Internal Daylight & Sunlight Assessments: A Complete Guide

Internal daylight and sunlight assessments are a critical component of building design and planning, ensuring that proposed developments provide adequate natural light to occupiers while safeguarding existing conditions in neighbouring buildings. These assessments serve both as a measure of design quality and as an indicator of environmental performance and wellbeing.

In this comprehensive guide, we explore what internal daylight and sunlight assessments involve, why they matter, how they are conducted, and how developers, architects, and planning consultants can use them to their advantage.

What Are Internal Daylight & Sunlight Assessments?

Internal daylight and sunlight assessments evaluate the availability and quality of natural light within a building’s interior spaces. They are typically required as part of the planning process to demonstrate that a proposed development will meet acceptable standards of amenity for future occupants and will not result in an unacceptable loss of light to neighbouring properties.

In the UK, assessments follow the guidance set out in:

BRE Report 209 (2022 Edition): Site Layout Planning for Daylight and Sunlight – A Guide to Good Practice

BS EN 17037: European Standard for daylight in buildings

CIBSE LG10: Daylighting – A Guide for Designers

Why Are Internal Daylight & Sunlight Assessments Important?

Natural light plays a significant role in health, wellbeing, energy efficiency, and the perceived quality of living or working environments. Assessments are vital for:

Occupant Wellbeing: Improves mood, supports circadian rhythms, and reduces reliance on artificial lighting.

Energy Performance: Reduces energy use for lighting and heating.

Planning Compliance: Required to meet policy and design guidelines.

Design Optimisation: Informs decisions on layout, glazing, and building form.

Key Assessment Metrics - Internal Daylight & Sunlight Assessments

1. Vertical Sky Component (VSC)

VSC measures visible sky from the window centre and indicates daylight potential. A VSC of 27% is typically the benchmark for good daylighting.

2. Daylight Factor (DF)

Daylight Factor is the ratio of indoor to outdoor illuminance under an overcast sky. Target levels include:

2.1% for living rooms

1.4% for kitchens

1.0% for bedrooms

3. Target Illuminance (ET and ETM)

BS EN 17037 specifies achieving:

300 lux over 50% of the space (ET)

100 lux over 95% of the space (ETM)

4. Annual Probable Sunlight Hours (APSH)

APSH quantifies how much direct sunlight reaches a window. A minimum of 25% annually and 5% in winter is considered acceptable.

Assessment Methodology

Internal daylight and sunlight assessments typically involve:

3D modelling of the proposed development and context.

Obstruction analysis from neighbouring buildings, trees, etc.

Lighting simulation using tools like Radiance or IES VE.

Grid calculation across a reference plane at 0.85m above floor.

Reflectance and glazing assumptions based on material finishes and transmittance data.

Common Design Influences

Window placement and head height

Room depth and ceiling height

Balconies and overhangs

Orientation of window walls

Glazing type and frame-to-glass ratio

Internal vs External Impact

Internal assessments ensure good amenity for proposed spaces. Complementary assessments check for impacts on neighbouring Vertical Sky Component (VSC), No Sky Line (NSL), and Annual Probable Sunlight Hours (APSH) levels.

Daylight and Sunlight in Conversions and Extensions

Conversions, especially in dense areas or basement settings, can struggle to meet standards. Planning authorities may allow flexibility where constraints are justified.

Best Practices

Engage early in the design process

Favour dual-aspect layouts

Use light-reflective finishes

Minimise depth of floor plates

Common Pitfalls

Ignoring future development context

Underestimating balcony or overhang impacts

Using average daylight factor instead of median

Overreliance on lightwells

FAQs: Internal Daylight & Sunlight Assessments

Q: When is an internal daylight & sunlight assessment required?

A: During planning for new builds, conversions, and significant extensions—especially near residential areas.

Q: Do all rooms need to meet the standard?

A: No. Only habitable rooms like living areas, kitchens, and bedrooms need to be tested.

Q: What happens if my scheme doesn’t meet the guidelines?

A: Guidelines are advisory. Planning may still be granted if design quality or policy goals justify a deviation.

Q: Can balconies be excluded from the analysis?

A: No. Balconies and overhangs must be considered as they affect light access.

Q: What’s the difference between BS EN 17037 and BRE 209?

A: BS EN 17037 focuses on internal conditions. BRE 209 also addresses site layout and neighbour impact.

Key Takeaways

Internal Daylight & Sunlight Assessments are vital for design quality and planning compliance.

They use metrics such as Daylight Factor, VSC, and APSH.

Standards from BS EN 17037 are now the accepted norm in the UK.

Assessments should be performed early in the design process.

Balancing good internal light with minimal neighbour impact is key.

Need Help with Internal Daylight & Sunlight Assessments?

At Anstey Horne, we specialise in delivering clear, accurate, and robust daylight & sunlight assessments for planning applications and appeals in London. If you’re dealing with a complex site or want to understand how room layouts affect daylight & sunlight within a development, get in touch with us today.

For more advice on how we can help support a planning application with a daylight & sunlight assessments 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 all aspects of Internal Daylight & Sunlight Assessments for planning see the collection of articles on our blog page.

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

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