VSC, NSL and ADF in a Daylight and Sunlight Assessment

Three metrics do most of the work in a vsc nsl daylight assessment, and they answer different questions. Vertical sky component measures how much sky a neighbouring window can see. No sky line measures how far daylight reaches into the room behind it. Average daylight factor, and the illuminance targets that have replaced it, measure daylight inside the proposed flats you are building.

Mixing them up is the commonest error in officer reports and objections alike, and it is worth being precise, because the numbers carry different weight in a committee.

Which metric answers which objection

Neighbour objections split into two kinds. "Your building will block the light to my window" is a VSC question. "My back room will go dark" is a no sky line question, because a room can keep reasonable VSC at the window and still lose daylight in its depth.

Applicant-side questions are different again. Whether the flats you are proposing will be pleasant to live in is assessed by internal illuminance under BS EN 17037, which the 2022 edition of BRE 209 adopted in place of the average daylight factor method that came from the withdrawn BS 8206-2.

Sunlight is a separate axis from daylight. Annual probable sunlight hours deals with direct sun to windows, and the sun-on-ground work belongs in an Overshadowing Study rather than in the window analysis.

Vertical sky component and the 27 per cent guideline

VSC is the ratio of direct sky illuminance falling on a vertical plane at the centre of a window to the horizontal illuminance under an unobstructed overcast sky, expressed as a percentage. The theoretical maximum for a vertical wall is around 40 per cent.

The BRE guideline has two limbs. Daylight to an existing window may be adversely affected if the VSC falls below 27 per cent and is less than 0.8 times its former value. Both limbs matter. A window already at 22 per cent that drops to 20 per cent has failed the first limb but comfortably passes the second, and that is a materially different case from a window falling from 30 to 20.

Three modelling conventions to state explicitly:

• VSC is measured at the outside face of the window, so window reveal and glazing are irrelevant to the number
• balconies and overhangs above a window reduce VSC sharply, and BRE 209 suggests reporting the result with and without the overhang so the reader can see how much of the loss is caused by the applicant's building
• the reference point is the centre of the window, so two windows in the same room can produce different results

The percentage looks precise. It is a modelled ratio under a standard sky, and a difference of one or two points is within the noise of how the surrounding context has been modelled.

No sky line and room depth

The no sky line, or daylight distribution, is the contour on the working plane inside a room dividing the area that can see some sky from the area that cannot. BRE guidance treats a reduction of the sky-visible area to less than 0.8 times its former value as noticeable.

Room depth drives the result more than window size does. A shallow room with a modest window can keep most of its area above the no sky line. A deep open-plan living and kitchen space with the same window will fail on the back third even before anything is built opposite.

Two practical points. Where the internal layout of a neighbouring property is unknown, assumptions have to be stated and justified, and an assumed layout that flatters the scheme will be challenged. And where the neighbouring building has been converted, the room use may have changed, so an office assessed as a living room produces a misleading result.

Non-habitable rooms are outside the guidance. Bathrooms, stores, circulation and, in most assessments, kitchens used only for cooking do not need to meet the targets, though a kitchen that is also the main living space does.

From average daylight factor to illuminance targets

The 2022 edition of BRE 209 moved internal daylight assessment onto BS EN 17037 and its UK National Annex. Rather than a single average daylight factor per room, the method sets a target illuminance to be achieved over a proportion of the reference plane for at least half the daylight hours in the year, with different targets by room type: lower for bedrooms, higher for living spaces, higher again for kitchens.

The consequence for a planning submission is that the calculation is a climate-based daylight model rather than a hand formula, and the reporting has to say which climate file, which reflectances and which reference plane height were used. Reviewers should ask for those inputs. Two consultants using different assumptions can produce noticeably different results for the same room.

Older local plan policies still refer to average daylight factor targets in percentage terms. Where that is the adopted policy wording, report both, and explain the relationship rather than quietly substituting one for the other.

Annual and winter probable sunlight hours

APSH applies to windows facing within 90 degrees of due south. The guideline is that such a window should receive at least 25 per cent of annual probable sunlight hours, including at least 5 per cent during the winter months from 21 September to 21 March.

As with VSC, the second limb concerns change: a loss is treated as noticeable where the window falls below both figures and receives less than 0.8 times its former sunlight hours. The winter test is the one that catches schemes, because a low winter sun is blocked by relatively modest buildings.

North-facing windows are excluded from the test, which is sometimes read as a loophole. It is not. It reflects the fact that a north-facing window never received direct sun to begin with, so there is nothing to lose.

Illustrative example: a window dropping to 18 per cent

Illustrative example. A first-floor living room window in a terrace opposite a proposed four-storey block currently has a VSC of 24 per cent. With the scheme modelled, it falls to 18 per cent.

The first limb is failed both before and after, since 24 per cent was already below 27. The second limb is what decides it: 18 divided by 24 is 0.75, below the 0.8 ratio, so the loss registers as noticeable under the guidance. The no sky line result for the same room shows the sky-visible area falling to 0.83 of its former value, which passes.

The honest report then says: one metric fails marginally, one passes, the room retains a usable outlook, and the existing baseline was already below the suburban target because the street is narrow. That is an argument an inspector can weigh. A report that presents only the passing metric is the one that loses at appeal. These figures are illustrative.

Applying the targets flexibly

BRE 209 states that its numerical guidelines are advisory and should be applied flexibly according to context, and the 2022 edition is more explicit about that than earlier versions. Targets derived from low-rise suburban layouts are not a sound basis for judging a site in a dense town centre where every existing window already sits below them.

The credible ways of showing context are a mirror-massing or notional-compliant-scheme comparison, an assessment of the existing baseline across the affected façades, and a comparison with the prevailing daylight conditions in the surrounding streets. State the alternative target you are proposing and why, rather than asserting that the guidelines do not apply.

Where a Daylight and Sunlight Assessment is presented alongside other amenity evidence, keep the receptor list, the addresses and the room references identical across documents. The same discipline applies to acoustic work, where the ProPG and BS 8233 sections of a residential noise assessment use their own receptor naming and frequently disagree with the daylight consultant's.

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