What Is a Daylight and Sunlight Assessment?

What Is a Daylight and Sunlight Assessment? When planning new buildings in the UK, one critical factor that developers, architects, and planners must consider is how the proposed development affects daylight and sunlight.

This process, known as a Daylight and Sunlight Assessment, ensures that new schemes provide good internal lighting conditions for future occupants and do not adversely impact neighbouring properties. But what is a daylight and sunlight assessment in practice—and why is it so important for planning applications?

This beginner’s guide explains what a daylight and sunlight assessment involves, outlines the principles behind it, describes how professionals conduct assessments, and highlights how the BRE Guidelines shape best practices.

Understanding the Purpose of a Daylight and Sunlight Assessment

At its core, a daylight and sunlight assessment evaluates how much natural light a building and its surrounding spaces can receive. Natural light contributes significantly to the comfort and energy efficiency of buildings. Access to daylight and sunlight reduces reliance on artificial lighting, supports physical and mental health, and enhances the enjoyment of internal and external spaces.

A typical assessment covers three main aspects:

Daylight access for new buildings – ensuring internal spaces are adequately lit.

Impact on neighbouring buildings – preserving the right to light and amenity.

Sunlight to open spaces – assessing whether gardens, courtyards, and play areas receive sufficient sunlight.

These assessments play a crucial role in densely built urban areas, where taller buildings and tight site constraints can significantly reduce light availability.

The Role of the BRE Guidelines

The UK's most widely recognised guidance for daylight and sunlight assessments is the Building Research Establishment’s (BRE) publication: "Site Layout Planning for Daylight and Sunlight: A Guide to Good Practice (BR209)", most recently updated in 2022.

While not statutory, local planning authorities across the UK rely heavily on the BRE Guidelines to assess planning applications. These guidelines aim to balance the need for development with the need to protect daylight and sunlight for existing and future buildings.

The BRE document sets out numerical criteria and recommended methodologies for assessing both daylight and sunlight, offering clear guidance on how to interpret and apply these standards depending on a site's location and design.

Key Principles of Daylight and Sunlight Assessments

To answer the question “what is a daylight and sunlight assessment,” it’s important to understand the core measures used in the analysis:

1. Vertical Sky Component (VSC)

The Vertical Sky Component (VSC) measures how much sky is visible from the centre of a window on an existing or proposed building. A high VSC indicates that the window will receive good daylight.

A VSC of 27% or higher generally provides sufficient daylight.

If a development reduces the VSC to less than 27% and to less than 80% of its former value, the resulting change is likely to be noticeable to occupants.

2. No Sky Line (NSL)

The No Sky Line (NSL) is another way of measuring daylight distribution inside a room. It plots the area of a room’s working plane (usually 0.85 m above floor level) that can see any part of the sky through a window.

If a new development reduces the portion of a room that sees the sky to less than 80% of its original value, the room is likely to appear gloomier and more reliant on artificial lighting.

3. Annual Probable Sunlight Hours (APSH)

For rooms that face within 90° of due south, the Annual Probable Sunlight Hours (APSH) is used to evaluate how much direct sunlight a window receives over a year.

The BRE recommends:

A room should receive at least 25% of total APSH annually.

At least 5% should be received in the winter months (21 Sept – 21 March).

A reduction below 80% of the former value may be noticeable and potentially unacceptable.

When Is a Daylight and Sunlight Assessment Required?

Most planning authorities request a daylight and sunlight assessment as part of a formal planning application for:

New residential or mixed-use developments

Major extensions to buildings

Developments close to site boundaries or neighbouring windows

Conversions of commercial buildings into residential use (e.g., under permitted development rights)

Authorities may require assessments for both existing properties affected by the proposed scheme and the new development itself.

Assessment of New Developments

For new buildings, the BRE Guidelines focus on whether the internal spaces will benefit from adequate daylight and sunlight. Two main tests are used here:

a. Daylight Factor or Illuminance Tests (BS EN 17037)

Daylight can be measured using either the daylight factor method or the more complex illuminance-based approach. These assess the likely indoor light levels and whether they meet target thresholds depending on the room type (e.g., kitchen, living room, bedroom).

b. Orientation and Overshadowing

The orientation of windows and the overshadowing from neighbouring buildings are critical. The BRE advises that at least one main window should face within 90° of due south, and rooms should receive a minimum of 1.5 hours of direct sunlight on 21 March (spring equinox).

Assessing the Impact on Existing Neighbours

One of the most sensitive aspects of a daylight and sunlight assessment is analysing how a new development might reduce natural light to surrounding properties.

This involves calculating:

Reductions in VSC to neighbouring windows

Shifts in the No Sky Line within affected rooms

Loss of APSH to habitable rooms of nearby dwellings

If any of these reductions exceed the BRE’s advisory limits, the design may need to be revised, or justification provided as to why the development should be allowed.

Sunlight to Gardens and Amenity Spaces

Daylight and sunlight assessments also examine outdoor areas such as:

Private gardens

Shared courtyards

Public spaces like parks or playgrounds

The BRE recommends that at least 50% of each garden or amenity space should receive two or more hours of sunlight on 21 March. If a new development reduces this to below 80% of its original area, the impact is likely to be significant and potentially unacceptable.

Flexibility and Contextual Considerations

The BRE Guidelines are advisory, not mandatory. Local authorities can apply them flexibly based on local context. For example:

In dense urban areas or conservation zones, achieving full compliance may not be feasible.

In historic areas, smaller windows or higher degrees of enclosure may be acceptable.

Where site constraints make the ideal levels unachievable, design justifications can support a more nuanced planning balance.

Appendix F of the BRE Guide sets out how to determine alternative target values in such circumstances to support design flexibility while maintaining fairness and consistency.

How Are Daylight and Sunlight Assessments Carried Out?

Professional assessments are conducted using a combination of:

3D modelling and CAD software

Site visits and photographic studies

Technical calculations using tools like Waldram diagrams or radiance-based simulations

The assessment results are typically presented in a formal report, with:

3D renderings and shadow plots

Technical tables showing VSC, NSL, APSH

Commentary on compliance with BRE criteria

Conclusions and recommendations for mitigation if required

Common Misunderstandings

“It’s a planning issue, not a legal one.”

Correct—daylight and sunlight assessments under the BRE Guide are for planning purposes. However, common law and the Prescription Act 1832 separately govern legal rights to light. Meeting BRE standards does not guarantee that a development won’t infringe a neighbour’s legal rights to light.

“If one window fails, the whole scheme is rejected.”

Not necessarily. Planning officers often look at the overall impact and weigh any failures against the wider benefits of the scheme. Design flexibility, housing need, or site constraints may justify minor breaches.

Conclusion: Why It Matters

So, what is a daylight and sunlight assessment? It’s a technical evaluation tool that plays a crucial role in shaping development that is fair, sustainable, and humane. It helps keep buildings well-lit, prevents neighbours from being unreasonably overshadowed, and preserves the usability and enjoyment of both public and private spaces.

As cities grow denser and planning becomes more contested, understanding and correctly applying the principles of daylight and sunlight is not just good practice—it’s essential to creating livable, healthy urban environments.

Key Takeaways

Daylight and sunlight assessments are essential for assessing light quality in and around buildings.

The BRE Guidelines (BR209) provide widely accepted best practice criteria.

Key tests include VSC, NSL, and APSH for windows, and sunlight hours for gardens.

Assessments must consider both new development performance and its effect on neighbours.

Flexibility is allowed where justified by site conditions, policy aims, or urban character.

Assessments help inform planning decisions and promote sustainable, light-filled design.

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