Vertical sky component: how the 27 per cent test works
A window only falls short of the guidance where the proposed value is below 27 per cent and also below 0.8 times the existing value. Both limbs have to fail.
The vertical sky component test is the first calculation in any neighbour daylight assessment, and it is the one most often misapplied. A window passes where the vertical sky component at the reference point is at least 27 per cent. Where it falls below 27 per cent, the guidance treats the loss as noticeable only if the proposed value is also less than 0.8 times the existing value. Both limbs have to fail before there is a breach, and reports that treat 27 per cent as a pass mark on its own produce objections that do not survive scrutiny.
What the vertical sky component measures
The vertical sky component is the proportion of direct light from the sky reaching a point on a vertical surface, expressed as a percentage of the light that would reach an unobstructed horizontal surface under a standard overcast sky.
Two things follow from that definition. It measures sky visibility rather than illuminance inside the room, so it says nothing about the internal layout, the wall finishes or the glazing specification. And its theoretical maximum for a vertical window with no obstruction at all is under 40 per cent, because a vertical plane can only see half the sky and the standard overcast sky is brighter overhead.
That maximum is the reason 27 per cent matters. It is roughly the value a window achieves with an obstruction subtending 25 degrees to the horizontal, which is the geometry BRE 209 uses as its low-rise rule of thumb. So the 27 per cent figure and the 25 degree line are two expressions of the same threshold.
BRE 209 was reissued in 2022, and the current edition should be the one cited, because the earlier editions differ in how interior daylight is treated even where the neighbour tests are consistent.
Where the reference point sits on the window
Getting the reference point wrong is the commonest technical error, and it changes the answer.
The point sits at the centre of the window, on the outside face of the window wall. Not inside the room, not at the glass line where the frame is deeply reveal-set, and not at the sill or the head. For a window with a deep reveal the difference between the external wall face and the glazing plane can be enough to move a marginal result across the threshold.
Two situations need care. Where a window is under a balcony, a walkway or an overhang, the overhang reduces the vertical sky component independently of anything the development does, so report the result both with and without it and say which figure the assessment relies on. And where a bay window projects, the measurement position needs stating explicitly because there is more than one defensible choice.
Record the reference point on a drawing for every assessed window. It is the single piece of information a reviewing consultant will check first.
The 27 per cent figure and the 0.8 times ratio
The test has two limbs, and a window only falls short of the guidance when both are engaged.
| Result | Position under the guidance |
|---|---|
| Proposed value 27 per cent or more | Adequate skylight retained |
| Below 27 per cent, ratio 0.8 or more | Change unlikely to be noticeable |
| Below 27 per cent, ratio below 0.8 | Reduction likely to be noticeable |
So a window dropping from 34 per cent to 26 per cent has a ratio of 0.76 and falls below the threshold, which reads as a failure on a simple pass and fail count while being a small real change. A window dropping from 30 per cent to 12 per cent has a ratio of 0.4 and will feel different to the occupier. Report the existing value, the proposed value and the ratio for every window, so the extent of each shortfall is visible rather than hidden inside a count.
Remember that the vertical sky component is only one of three neighbour tests. Daylight distribution, measured through the no sky line within the room, and sunlight availability, measured as annual probable sunlight hours with a separate winter figure, address different things. A window can retain adequate sky visibility while the room behind it loses daylight across its depth, and reporting only the sky component conceals that.
Working the test on a terraced back addition
The most frequent real-world case is a single-storey or two-storey rear extension next to a neighbour's ground-floor window. The 25 degree check settles most of them without modelling.
Take an illustrative example. A neighbour's ground-floor window has its centre 1.5 m above external ground level, and the proposed extension has a flank wall 3 m high standing 3 m from that window. The vertical difference is 1.5 m over a horizontal distance of 3 m, which subtends about 26.6 degrees. That is above 25 degrees, so the preliminary check does not clear the scheme and the vertical sky component needs calculating properly.
Change one variable and the answer changes. The same 3 m wall set 4 m away subtends about 20.6 degrees and passes the preliminary check. A 2.7 m wall at 3 m away subtends about 21.8 degrees and passes. This is why the depth and the boundary offset of a rear extension matter more to the daylight result than its height does at these scales.
