Interpreting BR 209 - Part 1

Over the past several months there has been some controversy over the interpretation of BR 209 Site Layout Planning for Daylight and Sunlight, A guide to good practice published by BRE in 2011. On one side there are the planning authorities who, despite the cautions placed within the guidance, tend to interpret the guidance as mandatory and on the other side are the consultants who see their duty to the client as involving the most flexible interpretation possible, in order to justify dense urban development.

In fact, the usual way in which information is presented is in the form of tables showing each individual window and each room and results are often presented with simplistic statements of PASS or FAIL i.e. they either meet or do not meet the standards recommended in BR 209 but the word fail implies that the subject room cannot be adequately daylit when in fact this is not the case. The outcome of these results is then summarised by the surveyor. For this reason, the results can be difficult to interpret by lay people including some planning officers and can be open to statistical manipulation. It is suggested therefore that some agreement is required as to the interpretation and presentation of results.

It is also worth noting that, under the London Plan, Supplementary Planning Guidance is provided to support statutory development plans and that, in March 2016 the Housing Planning Supplementary Guidance was updated and included the following paragraphs relating to daylight and sunlight design:

1.3.45 Policy 7.6Bd Guidelines should be applied sensitively to higher density development, especially in opportunity areas, town centres, large sites and accessible locations, where BRE advice suggests considering the use of alternative targets. This should take into account local circumstances; the need to optimise housing capacity; and scope for the character and form of an area to change over time.

1.3.46 The degree of harm on adjacent properties and the daylight targets within the proposed scheme should be assessed drawing on broadly comparable residential typologies within the area and of a similar nature across London.

2.3.47 Quantitative standards on daylight and sunlight should not be applied rigidly, without carefully considering the location and context and standards experienced in broadly comparable housing typologies in London.

BR 209 Daylight

The three main criteria considered, in BR 209, for daylighting are the Vertical Sky Component (VSC) measured on the face of the window, the Average Daylight Factor (ADF) measured within the room and the daylight uniformity measured using the No Sky Contour (NSC). This latter is the series of points within the room at 850 mm (tabletop height) above floor level where the sky is no longer visible through the window(s).

As a preliminary assessment BR209 suggests the use of a series of reference points around the proposed building, for new developments. If none of the surrounding obstructions subtends an angle to the horizontal, at the reference point, of greater than 25 degrees then there will be the potential for good daylighting in the interior and there should therefore be no need to produce further calculations to demonstrate the levels of daylighting available.

If an obstruction is taller than this then there may still be the potential to achieve a satisfactory level of daylighting if the obstruction is not continuous and is narrow enough to allow adequate daylight around its sides and here further calculations should be used.

The amount of skylight falling on a vertical wall or window is quantified using the Vertical Sky Component (VSC). This is the ratio of direct sky illuminance falling on the vertical wall at a reference point, to the simultaneous horizontal illuminance under an unobstructed sky. The maximum value is approximately 40% for a completely unobstructed vertical wall.

For a room with non-continuous obstructions, there is the potential for good daylighting provided that the VSC at the window position, 2 metres above ground is not less than the value for a continuous obstruction of altitude 25 degrees which is equal to a VSC of 27%. This is a complex way of saying that sufficient day-lighting can be achieved by other means and the guidance suggests that if the VSC of 27% is achieved within 4 metres horizontally from any window then sufficient daylighting is still likely to be achieved and, at paragraph 2.1.6 of BR209 it advises that where a room is served by a single window and the VSC meets or exceeds 27% then conventional window design will usually provide adequate daylighting. Where the VSC value is between 15% and 27% then larger windows or changes to room layout will normally be required and if the value is between 5% and 15% then large windows will almost certainly be required. Below this value the room is unlikely to benefit from adequate daylighting.

This statement implies that any value above 5% could provide adequate daylighting in the right circumstances.

