What is No Sky Line?
No Sky Line (NSL) refers to the line which separates areas of the working plane that can “see” the sky from areas that cannot.
What is NSL used for?
Within the BRE Guide NSL is used to assess the degree to which a proposed development could impact on the levels of skylight available within the accommodation in neighbouring properties.
What do you include in NSL testing?
According to the BRE, NSL testing should be carried out for habitable rooms within neighbouring dwellings in instances where internal rooms layouts are known (for example if they are available on the local authority’s planning portal). Rooms in non-domestic buildings should also be considered if it is likely that a daylit interior is expected by the associated occupants.
Where and how is NSL measured?
NSL is measured at an array of points which span the usable floor area of the room that is being investigated. The plane that these points sit on is referred to as the working plan and is located at 0.85m above floor level in houses and factories and 0.7m above floor level in office.
NSL testing is always carried out as a desktop study. While it is possible to calculate NSL using graphic based manual calculations it is more typical nowadays for this testing to be carried out using computer simulations.
Impact testing is carried out by predicting the position of the NSL within neighbouring accommodation for the existing scenario (with existing buildings on the development site) and then again with the proposed development in place.
What is an acceptable level of NSL?
According to the BRE if more than 20% of the working plane lies behind the NSL supplementary electric lighting will be needed.
How is the impact on neighbouring properties determined?
According to the BRE the diffuse daylighting of an existing building will be adversely affected if the area of the working plane in a room, which can receive direct skylight, is reduced to less than 0.8 times its former value.
How should the results of NSL testing be interpreted?
According to the BRE the assessment of impact will depend on a combination of factors and there is no simple rule of thumb that can be applied. The BRE Guide advises that the magnitude of the departure from advisory minimums, the sensitivity of the receptor to loss of light and the existence of mitigating factors are all aspects that need to be considered when attempting to determine the significance of an identified impact.
Beyond the guidance provided in the BRE Guide (which states that “the advice given here is not mandatory”) it is important to recognise that in most planning jurisdictions even significant impacts can be deemed acceptable in circumstances where a countervailing planning objective can be articulated.
Should trees be included in NSL testing?
According to the BRE, for the case of impact assessments, it is usual to ignore the effect of existing trees. The BRE make this recommendation because daylight is at its scarcest and most valuable in winter when most trees will not be in leaf. Having regard to this guidance it is reasonable to assume that the overshadowing effect of trees can be omitted from an impact assessment but that the presence of evergreen trees may be an exception to this rule.
Does NSL analysis have any shortcomings?
NSL helps to identify the areas within a room where adequate levels of diffuse daylight are likely to be present. As these areas are determined on the basis of a simple line-of-sight connection to the sky the measure does not provide a comprehensive indication of the luminous environment that an occupant would perceive.
A critical shortcoming of NSL analysis is that it is insensitive to many of the important factors that influence the daylighting of internal spaces. These factors include climate, window orientation, the presence of reflected light and the optical properties of window glazing.
Having regard to the above it is instructive to note that a room with dark furnishings, served by a heavily tinted north facing window, located in an open field in Edinburgh, will have the same NSL as an equivalent room, with light furnishings, served by a clear south facing window, located in an open field in Marseille.
