Vertical sky component and no-sky line explained

The vertical sky component no sky line pair are the two tests that decide most daylight objections from existing neighbours. Vertical sky component measures how much sky a window can see. The no-sky line measures how far into a room daylight reaches. The BRE site layout guidance on daylight and sunlight sets numerical targets for both, and the proportional test that sits alongside them, the 0.8 times former value rule, is where arguments are actually won and lost.

What the vertical sky component measures

Vertical sky component is the ratio of the direct sky illuminance falling on a vertical surface at a reference point to the simultaneous horizontal illuminance under an unobstructed sky, expressed as a percentage. Under the CIE standard overcast sky used in the calculation, a completely unobstructed vertical wall has a value just under 40 per cent, so that figure is the ceiling against which everything else is read.

It is a measure of sky visibility at a point, not a measure of daylight inside a room. A window with a healthy value can still serve a dark room if the room is deep, the window is small or the reveal is deep. That is why the guidance pairs it with daylight distribution rather than relying on it alone.

Two features make it useful in practice. It depends only on the geometry outside the window, so it can be calculated without knowing anything about the interior. And it is quick, which makes it the right test for screening many neighbouring windows before detailed work begins.

The reference point and the 25 degree screen

The reference point sits at the centre of the window opening in the plane of the window wall. State the convention you have used in the report, because reviewers compare like with like and a shifted reference point produces results nobody can reconcile.

Before any of that, apply the screening test the guidance offers. Where no part of the proposed development subtends an angle of more than 25 degrees to the horizontal, measured in a vertical section perpendicular to the affected window from a point 2 m above ground level, diffuse daylight to that window is unlikely to be adversely affected. That one section drawing removes most windows from the assessment.

Use it honestly. The 25 degree screen applies to the whole of the proposed obstruction, so a tall element set back behind a lower frontage still has to be tested. And it is a screen, not a pass, so a window that fails the screen then needs the full calculation rather than a conclusion.

The numerical guidance and the proportional test

The guidance works as a two-part test. If the vertical sky component at the reference point is at least 27 per cent, enough skylight should still reach the window. If it falls below 27 per cent and is also less than 0.8 times its former value, the loss of light will be noticeable.

Both conditions have to be met for the guidance to indicate a problem, which is the single most misread sentence in the whole field. A window dropping from 38 per cent to 26 per cent breaches the absolute figure but retains 0.68 of its former value, so it fails both parts. A window dropping from 30 per cent to 27.5 per cent stays above the absolute figure and passes, whatever the proportional change.

Three points of judgement sit on top of the arithmetic.

The guidance is advisory, and the BRE text says so. It is a set of numerical targets to be applied flexibly, and in dense urban areas a lower absolute value can be acceptable where the existing condition is already constrained.

The baseline matters. Where the site currently holds a building that will be demolished, the existing condition includes that building. Where a consented scheme has not been built, the report should present both the as-existing and the consented baselines rather than choosing the convenient one.

And a low retained ratio on a window that was always poorly lit says something different from the same ratio on a window with an open outlook. Say which.

Daylight distribution and the no-sky line

The no-sky line divides the part of the working plane in a room that can receive direct skylight from the part that cannot. For dwellings the working plane sits 0.85 m above floor level, and the test is whether the area able to see the sky after the development is less than 0.8 times its former area. If it is, the guidance indicates that the loss will be noticeable.

This test catches schemes that vertical sky component alone lets through. A new building opposite can leave an acceptable sky component at the window while pushing the no-sky line back past the rear half of a deep living room, which is exactly the effect occupants complain about.

It needs internal information: room dimensions, window head heights, floor levels and the position of partitions. Where internal layouts are unknown, the usual approach is to obtain floor plans from the authority's records or to model a reasonable assumed layout and state the assumption clearly. Assumed layouts invite challenge, so it is worth asking for access.

Where overshadowing of gardens and amenity space is also in issue, that is a separate exercise again, and the Overshadowing Study deals with sun on the ground rather than sky at a window.

Which rooms get assessed

The guidance ranks rooms by how much daylight they need. Living rooms, dining rooms and kitchens matter most, bedrooms less, and bathrooms, WCs, halls, landings, stores and garages are not assessed.

Where a room has more than one window, the assessment considers the room as a whole, and daylight distribution is the better test than the sky component at each window. Where a window serves a room used for two purposes, assess it against the more demanding use. And where the room beyond is a conservatory or an enclosed balcony, the position is more complicated, because a secondary space can reduce light to the room behind it and the guidance treats the inner room separately.

Non-domestic buildings are assessed where daylight is needed for the use. An office with a daylit workspace is in scope. A warehouse, a plant room or a retail unit lit entirely artificially is not, although occupied offices within them are.

Modelling assumptions that move the result

Reviewers should interrogate these, and authors should state them without being asked:

• Whether balconies, overhangs, brises-soleil and recessed reveals on the affected building are modelled, because each reduces the sky component independently of the proposal.
• Whether trees and hedges are included, excluded or modelled as semi-permeable. Vegetation is usually excluded, and that should be said rather than left implicit.
• The baseline used: as-existing, as-consented, or both.
• Window head and cill heights taken from survey rather than assumed from elevation drawings.
• Whether the site's own existing building is in the baseline model.
• Floor level datums, since a 150 mm error across four storeys changes several results.
• The treatment of windows serving rooms whose use is unknown.

Assumption lists are not padding. A reviewer who can see the assumptions can check the conclusion, and a reviewer who cannot will ask for the model.

Reporting so a reviewer can check the numbers

A table that works has one row per window or room and these columns: building and floor, room reference, room use, window reference, existing value, proposed value, the ratio of proposed to existing, and whether the guidance is met. Add a column for the baseline used where more than one is presented.

Pair it with drawings: a plan showing every assessed room with reference points marked, a section through the critical relationship showing the 25 degree line, and a plan showing the no-sky line position before and after for the rooms that fail. The convention for presenting those results is set out further in the note on reading the results tables in a daylight and sunlight report.

What a case officer needs on top of the numbers is a short written conclusion that distinguishes windows failing both parts of the test from those failing one, identifies the rooms with the worst retained ratios, and says what design change was made in response. A Daylight and Sunlight Assessment that reports 400 windows and recommends nothing is a data dump rather than an assessment.

Illustrative example: a four-storey infill block opposite a Victorian terrace. Thirty-one windows fail the 25 degree screen. Of those, 24 retain more than 0.8 of their former sky component. Four ground-floor living rooms fall below 27 per cent and below 0.8, with retained ratios of 0.71 to 0.78, and two of them also lose more than a fifth of their daylit area. Setting the top storey back 2.5 m lifts three of the four above 0.8, and the remaining window is reported honestly as a residual effect.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required, and daylight modelling should be carried out or checked by a specialist through the professional directory before submission.