Vertical sky component

Vertical sky component is the number that decides most daylight objections between neighbours in England. It is the ratio of the direct sky illuminance reaching a point on a vertical surface to the illuminance that would reach an unobstructed horizontal surface at the same moment under a standard overcast sky, expressed as a percentage. Vertical sky component explained in one line: it measures how much sky a window can see.

The value for a completely unobstructed vertical wall is just under 40 per cent, because a vertical surface sees only half the sky dome and the standard overcast sky is brighter overhead than at the horizon.

What the measurement describes

The measurement is taken at a point, not across a room. That point is the centre of the window, on the outside plane of the wall, so the result describes the availability of skylight at the window rather than the daylight inside the room.

Two consequences follow. A window with a good value can still serve a dark room if the room is deep, and a window with a poor value can serve a bright room if the room is shallow and has a second window. The measurement tells you about the obstruction outside, and nothing at all about what happens once the light passes the glass.

That is why the building research guidance pairs it with a second test looking at daylight distribution inside the room, and why assessments that report one without the other are incomplete.

The sky model matters too. The standard overcast sky is a defined luminance distribution, not a real day, which makes results comparable between schemes and between assessors. Nobody is claiming it describes the weather.

The benchmark figure and the proportional test

The guidance gives a two part test for an existing window affected by new development.

If the value at the window is at least 27 per cent, enough skylight should still reach it and the effect is not normally treated as a material loss. If it falls below 27 per cent, the second limb applies: where the new value is also less than 0.8 times the former value, the reduction is likely to be noticeable to occupants.

Both limbs have to be failed before a loss is treated as significant under the guidance. A window dropping from 32 per cent to 28 per cent has lost more than 0.8 of its value in ratio terms but remains above the benchmark, so the guidance does not treat it as a noticeable reduction. A window dropping from 25 per cent to 24 per cent is below the benchmark but retains far more than 0.8 of its former value.

The 0.8 ratio is a proportional test, which is why an existing window that is already heavily obstructed is hard to affect further in percentage terms. That is a real effect rather than a loophole, and it is also the point objectors find most counterintuitive.

Where the 27 per cent figure comes from

It derives from the skylight available to a low rise suburban context, which is the setting the original research addressed. Applied to a dense city centre, it produces results that almost nothing achieves.

Authorities have responded in different ways. Some apply the guidance as written and rely on the proportional limb. Some set an alternative target reflecting the prevailing character of the area, with the justification set out in a supplementary document or a design code. London policy takes a flexible approach, expecting the assessment to be interpreted in the context of the site and the prevailing pattern of development rather than applied mechanically.

The guidance itself says its numerical values are advisory, that they should be applied flexibly, and that different targets may be appropriate in particular circumstances. Quoting that passage is more useful than arguing about it.

So a report that presents the numbers and stops is doing half the job. The other half is explaining what the numbers mean for this location, against this baseline, for these rooms.

Daylight distribution as the companion test

The distribution test looks at the area of the working plane inside a room that can see the sky, bounded by the no sky line. Where the area receiving direct skylight after the development is less than 0.8 times the area before, the guidance treats the loss as noticeable.

It is the test that catches deep rooms and rooms lit by a single small window, and it frequently gives a different answer from the window based measurement. A room can retain an acceptable value at the window while losing a large part of its sunlit and skylit floor area.

Running both tests requires a room layout, which means existing floor plans for neighbouring properties. Where those cannot be obtained, assumptions have to be made and stated, and the approach to inaccessible properties is a recurring source of dispute between assessors.

Sunlight is assessed separately again, by annual probable sunlight hours for windows facing within 90 degrees of due south, and by the sun on ground criterion for external amenity space.

How officers read the results

Case officers look for four things, usually in this order.

How many windows fall below the benchmark, and in which rooms. Whether those rooms are habitable, since a loss to a bathroom or a hall carries little weight. What the proportional retention is for each affected window. And what the baseline was, because a scheme replacing an existing building is assessed against the existing obstruction rather than against an empty site.

The fourth point does most of the work in redevelopment cases. Where a site currently holds a building, the comparison is with that building in place, and a proposal that is taller but set further back can improve results at some windows while worsening them at others.

A summary table with a traffic light column is read. Forty pages of appendix output is not. Put the conclusions where an officer will find them, and keep the full output as an appendix for the neighbouring owner's consultant.

Presenting the numbers clearly

Report per window, with a stable reference for each, and give the existing value, the proposed value, the ratio, the room served and the room use.

Then give three summary figures: the total number of windows assessed, the number meeting the guidance on either limb, and the number failing both. State the method and the software, the massing included, the baseline used and any assumption made about room layouts.

Where windows fail, say so plainly and address the magnitude. Assessments that bury failures in a table and describe the overall effect as acceptable in the narrative attract exactly the scrutiny they are trying to avoid.

A Daylight and Sunlight Assessment covering the window based measurement, the distribution test, sunlight to windows and the sun on ground criterion for amenity space keeps the assumptions consistent in one document. For the method itself, including how the calculation is carried out and where the reference point sits, see our fuller explanation of the BRE 209 daylight test. Where amenity space rather than windows is the issue, the Overshadowing Study is the relevant document.

Councils expect a suitably qualified author for daylight work, and professional review before submission is strongly advised.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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