, overshadowing assessments are becoming more important. The move towards sustainable urban living means planners must balance housing need with liveable neighbourhoods.

Emerging trends include:

Climate-based daylight modelling (CBDM) being used alongside traditional BRE metrics.

Greater emphasis on outdoor amenity in the wake of Covid-19 and changing lifestyle patterns.

Integration with environmental impact assessments (EIA) for major schemes.

FAQs About Overshadowing Studies

What is the purpose of an overshadowing study?
To assess how a new development will affect sunlight in neighbouring gardens, amenity spaces, or windows, ensuring planning decisions are based on evidence.

When is an overshadowing study required?
Typically for larger developments, rear extensions, or any scheme where neighbouring amenity may be affected by reduced sunlight. Local planning authorities often request them during validation.

Which guidance applies in England and Wales?
The BRE 209 (2022) Guidelines are the main reference point, supported by the National Planning Policy Framework and local authority design guidance.

Does overshadowing apply to windows as well as gardens?
Yes. While overshadowing studies often focus on outdoor areas, daylight and sunlight assessments for windows use complementary methods such as Vertical Sky Component (VSC) and Annual Probable Sunlight Hours (APSH).

Can overshadowing justify refusal of planning permission?
Yes. If overshadowing causes a material loss of amenity contrary to BRE guidance and local policy, it can form the basis of refusal.

Do all councils interpret BRE guidelines the same way?
No. Some apply them strictly, while others take a more flexible, context-based approach. Developers should review local guidance early in the process.

Conclusion - Overshadowing Studies

Overshadowing studies play a pivotal role in planning, particularly in ensuring that development balances growth with the protection of neighbour amenity. For developers and architects, a robust overshadowing study can make the difference between approval and refusal.

By applying BRE 209 (2022) correctly, engaging with planners early, and presenting findings clearly, professionals can reduce risk, build stronger cases, and achieve better design outcomes.

Need Expert Advice?

At Anstey Horne, our daylight and sunlight specialists provide independent overshadowing studies that meet BRE 209 (2022) standards and support planning success. If you are preparing a planning application or facing objections, our team can help you navigate the technical and planning complexities with confidence.

For more advice on how we can help support a planning application with overshadowing studies please give us a call. If you would rather we contacted you please fill in our Contact Form and we will be in touch.

For more information on understanding all aspects of Daylight & Sunlight Assessments for planning see the collection of articles on our blog page.

For further advice on BRE 209 Daylight & Sunlight for planning, please call our Daylight & Sunlight Enquiry Line on 020 4534 3138.

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