Those figures are illustrative geometry, not a substitute for assessment, and the angle is measured from the centre of the lowest affected window. The practical consequences for this scenario are set out in daylight and a single storey rear addition next door.
Windows the test does not suit
The guidance is aimed at habitable rooms in existing buildings, and applying it elsewhere produces results nobody should rely on.
- Rooflights, where the sky visibility from a horizontal or near-horizontal plane is a different calculation entirely.
- Non-habitable rooms such as bathrooms, halls, stairs, stores and garages, which are outside the scope and should be listed as excluded rather than quietly omitted.
- Windows serving a room that also has another window, where the room is what matters and the individual window result can mislead.
- Windows in commercial premises, where the guidance's residential amenity framing does not transfer and local policy usually governs.
- Windows serving a room whose use is assumed from outside, which happens often and should be stated as an assumption.
Say which rooms you treated as non-habitable and why. That single decision moves the headline percentage more than any calculation in the report.
Reporting the result window by window
Build the table for the person writing the committee report, not for the appendix.
Each row needs the address, the floor, the room reference, the room use, the window reference, the existing value, the proposed value, the ratio and a plain statement of whether the guidance is met. Sort by ratio with the worst first. Add the reference point position to the accompanying drawing, and state the software and the BRE 209 edition in a method note.
Then give the totals with their denominators. Windows assessed, windows meeting the guidance, and the distribution of the shortfalls by extent. A Daylight and Sunlight Assessment that volunteers its worst result is treated as reliable, and the rest of its numbers are then accepted without argument.
Where amenity space is also affected, the sun-on-ground assessment belongs in a separate Overshadowing Study, because the method and the thresholds differ.
What officers do with a marginal result
BRE 209 describes itself as guidance and says its numerical values are not to be applied rigidly, and case officers use that flexibility every week.
In practice a marginal result is weighed against context. In a dense urban area, where the existing values are already low and the development plan supports making effective use of land, officers accept ratios that would be resisted in a suburban street. Where the affected room is a bedroom rather than a living room, the weight given is lower. Where the shortfall affects one window of a room with two, it carries less weight again.
What officers will not accept is a marginal result presented as a pass. State the breach, state its extent, explain the context and explain the design alternatives considered. A report that argues 0.78 is effectively 0.8 loses credibility on everything else it says, and the flexibility argument works far better when the numbers are stated plainly, as discussed in daylight in a dense urban block.
PlanScope drafts a Daylight and Sunlight Assessment structure from your project details and your own modelling results, with "TO BE CONFIRMED" flags where values are missing. 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 prepared by a suitably qualified consultant.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. Enter your postcode on PlanScope to see which reports your scheme is likely to need.
Frequently asked questions
Is 27 per cent a pass mark on its own?
No. A window with 27 per cent or more retains adequate skylight. Below that, the guidance treats the reduction as noticeable only where the proposed value is also less than 0.8 times the existing value. Both limbs have to be engaged before there is a breach, so a window dropping from 30 to 28 per cent is not a failure.
Where exactly is the reference point?
At the centre of the window, on the outside face of the window wall. Not inside the room, not at the glazing plane in a deep reveal, and not at sill or head level. For deeply reveal-set windows the difference between the wall face and the glass line can move a marginal result across the threshold, so record the point on a drawing.
How does the 25 degree line relate to the 27 per cent figure?
They express the same threshold. An obstruction subtending 25 degrees to the horizontal from the centre of the lowest affected window produces roughly 27 per cent, which is why BRE 209 offers the angle as a preliminary low-rise check. If the obstruction subtends less than 25 degrees, no further assessment is normally needed.
How are windows under balconies handled?
The overhang reduces the vertical sky component independently of anything the development does, so report the result both with and without it and state which figure the assessment relies on. Doing only the with-balcony calculation attributes an existing constraint to the new building, which a reviewing consultant will pick up.
What do officers do with a result just below the threshold?
They weigh it against context. In a dense urban area with low existing values and policy support for effective use of land, ratios are accepted that a suburban street would resist. Bedrooms carry less weight than living rooms. What officers reject is a breach presented as a pass, which undermines the rest of the report.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.