Vertical Sky Component (VSC) Explained

The Vertical Sky Component (VSC) is a key metric used to assess daylight availability for properties.

It is one of the primary tests recommended by the Building Research Establishment’s (BRE) publication : Site Layout and Planning for Daylight and Sunlight: A Guide to Good Practice, Third Edition, 2022 ("the BRE Guidelines").

While the Vertical Sky Component (VSC) is typically used to evaluate potential loss of light to an existing dwelling, it is also a valuable tool in other scenarios.

In essence, the VSC represents the amount of visible sky from the centre point of a window. Imagine pressing your face against the window with your eyeline aligned with the centre —what you see is your VSC.

What Exactly Is the Vertical Sky Component (VSC)?

The VSC quantifies the ratio of visible sky to physical obstructions when viewed from a hemispherical sky dome. For example, lying down in an open field and looking straight up might give you a VSC of 100%.

Conversely, when assessing the VSC from the centre of a vertical window in a building, and assuming no obstructions, the maximum achievable VSC is nearly 40%. This is because the building itself blocks about 60% of the sky dome from view.

VSC Guidelines and Recommendations

According to the BRE Guidelines, a Vertical Sky Component (VSC) of 27% is generally sufficient to ensure adequate daylight through standard window designs. This corresponds to a continuous obstruction angle of 25 degrees from the window's centre. Adhering to a VSC of 27% will result in the low density suburban street layouts similar to the image above.

The BRE Guidelines further suggest that:

VSC between 15% and 27% : Adequate daylight may still be achieved with larger windows or shallower rooms, corresponding to obstruction angles between 45 and 25 degrees.

VSC between 5% and 15% : Achieving adequate daylight becomes challenging and typically requires very large windows, with obstruction angles between 65 and 45 degrees.

VSC below 5% : It is often impossible to provide sufficient daylight.

Vertical Sky Component VSC Street Context

Vertical Sky Component (VSC) in Urban Areas

Urban environments and city centres present unique challenges. Achieving the suburban layout that a VSC value of 27% indicates (like the 25-degree angle akin to two rows of two-story houses) is often impractical. The existing layouts of some of the most desirable streets in denser urban areas have historically been constructed with much lower VSC values.

With this in mind many local authorities now accept VSC levels in the "mid-teens," and in some cases, even lower levels are permitted.

However, in such instances further justification of the daylight and sunlight levels will be required. Understanding the existing VSC levels in the surrounding area can assist in establishing local context VSC values. This sort of study is shown in the title image.

It is also important to understand that the VSC test is only one of a number of assessments described in the BRE Guidelines. In order to comply with the guidelines a local authority will also want to understand the distribution of daylight within a room using a No-Sky Line (NSL) assessment for instance. Failure to properly consider both tests has led to planning permissions being overturned at Judicial Review

Expert Guidance on Daylight for Planning Applications

Anstey Horne have a team of experienced experts that will assess what levels of light a local authority are likely to accept based on their experience of working on similar projects within that area.

We work closely with project teams to develop a strategy in order to effectively utilise alternative VSC targets in dense urban areas that fall outside the default BRE recommendations. This often involves speaking with the local authority at an early stage (such as pre-application) for instance.

Engaging with local authorities early in the process - such as during pre-application discussions - can help establish agreed-upon VSC targets that respect the existing context. This proactive approach ensures compliance and streamlines the planning process, reducing the risk of delays and additional costs associated with justifying VSC levels at the planning submission stage.

For more advice on how we can help support a planning application with a daylight & sunlight assessments please give us a call. If you would rather we contacted you please fill in our Contact Form and we will be in touch.