Vertical sky component: the BRE 209 daylight test explained

Vertical sky component is the daylight measure that decides most neighbour objections in England. It expresses, as a percentage, the amount of visible sky at the centre of a window on the outside face of the wall, compared with what an unobstructed vertical surface would receive under a standard overcast sky. The BRE guidance treats 27 per cent as the reference figure, and a proposal that leaves a neighbouring window with less than 0.8 times its former value is likely to produce a noticeable loss of light. Those two numbers do a great deal of work, and they are frequently misapplied.

What VSC measures at a window

The measurement point is the centre of the window on the outer face of the wall, not inside the room. VSC therefore tells you about the availability of skylight at the window plane, and it says nothing about what happens to that light once it enters the room.

The theoretical maximum for a completely unobstructed vertical wall under the standard overcast sky is just under 40 per cent, because a vertical surface can only see half the sky dome and the standard sky is brighter overhead than at the horizon. That is why the reference figure is 27 rather than something closer to 100.

Two consequences follow. A ground floor window in any urban street will have a lower VSC than a third floor window in the same building, purely because of the buildings opposite. And an improvement in VSC of one or two percentage points is a large relative change at the bottom of the range and a trivial one at the top.

The 27 per cent figure and the 0.8 ratio

The guidance sets out a two-part test for existing windows affected by new development. If the VSC at the window is at least 27 per cent after the development, skylight is likely to remain adequate and no further investigation is needed. If it falls below 27 per cent, the question becomes whether the new value is less than 0.8 times the former value.

A window at 30 per cent reduced to 26 per cent falls below the reference figure, but the ratio is around 0.87, so the loss is unlikely to be noticeable. A window at 20 per cent reduced to 14 per cent has a ratio of 0.7 and a real loss, even though the absolute reduction is smaller in one sense.

Report both figures for every window. A table showing existing VSC, proposed VSC, the ratio and whether the window meets either limb of the test is what an officer needs. Reporting only the percentage change, or only the absolute values, invites a challenge.

Windows serving non-habitable rooms, such as bathrooms, stores and circulation, carry much less weight, and windows serving rooms with a limited expectation of daylight should be identified as such rather than quietly excluded.

The 25 degree line as a first screen

Before any calculation, the guidance offers a section-based screening test. Draw a line at 25 degrees above the horizontal from the centre of the lowest window of the affected building. If the new development sits below that line, there is unlikely to be a substantial effect on daylight, and detailed calculation is not needed.

The test is quick, it is drawn rather than modelled, and it is the right way to scope an assessment. Applying it to the lowest window in each affected elevation identifies the windows that need proper analysis and rules out the rest.

Two cautions. The line is measured from the lowest window, so a scheme that clears it for the first floor may still fail at ground floor. And it is a screening test rather than a pass, so a development below the line still needs the section drawing in the report as evidence.

No sky line and room depth effects

Daylight distribution is the second test and it looks inside the room. The no sky line divides the points on the working plane that can see some sky from those that cannot, and the guidance compares the area receiving direct skylight before and after, using the same 0.8 ratio.

This is where room shape matters. A deep room behind a single window has a no sky line close to the window even before any development, so a modest new obstruction can move a large proportion of the room into the zone with no sky view. A shallow room with a wide window is far more resilient.

Distribution analysis needs internal layouts for the affected property, which you may not have. Where layouts are unknown, the assessment should say so, state the assumptions made about room use and depth, and explain how those assumptions were derived. An assessment that quietly assumes a shallow room in a Victorian terrace is not credible.

Where VSC misleads and what to say about it

VSC is a blunt instrument used precisely, and an honest report says where it does not fit.

• Balconies and overhangs above a window reduce VSC regardless of what happens across the street. The guidance recognises this, and the assessment should present results with and without the overhang so the effect of the proposal itself is visible.
• The reference figures were derived from a low-rise suburban context. In a dense town centre, existing values are often already well below 27 per cent, and the guidance itself says the numerical targets should be applied flexibly with regard to context.
• VSC ignores the room behind the window entirely, so two windows with identical VSC can serve rooms with completely different daylight conditions.
• Reflected light from light-coloured facing surfaces is not captured.
• A window with a very small opening area can have a high VSC and still light a room poorly.

Say these things in the report rather than leaving an objector to say them. A Daylight and Sunlight Assessment that acknowledges the limits of the method reads as competent. One that presents the numbers as absolute reads as advocacy.

Presenting results so an officer can weigh them

The officer has to write a report that a committee and, potentially, an inspector will read. Give them the material to do it.

1. A plan and elevation identifying every assessed window with a reference number.
2. A table per property: window reference, room served, room use, existing VSC, proposed VSC, ratio, and whether the test is met.
3. The same for daylight distribution, with the assumptions about room layout stated.
4. A short summary of transgressions, giving the number of windows affected, the magnitude of the shortfall, and the rooms involved.
5. Context: the character of the area, the existing values, and whether the affected windows were already below the reference figure.
6. Any mitigation incorporated, such as a set back, a chamfer to the massing, or a reduction in the upper storeys.

Where the scheme also affects gardens and amenity areas, sunlight to those spaces is a separate test, and an Overshadowing Study covering the equinox and the solstices answers a different question from the window analysis. Do not merge them.

Finally, resist rounding in your favour. Reviewers recalculate, and a table that rounds 0.79 up to 0.8 costs more credibility than the window was worth. Browse the report types on PlanScope if you are scoping the daylight evidence for a scheme next to existing homes.

This is general guidance, not legal advice, and the BRE guidance has been revised, so check the current edition and any local supplementary guidance before relying on any figure here. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.