Vertical sky component: what 27 per cent measures

The vertical sky component is the proportion of light from an overcast sky that reaches a point on the outside face of a window, expressed as a percentage of what would reach an unobstructed horizontal surface at the same moment. A vertical wall with nothing at all in front of it scores just under 40 per cent. The BRE guidance value of 27 per cent is therefore roughly two thirds of the theoretical maximum, not two thirds of anything a real window in a real street achieves.

That distinction is the whole argument in most daylight disputes. Twenty-seven per cent was derived for low-rise suburban conditions, and applying it as a pass mark in a dense urban block produces failures that mean very little.

What the measurement actually is

VSC is a ratio of illuminances. The direct sky illuminance falling on the vertical surface, divided by the simultaneous horizontal illuminance under an unobstructed CIE standard overcast sky, expressed as a percentage.

Three consequences follow from that definition, and each one gets forgotten regularly.

It is measured on the outside face of the window wall, not inside the room. It measures sky visibility, not daylight in the room, so a window with an excellent VSC can serve a room that is dark because it is deep or because the window is small. And because the reference sky is uniformly overcast, orientation makes no difference: a north-facing and a south-facing window with identical obstructions have identical VSC.

The room behind the window is assessed by a different measure, usually the no sky line or daylight distribution, which shows how far into the room a view of the sky penetrates. A Daylight and Sunlight Assessment that reports VSC alone has answered half the question.

Where 27 per cent comes from

The BRE guidance says that if the VSC at the centre of a window is at least 27 per cent, enough skylight should still be reaching it. Where the value falls below 27 per cent and is also reduced to less than 0.8 times its former value, the reduction in skylight will be noticeable to the occupants.

Both limbs matter, and they are cumulative rather than alternative. A window dropping from 32 per cent to 26 per cent fails the absolute test but passes the relative one, because 26 is more than 0.8 times 32. A window dropping from 20 per cent to 15 per cent fails both, even though it was already below the guidance value before anything was built.

Those figures are illustrative, used to show how the two tests interact.

How VSC is calculated at a window

The reference point is the centre of the window, on the outer plane of the wall. Modern practice is to calculate it from a three-dimensional model of the surrounding massing, though the traditional Waldram diagram method gives the same answer and is still useful as a check.

Model what is actually there. Existing buildings at their real heights, parapets, roof plant where it obstructs, and boundary walls. Then model the proposal in the same file, so that the two runs differ only by the proposed massing.

Two modelling decisions need stating explicitly in the methodology. Whether balconies and overhangs above the window have been included, because a projecting balcony can halve the VSC of the window beneath it and the guidance suggests presenting results both with and without so the effect of the proposal can be separated from the effect of the existing building's own geometry. And how trees have been treated, since the usual approach is to disregard them because they are seasonal and can be removed.

Assess habitable rooms. Bathrooms, lavatories, storerooms and circulation space are not normally assessed, and including them inflates the apparent scale of an effect.

The 25 degree check

Before commissioning a full study, run the quick check. If no obstruction opposite a window subtends an angle greater than 25 degrees to the horizontal, measured from the centre of the lowest window in the affected elevation, enough skylight is likely to reach the window and further calculation is usually unnecessary.

It takes two minutes on a section drawing and it settles a surprising number of householder and small infill cases. Where it fails, it tells you where to concentrate the detailed work rather than modelling every window in the street.

The check is a screening tool, not a defence. An obstruction that passes 25 degrees at the lowest window may still cause problems at a basement or lower ground floor window set behind a lightwell, which is a situation with its own analysis, covered in the note on basement lightwells and daylight to habitable rooms.

When a lower value is acceptable

The BRE guidance is guidance. It says so itself, and it invites flexible interpretation where the character of the area justifies it.

In a dense city centre, existing VSC values of 15 to 20 per cent are ordinary, and requiring a proposal to keep every neighbouring window above 27 per cent would prevent development on sites that national and local policy expects to be developed. Inspectors have taken that context into account for years, and the argument is stronger when it is made with evidence: a survey of existing values across the street, a comparison with the prevailing urban grain, and a clear statement of what the affected rooms are used for.

Make the case in the report rather than leaving it to the officer. Set out the existing condition, the absolute and relative results, the number and type of rooms affected, and the reason the departure is reasonable here. That approach is set out at more length for daylight assessment in dense historic townscapes.

Context cuts both ways. A suburban street of two-storey houses is exactly the condition the 27 per cent value was written for, and a departure there is much harder to justify.

What a report should say, and where studies go wrong

A useful VSC section gives the existing value, the proposed value, the ratio, the room type and the address, window by window, in a table. It states the method, the software, the reference sky, the treatment of balconies and trees, and the source of the survey information used to build the existing model.

The recurring errors are worth listing plainly. Measuring at the window head or at a room reference point rather than the window centre. Comparing against a cleared site rather than the existing situation. Omitting balconies that already restrict the window. Treating VSC as a measure of room daylight. Assessing non-habitable rooms. And reporting a pass or fail without reporting the ratio, which hides the relative test entirely.

One more, less obvious. Where a room is served by two windows, the effect has to be considered for the room as a whole, and reporting the worse window in isolation overstates the harm. That is also where the wider amenity argument about outlook and enclosure usually surfaces, and those are separate considerations from daylight, as set out in the note on outlook and sense of enclosure.

Daylight work is specialist, councils generally expect a suitably qualified author, and professional review before submission is strongly advised.

To see which amenity and design documents a scheme is likely to need, enter your postcode on PlanScope and answer the guided questions.

This is general guidance, not legal advice, so check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.