A modified form of these calculations is suggested for use when assessing existing buildings to determine the impact potential of new developments and BR 209 suggests that any loss would be noticeable if the VSC is reduced below 27% and is less than 80% of its original value. It is arguable firstly that the room may still be adequately daylit in accordance with the above and secondly and perhaps more importantly, that this measure is merely one of noticeability and should be treated accordingly.

From this it is possible to interpret that where an existing window receives 27% VSC, any reduction down to 80% of 27% i.e. 21.6% would not be noticeable and thus be considered negligible impact.

When considering new developments, the accepted measure of adequate daylighting is the average daylight factor (ADF), which is the average illuminance internally, divided by the unobstructed illuminance externally and multiplied by 100% and can be calculated as follows:

The diffusible visible transmittance of glazing multiplied by the net area of the window multiplied by the angle of visible sky measured at the face of the window (which is obtained using the VSC and checking the table in the guidance notes) divided by the total area of the room’s internal surfaces multiplied by One minus the average internal reflectance, squared and expressed as a percentage.

The BRE guidance accepts that, where supplementary electric lighting is available, an ADF figure of 2% will produce a predominantly daylit effect but that the recommendations for dwellings are 2% for Kitchens, 1.5% for Living Rooms and 1% for Bedrooms as minimum figures. One of the quirks of this is that many local authorities have decided that kitchens below about 10 square metres do not count as habitable rooms and so should not be included and the other issue that many modern developments of apartments have deep Living/ Kitchen/ Dining Rooms for which it is assumed there will be a requirement for 2% ADF which is often unachievable.

BRE are clear that the ADF should not be used as a comparative tool for assessing impact on existing buildings but that it may provide an indication of the availability of daylight after the proposed development provided that the correct values are used for glazing transmission and internal reflectance. It is of course possible also to use this where the existing surrounding buildings are in fact new and thus the information regarding internal layouts and reflectance may well be available.

At appendix I of the guidance it offers advice on how to assess overall impact of a development on surrounding existing buildings including the suggestion that where the loss of daylight to existing surrounding buildings fully meets the guidance then the impact is assessed as negligible or minor adverse and where the loss does not meet the guidelines then it should be classified as minor, moderate or major adverse and there is guidance as to the factors that would influence which classification is used but it is clear that this is open to considerable interpretation.

Practical Considerations

Where the surveyor has to estimate the internal arrangements and room uses in order to produce results then it is accepted that this will not affect the results for VSC or APSH because the reference point is at the centre of the window being tested and windows have been accurately drawn from the survey information but any errors will affect the daylight distribution assessment and of course any ADF results that may be produced, but in the absence of suitable plans, estimation is the conventional approach.

It is almost inevitable that when the general public learn that their room layouts have been estimated, they will seek to challenge the results and whilst their concern may be genuine it is acknowledged that in many cases there is little difference resulting from inserting the corrected information.

Interpretation of Results

For new buildings and their rooms, the interpretation is simply one of ‘do the results achieve the guidance levels given in BR 209 and if not, then can it be stated that the majority are satisfactory, and any shortfall may be considered minor or, acceptable in the circumstances. Whereas for assessing impact on existing surrounding buildings and their rooms the interpretation is more one of how noticeable the reduction is and does this reduction take the resultant values below BRE guidance and then, can it be decided if the impact is unacceptable in the circumstances.

On this basis I am suggesting that BR 209 be interpreted as follows:

Daylight Proposed Dwellings

VSC Results

> 27% Meets or Exceeds Guidance no further calculations necessary

> 21.6% and < 27% Negligibly below guidance, further calculation recommended

> 15% and < 21.6% Below Guidance but may be adequate with correct design. Further calculation required

> 5% and < 15% Below Guidance and requires significant design management to ensure adequate levels of daylight, further calculation required

< 5% highly unlikely to be adequately daylit.

