Method and limits

VSC is computed by numerical integration of the CIE standard overcast sky over the sky hemisphere visible from the window centre. Sky luminance follows L(θ) = L_z(1 + 2 sin θ)/3 for altitude θ, and illuminance on the vertical plane is integrated as ∫∫ L(θ) cos θ cos φ · cos θ dθ dφ over azimuth φ = ±90° from the window normal, with the lower altitude limit set by the obstruction in that direction. The result is expressed as a percentage of the unobstructed horizontal illuminance under the same sky, which gives 39.6% for a fully unobstructed vertical window — the accepted reference value, and a check that the integration is right.

BRE Site Layout Planning for Daylight and Sunlight (BR 209) treats a window as adequately lit if the VSC is at least 27%, or, where it falls below that, if the proposed value is at least 0.8 of the existing value. Both tests are applied here. Failing them is not automatically a refusal — it is the trigger for a fuller daylight assessment and a planning judgement.

This is a screening calculation, not a formal assessment. It models a single rectangular obstruction of uniform height at one distance, measured at the window centre; it does not handle stepped or irregular massing, multiple separate obstructions, balconies, overhangs, window reveals, reflected light from surrounding surfaces, or internal room geometry. A formal BRE assessment uses Waldram diagrams or validated software, and considers No Sky Line, Average Daylight Factor and Annual Probable Sunlight Hours alongside VSC. Rights to light as a legal matter is separate again and is assessed on Radiance-based adequacy, not on VSC.