Daylight Sunlight: Assessments for Tall Buildings in the City?... Think Again - Pager Power
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Daylight Sunlight: Assessments for Tall Buildings in the City?… Think Again

Daylight Sunlight: Assessments for Tall Buildings in the City?… Think Again
January 14, 2025 Danny Scrivener

Tall buildings in urban environments have large areas of influence, casting shadows, reducing sunlight and removing visible skylight from view of those windows in the surrounding buildings. It is therefore fairly expected that these buildings be assessed for daylight sunlight impacts. However, in reality, the development of these buildings is infrequent, taking years of planning and construction to finally reach completion. After all, there is only so much development space for tall buildings, not just in terms of defined planning areas but also in terms of what fits with the surrounding landscape – the tall buildings located in London and Manchester are two key examples of this. 

In comparison, the development of low-rise residential properties is much more frequent, and these developments have to abide by the same rules in terms of daylight sunlight on smaller budgets, whilst frequently having to manage the sensitivity of developing with a neighbour nearby, who will most likely want to review any plans with close scrutiny. 

Speaking to clients regularly means we know and understand the most common issues, and more often than not, daylight and sunlight issues revolve around residential properties and extensions, rather than their tall inner-city counterparts. This article explores the similarities and differences between assessing tall city developments and much more modest housing and extensions.

Assessment similarities and variations

From tall buildings to domestic extensions, the assessment parameters broadly remain the same. The assessment will need to look at Vertical Sky Component (VSC), Annual Probable Sunlight Hours (APSH), and No Sky Line in each case. An assessment of the overshadowing of green spaces may also be required.

The biggest difference in the assessment characteristics between the two lies in the assessment area and the typical assessment considerations, including mitigating factors, both of which are explored below.

Assessment area

Inherently, the assessment area for tall developments is much larger than two/three-storey properties. A bigger assessment area, however, does not necessarily mean more significant daylight sunlight impacts. Most tall buildings are developed near existing tall buildings. Of course, there is always the case where the first tall building breaks new ground, however even in these cases, their locations is typically well thought out or has been previously earmarked as iconic development – think the Shard in London, One Eastside in Birmingham, or both the Beetham Tower or Deansgate Square in Manchester.

Assessments for Tall Buildings

Figure 1: Iconic: The Shard and the Beetham Tower.

As per BRE 209, the assessment area for a building extends 25 degrees down from the top of the building to the centre of any surrounding windows (this is also known as the 25-degree rule), and conservatively, this means an assessment area typically extends to three times the height of the proposed development. Naturally then, taller buildings have larger assessment areas, but with distance, impacts are typically lessened. Furthermore, consideration and analysis of the baseline environment, which typically includes other tall buildings, is required and often plays a key part in determining the overall significance of the results.

In number terms, new tall building developments are much less frequent than new two or three-story residential developments or extensions, therefore while the assessment area may be larger for tall buildings, the frequency of assessments is less and the mitigating factors by considering the surrounding baseline environment are more obvious i.e. a tall building located amongst other tall buildings may be deemed more acceptable than a proposed large extension to an existing property which subsequently overshadows a single neighbouring property. The prospect of more urban development in a city may be more routinely accepted than in more suburban or rural locations, where additional development in a resident’s vicinity is rare and often more sensitive to neighbouring occupants on a personal level, especially in areas around property value and saleability.

Assessments for Tall Buildings


Figure 2:
Assessment area for buildings, as defined by BRE 209

If a new development extends above the dotted line in Figure 2 above for a particular window in an existing building, the new development needs to assess daylight sunlight impacts for that particular window.

Assessment considerations

As touched upon above, tall buildings are very often located in proximity to other tall buildings and this in itself is a mitigating factor. The daylight sunlight impacts upon those surrounding it are considered on the basis that there are already tall buildings present i.e. you cannot have a collection of tall buildings without there inherently being overshadowing from one to the other, and on the surrounding environment. Therefore, it is more common for surrounding commercial or residential properties to have skylight and daylight levels below the recommended levels, which according to BRE 209, are a VSC of 27% and an APSH of 25%. If the results of the analysis for existing properties show that VSC and APSH are below their respective thresholds, the next step is to look at whether the new threshold is 80% of the baseline threshold.

The same principle is perhaps less applicable for residential development and extensions in suburban and rural locations, where surrounding properties will more commonly already exceed the relevant skylight and daylight levels required to meet the BRE guidance. In this instance, it is important to ensure that baseline levels are maintained following the completion of any new development or extension. This will be particularly sensitive to housing developments where the land use has been maximised to develop as many properties in one location as possible. In general, developments in suburban and rural locations are more sensitive to change than inner-city development.

Of course there are those building developments that exist somewhere between the two i.e. multi-storey apartment blocks or locations that are more difficult to define. It would likely be the case that a development that proposed to push the envelope in terms of size and scale in a given area would experience more significant daylight sunlight impacts, such as a new multi-storey development in a low-rise area.

Final thoughts

Daylight sunlight assessments are not just a significant consideration for high-rise developments in city centres but also much smaller developments in the suburbs and more rural locations, whether it be a single dwelling property or extension. In fact, the assessment of smaller projects is more common due to the frequency with which they are developed. Assessment experience dictates that high-rise developments are typically located in areas where their daylight sunlight assessment requirements are well known and issues are common (and more acceptable) considering the baseline conditions. Developers of residential properties and/or extensions may never have encountered the assessment requirement before, and the sensitivity of the surrounding receptors may not be fully understood.

There are nuances in how the assessment is approached in each case, but it may not necessarily be the case that smaller developments have no or little effect, and in fact, the overall impact may be more sensitive due to the nature of its location. 

Speak to an expert

If you are interested in finding out more about our daylight sunlight assessments, follow the link here to make an enquiry, or give us a call on +44 (0)1787 319001.

About Pager Power

Pager Power is a dedicated technical consultancy that has been providing independent guidance and advice regarding solar developments, wind farms, and building developments internationally since 2002. 

Our Daylight Sunlight Assessments utilise our own bespoke model that complies with BRE and British Standards guidance to ensure accuracy and compliance with best practices. More information on the guidance can be found here.

Further details about the services we provide can be found here. Pager Power has completed over 1,500 glint and glare assessments, over 1,000 aviation/radar impact assessments, over 500 television and radio reception surveys, and over 500 telecommunications impact assessments.

Finally, a link to our testimonials page can be found here.

Notes

1. Typically only when room dimensions of surrounding buildings are known, otherwise the assessment can lack meaningful accuracy.
2. The most relevant factor here is the ratio of height to width, so whilst the assessment area is larger for tall buildings, effects drop off greatly with distance as the tall building takes up a relatively small proportion of visible sky. For small buildings, the height to width ratio is more proportional, therefore effects throughout the whole assessment are more likely.

Figure 1 image accreditation:

Fredseigenkijks (2014) on flickr.com. Last accessed on 14th January 2025. Available here.
Les Pounder (2011) on flickr.com. Last accessed on 14th January 2025. Available here.

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