ADF Results

Living/ Dining/ Kitchens as a single room to be assessed at 1.5% ADF or kitchen to be excluded from calculation

Living Room or Dining Room at 1.5%

Bedroom at 1%

Kitchen/ Dining Room at 2%

Kitchens below 10 square metres classed as non-habitable rooms

Daylight Distribution

Since this is an assessment of the depth to which a room may receive direct light from the sky it is reasonable to consider the fact that light is internally reflected and so there would not, in practice, be a line of shadow in the room. Whilst BRE use 80% of room at work surface level as being an indicator of reasonable distribution, surveyors who work in the rights to light field have long accepted that a room could be adequately daylit if the no sky area was less than about 45%. (55% at 0.2).

On this basis

> 80% Meets or Exceeds Guidance

> 55% and < 80% Negligibly below guidance but acceptable if ADF adequate

< 55% Unlikely to be acceptable

Daylight Existing Dwellings

VSC Results

The importance of this measure being used for existing rooms is that it does not require knowledge of the internal arrangements. However, as stated above, the measure is more one of whether the reduction will be noticeable to the occupant. In many cases within the urban environment the existing VSC will be below 27% even before a development takes place. This does not mean that the room will necessarily be poorly daylit as has been established above and it would only be possible to establish actual impact by assessing the room in the field. Without this it is, in my opinion, more appropriate to discuss degree of noticeability.

There is no published research into how the levels of noticeability might be categorised although some practitioners have independently adopted their own approaches. The importance of a mathematical approach is that rather than use the overarching term FAIL, the ordinary person can see the percentages of rooms that will fall into each category of noticeability.

One possible sub division is as follows:

Pass/ Unaffected =        Meets or Exceeds BRE target value or is unaffected by the proposals

Negligible                =        The remaining VSC will be between 21.6% and 26.9% even though the reduction is greater than 20%.

Noticeable 1           =        The remaining VSC will be between 15% and 21.5%. The room may still be adequately daylit, but the reduction would be noticeable.

Noticeable 2           =        The remaining VSC will be between 5% and 14.9%. The room is less likely to be adequately daylit and the reduction would be noticeable.

Noticeable 3           =       The remaining VSC will be less than 5%, the room is unlikely to be adequately daylit and the reduction would be noticeable.

By adopting the terminology in Appendix I of BR 209 it might be said, for example, that where the number of rooms categorized in Noticeability Levels 2 and 3 exceeds 10% of the total number of rooms tested then the impact would be considered major adverse. Where the number categorized in Noticeability Level 1, 2 and 3 exceeds 10% of the total but the number at level 3 is less than 10% then the impact might be considered moderate adverse. Finally, where the number categorized at noticeability level 1 or negligible is less than 10% then the impact would be considered minor adverse.

ADF Results

Where the internal arrangements, finishes etc are known then the use of ADF may be considered appropriate by the planning officers. In these cases, the target values are presently those given by BRE. In theory, it would be possible to adopt noticeability criteria for reductions where the existing value far exceeds BRE guidance, but this would, I think, place an undue pressure on developers when compared to the criteria used in rights to light cases for example.

Daylight Distribution

If we accept the modified criteria described above for new developments, then it is only reasonable to consider the same approach for existing dwellings. However once again we must look at the noticeability of any reduction as many rooms will have less than 80% of the room area receiving direct light form the sky in their existing condition.

On this basis it is suggested that the following noticeability levels might be applied:

Pass/ Unaffected =         At least 80% of the room can receive direct daylight or the existing value will be unchanged.

Negligible              =         Between 67% and 79% of the room will be able to receive direct daylight and the existing value will be reduced by no more than 20%.

Noticeable 1         =         Between 55% and 66% of the room will be able to receive direct daylight but the existing value will be reduced by no more than 20%

Noticeable 2         =         Between 45% and 54% of the room will be able to receive direct daylight but the existing value will be reduced by no more than 20%

Noticeable 3         =         Less than 45% of the room will be able to receive direct daylight or the reduction is greater than 20%

Conclusion

Whilst the categorisation used in this paper may be open to discussion, it nevertheless demonstrates that it is possible to provide an analysis of daylight results in a way that can be understood by lay people. Perhaps the BRE would care to comment?