Daylight & Sunlight Assessments

A thorough daylight sunlight assessment comprises several technical components, each measuring different aspects of natural light access:
- Vertical Sky Component (VSC)VSC calculations measure the diffuse daylight reaching windows before and after development.

According to BRE guidance, a VSC of 27% or more indicates good access to daylight. If a proposed development would reduce the VSC to less than 0.8 times its former value, the loss of light is considered significant and may require mitigation measures.

Annual Probable Sunlight Hours (APSH)APSH analysis evaluates the sunlight hours a window receives throughout the year, with particular attention to living rooms that have the highest sunlight needs. The BRE guidelines recommend:
- At least 25% of annual probable sunlight hours overall
- At least 5% of annual probable sunlight hours during winter months

No Sky Line
Also known as the Daylight Distribution test, this assessment determines how much of a room’s floor area receives direct skylight. If the proposed development would reduce this area to less than 0.8 times its former value, the impact is considered adverse.

Shadow Studies
These track overshadowing impacts on gardens, amenity spaces, and neighbouring buildings throughout the year. The BRE guidance suggests that at least half of any garden or amenity space should receive at least two hours of sunlight on March 21st (the spring equinox).

Site Survey and Data Collection
Every daylight and sunlight assessment begins with a detailed site survey to document existing conditions. This typically includes:
- Building orientations and window positions in surrounding properties
- Photography and measurements of neighbouring buildings’ windows
- Assessment of trees, existing obstructions, and reflective surfaces
- Documentation of outdoor spaces that might be affected

The data collected forms the foundation for 3D modeling of the site and its surroundings using specialist software. This digital model enables accurate simulation of light conditions before and after the proposed development.

Local authorities typically require daylight and sunlight assessments in the following scenarios:

- Planning applications for developments that may impact neighbouring properties’ natural light access
- High-density urban developments where overshadowing risks are elevated
- Extensions or alterations to existing buildings affecting adjacent properties
- Commercial and residential developments exceeding specific height or proximity thresholds

The exact requirements vary between local planning authorities, but assessments are most commonly needed when:
- The proposed development is substantially taller than surrounding buildings
- The site is in a dense urban environment
- The development is in close proximity to residential properties
- There are sensitive receptors nearby (like schools or hospitals)Early consultation with planning authorities can help determine whether an assessment is necessary for your specific project.

A comprehensive daylight sunlight assessment typically includes:
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- Technical Reports - Detailed documentation with visual data, calculations, and compliance analysis
- 3D Models - Visual representations of the development and its surroundings
- Shadow Studies - Seasonal and daily overshadowing patterns showing impact throughout the year
- Recommendations - Design modifications if BRE guidelines are not met
- Planning Documentation - Supporting materials formatted for submission to local authorities

These deliverables help demonstrate to planning authorities that the development has been designed with due consideration for its impact on the surrounding built environment.

A professional daylight sunlight assessment is not merely a planning requirement—it’s a valuable tool that supports better design decisions and smoother project delivery. By understanding the technical aspects and benefits of these assessments, developers can transform a regulatory requirement into a strategic advantage.

For complex projects in sensitive locations, commissioning an assessment early in the design process can identify potential issues before they become costly problems. This proactive approach helps ensure your development not only meets regulatory requirements but also contributes positively to the built environment through thoughtful consideration of natural light.

The most successful developments balance maximizing site potential with protecting the amenity of existing properties—and a thorough daylight sunlight assessment is key to achieving this balance.