BSI publishes its recommendations for a UK National Annex to the new European Standard for Daylight in Buildings

The British Standards Institution (BSI) has published, for public comment, its proposed endorsements to the new European Standard for Daylight in Buildings (EN 17037:2018). The endorsements will take the form of a UK National Annex.

The UK committee’s recommendations can be accessed and commented upon at:

The closing date for comments is 25 March 2019.


Background to the new European Standard

There has been considerable variance in daylight standards across Europe, with some countries concentrating on occupier amenity and comfortable enjoyment of the space whilst others have approached it from the point of view of saving energy by reducing the need for artificial lighting. The new European Standard (“the Standard”) seeks to introduce a common approach to calculation methods and targets, with the latter being adapted to account for the prevailing climate in the locality of the site in the country concerned.

Published in December 2018, the Standard encourages building designers to design successfully daylit spaces. It allows designers and developers to target good daylight provision and to address other related issues, including view out, exposure to sunlight and protection from glare. So, what do the Standard and proposed UK National Annex recommend in the way of daylight provision and what would this mean for developers and designers of buildings?

Good daylight provision

The primary objective of the Standard is to set a range of targets for daylight provision in buildings – minimum, medium and high – to aid a better understanding of daylight performance.

In the UK, the most commonly used daylight metric for new buildings has been the average daylight factor. One of its advantages is that it is relatively easy to calculate; however, it is also rather simplistic, works less well in clear-sky climates, doesn’t convey a clear sense of daylight performance over the course of the year and doesn’t correlate well with users’ impressions of how well daylit a space is.

The Standard instead adopts targets based on daylight illuminance (lux).  The recommendations for rooms with vertical or sloping windows are:

  • Minimum level: 300 lux median illuminance over 50% of the space and 100 lux minimum over 95% of the space, both for more than half of the daylight hours
  • Medium level: 500 lux median illuminance over 50% of the space and 300 lux minimum over 95% of the space, both for more than half of the daylight hours
  • High level: 750 lux median illuminance over 50% of the space and 500 lux minimum over 95% of the space, both for more than half of the daylight hours

(Note: 300 lux is what most people feel is a good level of light, according to leading published research.)

The Standard also permits an alternative approach based on median and minimum daylight factors (DF). The proportion of the space to which they apply would be the same as above. The recommendations are dependent on location and in London they are:

  • Minimum level: 2.1% median DF and 0.7% minimum DF
  • Medium level: 3.5% median DF and 2.1% minimum DF
  • High level: 5.3% median DF and 3.5% minimum DF

The alternative recommendations would generally be easier to meet than the corresponding illuminance-based criteria, except in south-facing rooms. However, make no mistake, these recommendations represent a materially higher level of daylight than those recommended in the current British Standard, which was published in 2008 and will be withdrawn. The latter recommends minimum average daylight factors (ADF) in dwellings, namely 1% ADF in bedrooms, 1.5% ADF in living rooms and 2% ADF in kitchens.

The feeling among experts and the BSI committee is that the recommendations in the new Standard may be difficult to achieve for some buildings, particularly dwellings in dense urban areas and in basements or conversions. Also, they could result in overheating in summer, particularly in single-aspect flats where cross-ventilation is not possible.

A concern is that to achieve even the new minimum recommendations could potentially require a reduction in density, a knock-on effect of which could be a reduction in housing delivery. That would run counter to the National Planning Policy Framework, which promotes making efficient use of land for housing and flexible application of daylight and sunlight policies or guidance, “as long as the resulting scheme would provide acceptable living standards”. But will daylight levels that have been held to be “acceptable” in the past, for example in the recent planning appeal at 21 Buckle Street, London E1 (reference: APP/E5900/W/17/3191757), remain so in the context of increased daylight recommendations in the new Standard?

Endorsement for adoption in the UK

To address these concerns the UK committee at the BSI tasked with implementing the Standard is proposing a number of endorsements to be published as a National Annex. Its key recommendation is that even if a predominantly daylit appearance is not achievable for a room in a UK dwelling, the following target illuminance values are exceeded over at least 50% of the room area for at least half of the daylight hours:

  • Bedroom – 100 lx (~0.7% DF in London)
  • Living room – 150 lx (~1.1% DF in London)
  • Kitchen – 200 lx (~1.4% DF in London)

Furthermore, the additional recommendation that a target illuminance level should be achieved across 95% of the room area would not have be applied to UK dwellings. Finally, it is recommended that any room in a dwelling where the illuminance of 500 lx is exceeded over 50% of the room area for more than half of the daylight hours is checked for overheating.

Whilst the proposed UK National Annex may come as a disappointment for those who see it as downgrading the bold aspirations in the Standard for better daylit buildings, there will be plenty who will breathe a sigh of relief, because it essentially introduces a more readily achievable minimum standard that is more suited to dense urban environments and is not dissimilar to that of the current British Standard, which it will replace when implemented in June 2019.

Daylight and sunlight guidelines used by local planning authorities

The guidance on daylight and sunlight provision in new buildings that is most widely cited in local authority planning guidance is BRE Report 209, ‘Site Layout Planning for Daylight and Sunlight – A guide to good practice’ (2nd edition, 2011), published by the Building Research Establishment. The author of the guide, Dr Paul Littlefair, has confirmed that the BRE publication will be updated to reflect the Standard and the UK National Annex. The update is likely to take around a year, so the 3rd edition of the BRE guide should be available around mid-2020.

In theory, that should provide time for developers, designers, consultants and planning officials to get to grips with the new targets and calculation methods introduced by the new Standard and the UK National Annex and for planning authorities to consider whether developments in their area should be assessed against the recommendations of the Standard or whether the lower minimum targets of the UK National Annex suffice.

In reality, developers and designers face a period of uncertainty as the new standards and guidance are implemented. The benefit of expert daylight advice from a consultant capable of translating the technical jargon and experienced in presenting and defending daylight and sunlight assessments to planning authorities is likely to become increasingly important.

If you would like to discuss the potential implications of the new European Standard for your projects, please contact our Daylight and Sunlight team.