Vertical sky component and no-sky line: the two tests
Vertical sky component and the no-sky line answer different questions: how much sky reaches the window, and how much of the room can still see sky. Objections routinely conflate them.
Vertical sky component and the no-sky line are not two versions of the same check. Vertical sky component is measured on the outside face of the window wall and tells you how much sky is available at that window. The no-sky line is drawn inside the room, on the working plane, and tells you how much of the floor area can still see a patch of sky. A neighbouring window can hold a perfectly respectable vertical sky component while the back half of the room behind it goes dark, and the reverse happens too. That is the whole reason both tests exist in BRE 209 (2022 edition), and the reason a report that gives only one of them invites a challenge.
Both are guidelines, not standards. BRE says its numerical values should be applied flexibly and that other target values may be appropriate in dense urban settings. Officers and inspectors take that seriously, which cuts both ways for an objector and an applicant.
What each test measures and where it is taken
Vertical sky component is the ratio of the direct sky illuminance falling on a vertical surface at a reference point to the illuminance on an unobstructed horizontal surface, under a CIE standard overcast sky, expressed as a percentage. The reference point is the centre of the window, on the outside of the wall. Because it sits outside the building, the glazing, the room depth, the internal finishes and the window size have no effect on the result at all. It is a measure of the sky the obstruction leaves behind.
Daylight distribution works the other way round. A no-sky line is plotted across the working plane, taken at 0.85 metres above floor level in dwellings, separating the part of the room that has a direct view of sky through the window from the part that does not. What is compared is the area on the sky side before and after the development.
So one test is about the availability of daylight, the other about its penetration into the room.
The window test and its numerical guideline
The guideline most people quote is 27 per cent. If the vertical sky component at the centre of a neighbouring window is 27 per cent or more after the development, the daylight effect is unlikely to be noticeable. An unobstructed vertical wall reaches roughly 40 per cent, so 27 per cent is already a meaningful degree of obstruction: it corresponds broadly to a continuous obstruction at about 25 degrees above the horizontal from the window centre.
Where the result falls below 27 per cent, the second limb applies: the retained value should be no less than 0.8 times the former value. A window at 30 per cent falling to 24 per cent retains exactly 0.8 and sits on the guideline. A window at 12 per cent falling to 10 per cent is well under 27 per cent but retains 0.83, and BRE would treat that as an unlikely-to-be-noticed change.
That second limb is where low-rise suburban schemes and dense city schemes part company. In a terraced street with deep rear projections, plenty of windows already sit in the teens, and the proportional test carries the analysis. A Daylight and Sunlight Assessment that reports only absolute values in that context is doing half the job.
The room test inside the room
Daylight distribution is reported as the retained proportion of the sky-lit area on the working plane. The same 0.8 figure appears: if the area of the room that can receive direct skylight is reduced to less than 0.8 times its former size, the change is likely to be noticeable.
The inputs are different, and that matters when you are checking someone else's report. A distribution calculation needs the internal geometry: room depth, window head height, cill height, wall thickness, reveal depth and the position of any internal partitions. Those come from a measured survey or, more often on objection work, from assumption. A report that models neighbouring interiors from estate agent photographs should say so plainly, and the assumptions should be listed.
Room use also matters. BRE treats living rooms as more sensitive than bedrooms and kitchens, and excludes bathrooms, WCs, halls, garages and storage. An objection that counts a downstairs cloakroom window as a daylight failure is easy to answer. Our article on average daylight factor and where it still applies covers the third, less common measure that sometimes appears alongside these two.
Why a window can pass one and fail the other
Window head height is the usual explanation. Light penetration into a room is governed mostly by how high the top of the window sits, because the no-sky line is set by the sightline from the working plane over the obstruction and through the window head. A tall obstruction opposite reduces vertical sky component evenly across the window. A low, close obstruction, or a deep reveal, or a balcony soffit, can leave the upper part of the window clear, keeping vertical sky component respectable while pushing the no-sky line forward in the room.
The opposite case is a shallow room with a large window facing a distant tall building. Vertical sky component drops, because the sky really is obstructed. The room is shallow enough that almost all of it still sees a strip of sky, so the distribution result barely changes.
