Daylight Factors and BS EN 17037: Understanding Internal Daylighting Standards

This article explores the core principles of Daylight Factors and BS EN 17037, explaining how the standard is applied, what it means for building design, and how developers and planners can meet or exceed its requirements.

Daylighting plays a crucial role in shaping the quality, comfort, and sustainability of indoor environments. With growing attention to occupant wellbeing, energy efficiency, and environmental impact, internal daylighting standards have become increasingly important in architectural design and planning. In the UK and across Europe, the benchmark for internal daylighting is set by BS EN 17037, a comprehensive standard that replaced the outdated BS 8206-2.

What Are Daylight Factors?

Daylight Factor (DF) is a measure that expresses the ratio of indoor illuminance from natural light to the illuminance outside under an unobstructed overcast sky, expressed as a percentage. For example, a daylight factor of 2% means the indoor space receives 2% of the natural daylight available outside.

Why is this important? Because it offers a consistent, climate-independent way to assess daylight levels inside a building. Daylight Factors help architects and consultants predict how much daylight a space will receive without the variability of weather conditions.

There are several types of daylight factors used in internal lighting assessments:

Point Daylight Factor: Illuminance at a specific location.

Average Daylight Factor (ADF): Mean of several DFs in a room (deprecated in BS EN 17037).

Median Daylight Factor: Used in BS EN 17037 to assess compliance across the reference plane.

Minimum Daylight Factor: Indicates performance in the least daylit part of a room.

BS EN 17037 replaces the old ADF approach with a more nuanced method based on illuminance and daylight factors calculated over a grid.

Overview of BS EN 17037

Published in 2018, BS EN 17037 is the UK and European standard for daylight in buildings. It was developed by the European Committee for Standardization (CEN) and adopted by the British Standards Institution (BSI).

The standard introduces a modern, performance-based approach. It focuses on actual daylight provision, rather than theoretical averages, by using either:

Climate-Based Daylight Modelling (CBDM): Hourly analysis using real-world climate data.

Daylight Factor Method: A simpler, overcast-sky-based method for early design or less complex schemes.

BS EN 17037 assesses four core aspects:

Daylight Provision: Whether sufficient light is available across a defined area.

View Out: The quality and quantity of external views from a room.

Access to Direct Sunlight: Particularly relevant for spaces like living rooms.

Glare Control: Managing daylight to avoid discomfort.

Daylight Provision Under BS EN 17037

Illuminance Method

This approach calculates daylight using local weather data to assess how often a space achieves target illuminance levels during daylight hours. There are three recommendation levels:

Minimum	300 lux	100 lux
Medium	500 lux	300 lux
High	750 lux	500 lux

Daylight Factor Method

For simpler projects or where climate-based modelling isn't possible, the Daylight Factor Method is an accepted alternative. This uses a CIE Standard Overcast Sky and the ratio:

DF = (Indoor Illuminance / Outdoor Illuminance) × 100

In the UK, the following equivalent daylight factor values are used to meet minimum illuminance levels:

Target DF

0.7%

1.1%

1.4%

Calculation and Modelling

To comply with BS EN 17037, design teams must calculate daylight across a grid, generally 0.3m spacing on a reference plane 0.85m above floor level (0.7m in offices). Calculations should:

Use appropriate glazing transmittance and frame factors

Include internal surface reflectance (ceilings ~0.7, walls ~0.5, floors ~0.2)

Account for external obstructions, including trees and balconies

Be validated using approved simulation software or physical measurement techniques

National Annex and Dwellings

BS EN 17037 is a flexible framework. In the UK, National Annex provides minimum illuminance levels specifically for dwellings, particularly “hard-to-light” spaces such as basements or converted units:

Bedroom	100 lux	0.7%
Living Room	150 lux	1.1%
Kitchen	200 lux	1.4%

Key Design Strategies for Compliance

Maximise window size and height, especially raising the window head

Use light-coloured internal finishes to enhance reflectance

Avoid balconies directly above windows unless offset or translucent

Reduce room depth or incorporate dual-aspect layouts

Include open courtyard designs or daylight shafts in dense urban sites

How Daylight Factors Fit Into Planning Applications

In many UK planning authorities, BS EN 17037 has replaced BS 8206-2 and BRE 2011 ADF-based assessments. However, BRE Report 209 (2022) remains a vital companion, especially for site layout, VSC, and No-Sky Line assessments. Together, they provide a comprehensive view of internal and external daylight quality.

Where a daylight and sunlight report is required to support a planning application, referencing compliance with BS EN 17037 can:

Demonstrate high design quality

Support healthy homes agendas

Satisfy local validation requirements

Daylight Factors vs Climate-Based Modelling

Sky Type	CIE Overcast	Real weather data
Output	Static % of external light	Dynamic, hourly illuminance
Best for	Simpler or early-stage design	Detailed compliance and optimisation
Limitation	No sunlight or climate data	Requires specialist software

FAQs

What is the purpose of BS EN 17037?

BS EN 17037 sets the benchmark for daylighting in buildings across Europe. It helps designers ensure good internal lighting levels, access to sunlight, and external views, all of which contribute to occupant wellbeing.

Is BS EN 17037 mandatory?

While not legally binding, BS EN 17037 is often referenced in UK planning guidance and adopted as best practice. Many local authorities require its use in daylight and sunlight reports.

How is daylight factor different from illuminance?

Daylight Factor is a ratio of internal to external light under standard overcast conditions. Illuminance measures actual light levels (in lux) using climate data. Both methods can demonstrate compliance under BS EN 17037.

Can I still use ADF?

No. ADF (Average Daylight Factor) was part of the outdated BS 8206-2. BS EN 17037 uses median and minimum daylight factors or climate-based illuminance modelling instead.

Does BS EN 17037 apply to non-residential buildings?

Yes. While residential targets are given in the UK National Annex, BS EN 17037 also covers schools, offices, hospitals, and commercial buildings. Designers should tailor targets to usage.

What’s the role of BRE 209 (2022) with BS EN 17037?

BRE 209 focuses on site layout and external obstructions (e.g., VSC, No-Sky Line), while BS EN 17037 assesses internal daylight provision. Both are often used together in planning reports.

Conclusion

Understanding Daylight Factors and BS EN 17037 is essential for delivering daylight-compliant developments that support occupant wellbeing, secure planning consent, and meet the expectations of modern sustainability standards.

By moving away from ADF and embracing performance-based methods, BS EN 17037 encourages thoughtful, climate-conscious design. Whether using climate-based simulations or daylight factor methods, compliance with this standard demonstrates design quality, boosts marketability, and enhances living and working environments.

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.

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