Vertical sky component and when a departure is justified

Vertical sky component measures how much of the sky a window can see. It is expressed as a percentage of the illuminance that would reach the same point from an unobstructed overcast sky, and the theoretical maximum for a vertical wall is around 40 per cent. The Building Research Establishment guidance on site layout planning for daylight and sunlight treats 27 per cent as the figure above which a window receives enough skylight, with a secondary test applying where the value falls below it and also below 0.8 times its former value.

Both numbers are guidance. Neither is a standard, and the guidance says so in its own text.

What the measure records

The calculation is made at the centre of the outside face of the window, not inside the room. That is the first thing to understand about it, because it means vertical sky component says nothing about room depth, window size, internal reflectance, glazing transmittance or what the room is used for.

It measures obstruction. A large window and a small window in the same wall plane return the same value. A deep single aspect flat and a shallow living room behind identical windows return the same value. That is a strength when comparing before and after on the same facade, and a weakness when it is used as a proxy for the quality of a space.

The measure works well for what it was designed to do: screening a scheme quickly to find where an obstruction has changed materially. It works badly as a pass or fail gate, which is how it is often deployed at committee.

The reference value and the proportional test

The guidance sets up two tests that run together.

The absolute test asks whether the retained value reaches 27 per cent. If it does, the window has enough skylight and no further analysis is needed for that window.

The proportional test applies where the value falls below 27 per cent. If the retained value is also less than 0.8 times its former value, meaning a reduction of more than 20 per cent, occupants are likely to notice the loss.

The trap is treating the second row as a failure. It is not. A window dropping from 24 per cent to 21 per cent retains 0.875 of its former value and complies with the guidance on its own terms, even though it never reached 27 per cent at any point.

The other common error is averaging. Reporting a scheme average of 26 per cent tells a committee nothing about the one flat that went from 22 to 9. Report by window and by room, and say which rooms they serve. The article on presenting daylight results a committee can read deals with the format.

Why the figure is guidance rather than a rule

The guidance itself states that its numerical values are not to be applied rigidly, that they should be interpreted flexibly, and that in dense urban settings different targets may be appropriate where they are justified by the context. That wording is doing a lot of work in appeal decisions.

National policy supports the same reading. The Framework expects decisions to avoid rejecting well designed development for reasons that could be addressed through minor changes, and it expects an approach to daylight that recognises the benefits of making efficient use of land in accessible locations.

None of this means the measure can be ignored. It means the argument runs on evidence about context rather than on the number alone.

Evidence needed to justify a departure

A justified departure looks like this. A bare assertion that the site is urban does not.

1. The existing baseline, stated honestly, including whether the neighbouring window is already obstructed and by what. Where the adjoining site is undeveloped or underdeveloped, a mirror image or notional compliant baseline puts the change in context, and the reasoning is set out in mirror image baseline and existing obstructions.
2. The room served by each affected window, its use and whether it has other windows. A bedroom losing skylight matters less than a living room, and a room with two aspects is not the same case as a single aspect one. The consequences of these assumptions are covered in room use assumptions and how they change results.
3. Internal daylight results for the worst affected rooms, so the committee sees the actual condition rather than an external proxy.
4. A comparison with the prevailing pattern of development nearby, with measured values for existing buildings in the same street. Showing that surrounding windows already sit at 18 per cent reframes the whole discussion.
5. A tested alternative that would comply, with its consequences set out, whether that is fewer homes, a poorer street frontage or an unviable scheme.
6. The planning balance, stating plainly what the scheme delivers against the amenity effect.

Point four is the one most often missed and the one that carries most weight. If the local norm is already well below the reference figure, a proposal meeting the local norm is doing what the guidance asks.

How departures are weighed in practice

Inspectors and officers have accepted values below the reference figure where the site sits in a dense urban context, where the affected rooms are secondary, where the surrounding pattern already shows low values, and where the scheme delivers housing or other benefits of weight. These outcomes are indicative of how the issue has been approached rather than binding precedent, and each turns on its facts.

Departures fare badly where the applicant has not modelled the alternative, where the affected rooms are principal living rooms in family housing, where the reduction is severe in absolute terms, and where the baseline has been manipulated. Presenting a mirror image baseline without explaining it is the fastest way to lose an officer's trust.

Be honest about the worst case. A report that leads with the two windows that fail badly, explains them and then shows the rest of the scheme performing well reads as competent. One that buries them in an appendix reads as evasive, and officers do read appendices.

Presenting the result alongside other measures

Vertical sky component belongs in a set. Daylight distribution through the no sky line, annual probable sunlight hours for sunlight, and overshadowing to gardens and amenity space each answer a different question, and a Daylight and Sunlight Assessment that reports only one of them invites a request for the others.

Where the scheme affects amenity space rather than windows, an Overshadowing Study is the right document, and the two are often confused in validation correspondence.

This is general guidance, not legal advice. 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. For specialist technical reports, qualified professional input may be required.