Vertical sky component and sunlight hours explained

Vertical sky component is a measure of how much sky a window can see, expressed as a percentage of the light available from an unobstructed sky at a vertical surface. The reference value in the BRE guide is 27 per cent, with a secondary test that a retained value of 0.8 times the former value is unlikely to be noticeable. Those two numbers carry most of the argument in a neighbouring amenity daylight assessment, and they are routinely misquoted as a standard. They are not. BRE 209 (2022) describes itself as guidance, and says the numerical values should be interpreted flexibly according to context.

Getting that framing right in the report changes how an officer reads the results.

What vertical sky component actually measures

VSC is calculated at the centre of the outside face of the window, on the plane of the glass, under a CIE standard overcast sky. It takes no account of the room behind the window, the glazing, the frame, internal reflectance or what the window is used for.

A completely unobstructed vertical wall gives just under 40 per cent. The 27 per cent reference value corresponds roughly to a continuous obstruction subtending 25 degrees at the window centre, which in a typical suburban street is a two-storey terrace across the road.

Because the calculation ignores the room, a bedroom window in a deep plan flat and a kitchen window in a shallow room return the same VSC for the same obstruction. That is a limitation of the test, and it is the reason the guide pairs it with a daylight distribution check.

The 27 per cent value and the 0.8 times test

The guide gives two conditions. Either the retained VSC is at least 27 per cent, or the retained value is at least 0.8 times the former value. Meet either and the loss of light to the existing window is unlikely to be noticeable.

The second limb does the heavy lifting in cities. A window that already sits at 14 per cent because of an existing building opposite does not need to reach 27 per cent after your scheme is built. It needs to retain 11.2 per cent.

The common mistake is reporting the percentage reduction without the absolute figures, or reporting the absolute figures without the baseline. Give both, in a table, window by window:

Figures here are illustrative and show the format rather than any real site.

Where a window fails, say what room it serves, how many other windows that room has, and whether the room is a bedroom, because the guide treats bedrooms as less demanding of daylight than living rooms and kitchens.

Daylight distribution and the no sky line

The no sky line is the boundary on the working plane between the part of a room that can see some sky from the plane and the part that cannot. Daylight distribution compares the area that can see sky before and after the development, with the same 0.8 reference ratio.

This is the test that catches deep rooms and rooms lit only by a window under a balcony or a recessed inset. A scheme can pass VSC comfortably at every window and still push the no sky line most of the way across a living room, because the obstruction geometry sits low and close.

Running distribution requires internal room layouts. On a neighbouring amenity assessment you often do not have them, and the honest answer is to say so, assume a plausible room depth from the elevation and the planning history, and label the assumption. Measured surveys of neighbouring interiors are rarely obtainable, and an inspector will accept a stated assumption more readily than a silent one. The companion article on annual probable sunlight hours deals with the sunlight side of the same study.

Annual probable sunlight hours and the winter proportion

APSH applies only to windows facing within 90 degrees of due south. A window facing north-east is simply out of scope, and saying so clearly saves an officer reading a page of irrelevant results.

The test has three parts working together. The window should retain at least 25 per cent of annual probable sunlight hours, of which at least 5 per cent should fall in the winter months from 21 September to 21 March. Where it falls below either figure, the loss is still considered acceptable if the retained value is at least 0.8 times the former value and the absolute reduction is no more than 4 percentage points of annual probable sunlight hours.

That last condition is the one most often dropped from reports. A window going from 70 per cent to 55 per cent retains 0.79 of its former value, which fails the ratio, but the practitioner who reports only the ratio has not told the officer the whole rule.

Gardens and amenity areas have their own sunlight test, based on the proportion of the area receiving at least two hours of sun on 21 March, which is a separate calculation and should be reported separately.

Target illuminance methods alongside the older tests

BRE 209 (2022) brought in the illuminance-based approach from BS EN 17037, which assesses the daylight actually reaching the working plane rather than the sky visible outside the window. For new residential space the UK National Annex sets minimum target illuminances of 100 lux in bedrooms, 150 lux in living rooms and 200 lux in kitchens, to be achieved over at least half the space for at least half the daylight hours.

These methods apply to the quality of the new accommodation you are creating. VSC, distribution and APSH remain the tools for assessing what your scheme does to existing neighbouring windows. Mixing the two is a frequent source of confusion at committee.

Illuminance modelling needs reflectances, glazing transmittance, maintenance factors and a credible room model, so it is only as good as the inputs. State them.

Flexible application in dense urban settings

The guide says explicitly that in historic city centres and areas of high density, different criteria may be appropriate, and that the numerical values are not mandatory. Several development plans, including the London Plan, pick this up and ask for an assessment appropriate to the context rather than mechanical compliance with the reference values.

In practice that means setting a local baseline. Survey the VSC of comparable existing windows in the same street or block, and show what the area's prevailing daylight condition already is. A proposal that leaves a window at 15 per cent in a street where the median is 16 per cent is a different proposition from the same figure in a suburb at 32 per cent.

Inspectors have been receptive to that reasoning where it is evidenced and unreceptive where it is asserted. Do the survey.

Presenting results so an officer can weigh them

Lead with a one-page table of every window tested, the room it serves, the three results and a plain pass or fail against each limb. Put the modelling methodology, sky model, software and survey basis in an appendix rather than the front of the report.

Then write the planning judgement separately: how many windows fail, by how much, to which rooms, whether alternative light is available, and what the context baseline is. That paragraph is what goes into the officer report, and if you do not write it the officer will write it for you.

Where numbers are close to a reference value, say so rather than rounding in your favour. The credibility of the whole document rests on the tests you report honestly as failing.

A Daylight and Sunlight Assessment drafted to this structure gives the case officer the arithmetic and the judgement in separate places, which is what makes it usable. Councils normally expect a suitably qualified author for this work, and professional review is strongly advised before submission.

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This is general guidance rather than 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.