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Environmental Limited| London |
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| Exeter |
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Sunlight & Daylight are both forms of natural light. In modern planning and building design, it is important that natural light is sufficient. Not only is it vital for energy efficiency (reducing reliance on artificial lighting), but natural light is also deeply tied to environmental psychology and occupant well-being and mood.
Developers and architects regularly commission SWEL to accurately calculate the volume of natural light a new room or amenity space will receive. Crucially, we also assess whether a proposed development will overshadow existing neighboring properties, depriving them of their established natural light.
A comprehensive Sunlight & Daylight Assessment guarantees adequate light in new builds and mitigates planning risks associated with neighboring impacts.
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There are two primary reasons why a local planning authority will request a Sunlight & Daylight Assessment:
When a new building is erected, it casts a "shadow". If that shadow falls on a neighbour's land or building, it can create an adverse impact. It is important to note that developers are "allowed" some level of impact. The BRE BR 209 guidelines permit up to a 20% reduction in light before an impact is formally described as adverse. This threshold applies to:
If a planning officer or architect suspects an impact on a neighbour, a formal assessment is usually mandated.
There is a foundational rule in BRE 209 for establishing if a deeper assessment is needed: Taking a reference point from the center of the lowest existing windows, draw a line upwards at a 25-degree angle. If this line intersects with the proposed new building, further detailed assessment is required.
How much light is "enough"? While your local retail store might be brightly lit at over 1,000 lux, a typical residential living room feels comfortable at just 150 lux. Outdoor daylight can range from 1,000 lux on an overcast day to over 10,000 lux on a sunny afternoon. The key metric is how much of this light penetrates indoors.
For example, the British Standards Institution (BSI) requires kitchens to achieve a lighting level of 300 lux across at least 50% of the space for half of the daylight hours. To determine this, we utilize advanced 3D computer modeling software (such as AutoCAD and specialized lighting simulation tools) to precisely calculate light ingress and internal distribution.
Several architectural factors influence indoor daylight availability:
While most purpose-built modern homes easily meet these standards, challenges frequently arise in retrofits and conversions. Office-to-residential conversions often perform well due to expansive glazing; however, converting disused basements or deep-plan commercial spaces into residential units typically requires careful daylight modeling.
We understand that planning deadlines are often rigid. Standard BRE 209 daylight and sunlight assessments typically take 1 to 2 weeks to complete from receipt of full architectural plans, ensuring your application stays on schedule.
The core methodology utilized aligns with BRE BR 209 ("Site Layout Planning for Daylight and Sunlight"). The traditional method involves calculating height-to-distance ratios using a transparent indicator overlaid onto project plans. Vertical Sky Component (VSC) is presented as points, and "obstructed" point numbers are compared pre- and post-development. A 20% reduction is allowable, provided the VSC value for any affected window does not fall below 27%.
Calculations target ground-floor windows to establish the worst-case scenario. Higher-level windows will naturally experience higher VSC levels, making their assessment secondary once ground-floor compliance is proven. The internal layout of neighboring rooms is largely irrelevant when conducting external VSC impact assessments.
When assessing sunlight, we are referring to direct solar rays. We must evaluate whether a new development will cast a shadow over existing neighbors' windows. This is measured using Annual Probable Sunlight Hours (APSH).
Sunlight values are strictly dictated by the site's latitude zone—the sun tracks a noticeably different path across the sky in Aberdeen compared to London.
Using 3D analytical tools, we quantify obstruction effects into probable sunlight values. If the reduction in sunlight post-development exceeds 20%, the proposed design may require alteration (e.g., massing adjustments or stepping back upper floors) to allow more light passage. With years of consulting experience, we can guide developers through these necessary design optimizations.
Assessing ambient sunlight and direct "sun on the ground" utilizes distinct methodologies and indicators. Sun on the ground assessments evaluate the hours of sunlight enjoyed by an amenity space on a notional day (typically the spring equinox, March 21st).
Indicators are overlaid onto a fixed-scale plan. By inputting known obstruction heights, we calculate the probable hours of sun hitting the ground.
As with window assessments, a 20% reduction in existing sunlight to neighboring gardens is generally permissible. We also calculate the total sun on the ground hours for proposed new amenity spaces to ensure they meet modern livability standards.
For over a decade, the Average Daylight Factor (ADF) was the primary method used to calculate whether natural lighting was adequate inside a new room. However, following a transition period across local planning authorities, ADF has now been firmly superseded. The current accepted standard relies on Illuminance (Lux) testing in accordance with BS EN 17037 (Daylight in buildings).
As noted earlier, compliance now dictates that we construct precise 3D models of the proposed building and its rooms. This allows us to simulate the complex spread of natural light through windows or light tubes and extract specific lux values across a working plane. This modern methodology is significantly more rigorous than the outdated ADF approach, providing planning officers with a much truer reflection of a room's visual comfort.
There are multiple ways to assess daylight. Below are the primary metrics and units utilized:
In the sketch below, two opposing buildings sit on either side of a street. By drawing a 25-degree line up from the lowest window, you can see the building on the left is very close to the maximum allowable height before triggering further testing. Exceeding this 25-degree angle mandates the use of more complex assessment methods.
The Vertical Sky Component is the metric used to describe the availability of diffuse daylight originating from the sky.
As the diagram below illustrates, tall buildings situated close together yield a lower VSC value than low-rise buildings spaced far apart. In layman's terms, this is the core measurement of "overshadowing."
The image below demonstrates the legacy "paper" method for calculating VSC, which involves overlaying a transparent indicator onto site plans. While still technically accepted, the vast majority of our reports are now generated utilizing highly accurate 3D computer modeling.
While Rule of Thumb and VSC cover most standard planning reports, Rights to Light assessments require a different approach. These calculations are rooted in the "Waldram methodology," a traditional standard for determining if a room receives adequate natural daylight (defined as receiving at least 0.2% of the sky dome illumination at working plane height).
Because natural light is not distributed evenly across a room, the EFZ system assigns distinct values to different areas:
The resulting sum is the Equivalent First Zone (EFZ) loss.
Read more about our past projects on our Project Blog:
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