Reporting VSC and NSL results in a daylight assessment

Vertical sky component and no sky line answer different questions, and a daylight report that presents them in one undifferentiated table invites an officer to treat a technical pass as a failure. VSC measures how much sky a window can see. NSL measures how much of the room behind it can see any sky at all. A scheme can comfortably satisfy one and breach the other, and the planning consequence of each breach is not the same.

The reporting job is therefore to make the distinction legible, to give absolute values alongside ratios, and to say plainly where the guidance is advisory rather than fixed.

What the two measures describe

Vertical sky component is the ratio of the direct sky illuminance falling on a vertical plane at a reference point, usually the centre of the window on its outside face, to the illuminance on an unobstructed horizontal plane under a standard overcast sky. It is expressed as a percentage, and an entirely unobstructed vertical window gives just under 40 per cent.

No sky line, also called daylight distribution, is a contour drawn on the working plane inside the room. On one side of the contour a point can see some part of the sky through the window. On the other side it cannot. The measure reported is the area of the room that can see sky, before and after the development.

The difference matters because they respond to different geometry. A tall obstruction directly opposite a window cuts VSC sharply. A deep room with a small window has poor daylight distribution whatever is outside it. And a new building set further back but much wider can leave VSC almost unchanged while pushing the no sky line deeper into the room.

Both are set out in the Building Research Establishment's site layout guidance, which is the reference the Daylight and Sunlight Assessment should name by edition.

The numerical criteria and where they come from

The guidance sets thresholds rather than standards.

Two points about that table are routinely misread. The 27 per cent figure is not a pass mark that planning policy requires; it is the level at which the guidance suggests skylight is adequate. And the 0.8 ratio applies as a second test where the absolute figure is not met, not as an independent requirement.

The guidance itself says its numerical values are advisory, that they should be applied flexibly, and that in dense urban areas different target values may be appropriate to the local context. Quote that in the report. It is the sentence that allows a scheme in a city centre to be assessed against the pattern of its surroundings rather than against a suburban expectation, and leaving it out concedes ground that the guidance does not require you to concede.

Absolute values against relative change

Report both, side by side, always.

The reason is arithmetic. Where an existing VSC is 30 per cent and falls to 23, the room has lost a meaningful amount of sky and the ratio is 0.77. Where an existing VSC is 6 per cent and falls to 4.5, the ratio is 0.75, formally a worse retained proportion, and the real change is a percentage and a half of sky at a window that was already deeply obstructed.

A table showing only ratios makes those two look comparable. A table showing existing value, proposed value, absolute change and ratio lets the reader see that they are not, and lets the planning judgement follow the substance.

The same applies to no sky line. A room losing 30 per cent of a large well lit area is different from a room losing 30 per cent of an area that was already a strip by the window. Give the areas in square metres as well as the ratio.

Room use and the weight it carries

The guidance applies to main habitable rooms, and it is explicit that they are not equally sensitive. Living rooms carry most weight. Dining rooms and kitchens carry less, with kitchens sometimes treated as secondary where a separate living room exists. Bedrooms are less sensitive again. Bathrooms, cloakrooms, halls, landings, stores and circulation space are not assessed.

That hierarchy is one of the most useful tools in a report, and it is frequently thrown away by presenting every room in one list. Group the results by sensitivity, and where a breach falls entirely in bedrooms and kitchens, say so in the summary rather than burying it in an appendix.

Where the room use in a neighbouring property is unknown, state the assumption and state its direction. Assuming every room is a living room is a cautious approach and it should be labelled as one, because an officer reading a list of living room breaches will weigh them differently if they might be bedrooms.

Rooms already below the target

Existing sub-standard conditions are where most arguments happen, and they are common in dense areas, in basement flats, and behind balconies.

Where a window already has a VSC well below 27 per cent, the absolute test cannot be met by any scheme, so the relative test is the only live question. Report it that way explicitly: state the existing value, note that the window is already below the guidance figure, and then discuss the proportional change and its practical effect.

Two things help the reader. First, a short note on why the existing condition is poor, whether that is an opposing building, a deep balcony above, a narrow street or the room's own depth. Second, a sensitivity check where the ratio marginally fails, showing what the retained figure would be with a modest design change, because that is the information an officer needs before asking for one.

Resist the temptation to describe an already obstructed window as therefore unaffected. It is a judgement for the decision maker, and a report that appears to argue away every breach loses credibility on the ones that matter.

Balconies, unknown interiors and stated assumptions

Balconies above a window reduce VSC substantially, and where the balcony belongs to the same building as the window it is arguable that its effect should be isolated from the effect of the new development. The usual approach is to report both: the window as it exists with the balcony, and the same window analysed without the overhang, so that the change attributable to the proposal is visible.

Interiors of neighbouring buildings are almost never known. Room depths, partition positions and window heads are inferred from external survey, from historic drawings if they exist, and from the building's typology. Say so, in a methodology section, with the assumptions listed. An assessment that presents inferred room layouts as measured fact is vulnerable to a single objector with a tape measure.

State the model extents, the software, whether the assessment is against the existing situation or an extant consent, and whether cumulative effects with other consented schemes have been included. Each of those changes the numbers.

The table format officers can follow

A format that survives a committee report has five features.

1. One summary table on a single page, listing each affected property with the number of rooms assessed and the number meeting the criteria, grouped by room sensitivity.
2. A detailed schedule with one row per room, giving a reference keyed to a plan, room use, existing VSC, proposed VSC, absolute change, ratio, existing and proposed no sky line areas and the NSL ratio.
3. Plans and elevations showing every assessed window with its reference, so that a reader can find room R2/14 without reading the appendix.
4. A short narrative for each property where breaches occur, explaining the geometry that causes them.
5. A methodology section with the guidance edition, the software, the assumptions and the baseline.

Where open space rather than windows is affected, the analysis moves to sun on ground and belongs in an Overshadowing Study rather than in the VSC and NSL tables. The dates and plot conventions for that work are covered in shadow plots and dates in an Overshadowing Study.

One honest caveat to include in every report of this kind: the guidance is advisory, the room layouts behind neighbouring windows are usually assumed rather than measured, and the numbers are a tool for judgement rather than a test with a pass mark.

This is general guidance, not legal advice. Daylight and sunlight assessment is a specialist exercise and local policies vary, so confirm the approach with the local planning authority or a qualified consultant before relying on it.

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