Illustrative example. A ground-floor living room 6 metres deep with a 2.1 metre head height sits opposite a new four-storey block 12 metres away. The vertical sky component at the window falls from 28 per cent to 22 per cent, a retained ratio of 0.79, marginally outside the guideline. The sky-lit area on the working plane falls from 84 per cent of the room to 71 per cent, a retained ratio of 0.85, inside the guideline. Both numbers are illustrative, but the pattern is one you will meet repeatedly: a marginal window result alongside an acceptable room result, which is a materially different planning judgement from both failing.
Retained ratio and the proportional guideline
The 0.8 multiplier appears in both tests and in the annual probable sunlight hours test as well. It is not a compliance threshold and it is not defined as a percentage of windows that may fail. It is the point at which BRE considers the change perceptible to an occupant.
Two habits make that more useful in a report. State the absolute before and after values as well as the ratio, because a ratio alone hides whether a room started well lit or badly lit. And group the results by property and room use rather than by window number, since the decision maker is judging harm to homes.
Reporting both tests in one table
| Measure | Where measured | Guideline | Sensitive to |
|---|---|---|---|
| Vertical sky component | Centre of window, outside face of wall | 27 per cent, or 0.8 times former value | External obstruction only |
| Daylight distribution (no-sky line) | Working plane, 0.85 m above floor | 0.8 times former sky-lit area | Room depth, window head height, reveals |
| Annual probable sunlight hours | Centre of window, within 90 degrees of due south | 25 per cent annual and 5 per cent winter, or 0.8 times former | Orientation and obstruction |
Presenting the three together, per room, with the room use named, is the format officers find easiest to use. Where an Overshadowing Study has also been prepared for gardens and amenity space, keep it separate rather than merging it into the window tables.
How decision makers weigh a marginal failure
A breach of a BRE guideline is a material consideration, not a refusal reason in itself. What tends to move an officer is the pattern: how many rooms, in how many homes, by how much, in what kind of area, and whether the affected rooms are living rooms or secondary spaces. A scheme that takes one bedroom window from 0.82 to 0.78 retained vertical sky component in an inner London context is on very different ground from one that halves daylight to living rooms across a whole terrace.
Context arguments have to be evidenced rather than asserted. If you are arguing that lower target values suit the area, show the existing baseline: the vertical sky component of nearby windows in the same street pattern, or the density and storey heights of the surrounding block. That is the part most objections and quite a few applicant reports skip. Advice on framing this for the submission is in our guide to writing the amenity section of a design and access statement.
A daylight and sunlight report is specialist work, and councils expect a suitably qualified author. PlanScope can draft the surrounding planning documents and flag where the daylight evidence is missing, and the report should be reviewed before submission. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.
Frequently asked questions
Is 27 per cent vertical sky component a legal requirement?
No. It is a numerical guideline in BRE 209, which states that its values should be applied flexibly and that different targets may suit dense urban areas. Councils adopt it through local plan amenity policies and supplementary guidance, so the weight it carries varies. A result below 27 per cent is a material consideration to be weighed, not an automatic reason for refusal.
Which test matters more to a planning officer?
Neither on its own. Officers look for a consistent picture across both tests and across the affected homes, weighted by room use. A window failing the vertical sky component guideline while the room retains most of its sky-lit area reads very differently from a case where both measures drop sharply. Report both, per room, with the room use stated.
What working plane height is used for the no-sky line?
For dwellings the working plane is taken at 0.85 metres above floor level, which represents table height. Commercial assessments commonly use 0.7 metres for desk height. The plane is set 0.5 metres in from the surrounding walls when the sky-lit area is calculated, so the geometry of the room, not just the window, changes the result.
Do balconies affect vertical sky component?
Yes, and often sharply. An existing balcony or deep overhang above a window blocks a large part of the sky the window would otherwise see, so the absolute value starts low and a modest new obstruction produces a large proportional loss. BRE suggests analysing the window both with and without the balcony so the decision maker can see the effect of each.
Does BRE 209 apply to bathrooms and hallways?
Not usually. The guidance is aimed at rooms where daylight is expected, chiefly living rooms, dining rooms, kitchens and bedrooms, with living rooms treated as the most sensitive. Bathrooms, WCs, halls, landings, garages and storage are normally excluded. Check whether an objection or a report has included non-habitable rooms in its failure counts.
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.