Vertical sky component and the 27 per cent guideline

Vertical sky component is the proportion of light from an unobstructed overcast sky that reaches a point on a vertical surface, expressed as a percentage of the light that would reach an unobstructed horizontal surface at the same time. The measurement is taken at the centre of a window, on the outside face of the wall, and it takes no account of what happens inside the room. A completely unobstructed vertical wall reaches a little under 40 per cent, which is why 27 per cent is a guideline rather than a high bar.

Two tests run together. The BRE guidance suggests that daylight to an existing window may be adversely affected if the vertical sky component falls below 27 per cent and is less than 0.8 times its former value. Both limbs matter, and the second is the one that decides most cases.

What the number is measuring, and what it is not

VSC is a measure of the sky available at the window, nothing more. It says nothing about room depth, window size, internal reflectance, or whether the room has a second window on another elevation. A single deep room with one small window can have an excellent VSC and be gloomy inside.

That limitation is deliberate. VSC isolates the effect of external obstruction, which is exactly the question when assessing what a new building does to a neighbour. Room-based measures such as daylight distribution and the illuminance targets used for proposed interiors answer the other half.

So a report that presents VSC alone has answered one question. Where a room is deep or unusually configured, the Daylight and Sunlight Assessment should carry the daylight distribution results as well, and say what the room is used for.

The two tests and how they interact

Take a window with an existing VSC of 32 per cent, reduced by a proposal to 26 per cent. The absolute value is below 27, so the first limb is engaged. The ratio is 0.81, above 0.8, so the second is not. On the guidance as written, the effect is not considered adverse.

Now take a window with an existing VSC of 18 per cent falling to 13 per cent. The absolute value was already below the guideline before anything was built, and the ratio is 0.72. Here the proportional test governs and the loss is material, even though the absolute change is smaller.

That second case is the common one in urban areas and it is routinely misreported. Applying the 27 per cent figure as a pass mark to a window that never achieved it produces a nonsense result. Report the existing value, the proposed value and the ratio together, always.

Where the measurement is taken

At the centre of the window, on the external plane of the wall. Not inside the reveal, not at the head, not averaged across the elevation.

Three details change results and are worth checking in any report you receive.

Window position on the elevation matters. A ground floor window has a lower VSC than an identical window three storeys up, so a scheme affecting the lower floors of a terrace will always show the worst numbers there.

The model has to reflect what is actually built. Existing extensions, boundary walls above eye level, mature evergreen screening and neighbouring outbuildings all obstruct sky and all get omitted from hurried models. Omitting them inflates the existing baseline and makes the proposed loss look larger than it is, which is a rare case of an error that harms the applicant.

And the assessed window must be a real window. Assessing a rooflight, a door with a glazed panel or a window into a bathroom and reporting it alongside habitable rooms muddies the picture.

Balconies and overhangs

A projecting balcony above a window is the single most effective way to reduce its VSC, because it removes the upper part of the sky which contributes most of the light. Deep reveals, brise soleil, canopies and recessed entrances do the same at a smaller scale.

This matters in two directions. Where an existing building has balconies, the baseline VSC is already suppressed and the proportional test should be applied to that suppressed figure rather than to a hypothetical unobstructed one. Where the proposed building has balconies, its own rooms may fail the illuminance targets for reasons that have nothing to do with the neighbouring context.

The BRE guidance addresses this by suggesting an analysis with and without the balcony, so the effect of the new obstruction can be separated from the effect of the building's own overhang. Where balconies are relied on for private amenity space and cause a daylight problem, recessing them, using glass balustrades or setting the window head below the soffit are the usual fixes.

Reading a results table properly

A well-constructed table lists each window with a reference number tied to a plan, the room it serves, the room use, the existing VSC, the proposed VSC and the ratio. Anything less is not auditable.

Look for three things. Whether the failures cluster in one property or spread across many. Whether the affected rooms are primary living spaces or secondary. And whether the same room has another window that passes, because a room served by two windows where one fails is a different case from a single-aspect room losing its only source.

Percentages of windows meeting the guideline, quoted without the underlying table, tell you nothing. Ask for the table.

When a low result is still acceptable

The guidance is explicit that its numerical values are not mandatory and that different targets may be appropriate in dense urban areas. Decision-makers apply that in practice.

A shortfall is more readily accepted where the existing condition is already constrained, where the affected rooms are secondary or non-habitable, where the plan actively seeks higher density in that location, and where the scheme delivers benefits that carry weight in the balance.

It is accepted less readily where the loss falls on a single-aspect family home, where an alternative massing would avoid it, and where the applicant's own new homes would suffer the same conditions. A council can weigh harm to an existing neighbour against public benefit. It has less room to do that for future occupiers of the scheme itself.

Design changes that move the number

1. Set back the upper storeys facing the affected windows. The upper sky contributes disproportionately, so a two-metre setback at the top often does more than four metres at the base.
2. Increase separation across the boundary rather than uniformly across the site.
3. Chamfer or step the corner nearest the affected elevation instead of holding a flat facade.
4. Reduce parapet and plant enclosure heights where they sit above the affected sightline.
5. Break the mass with a light well or a gap on the boundary line, which helps daylight distribution as well as VSC.
6. Move the use rather than the building where the layout allows, so the affected neighbour faces circulation rather than a habitable elevation.

Where amenity space rather than windows is the concern, a separate Overshadowing Study covers sun on ground and is a different calculation entirely.

Illustrative example: a four-storey scheme opposite a converted terrace, where six ground-floor windows fell to a ratio of 0.74. Setting the third and fourth floors back by 2.5 metres on that elevation lifted four of the six above 0.8 and left two at 0.78, both serving rooms with a second window on the return. The revised report explained that, and the harm was assessed as limited.

Councils usually expect a suitably qualified author for daylight work, and professional review before submission is strongly advised. Reviewers can be found through the professional directory.

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.