No sky line: daylight distribution inside a room
Daylight distribution measures how much of a room's working plane can still see sky after development. Room depth and window head height decide the answer, not the size of your building alone.
The no sky line daylight distribution test asks a simple question about a neighbouring room: how much of the working plane can still see a patch of sky after your building goes up. BRE 209 treats a reduction to less than 0.8 times the former area as noticeable to the occupants, and that ratio is the number a case officer will look for in your assessment.
It is a different test from the vertical sky component, which measures light arriving at the outside face of the window. Distribution is about what happens inside the room, and the two results can disagree in either direction.
What the contour actually shows
Plot a horizontal plane across the room at working plane height, which BRE takes as 0.85 metres above floor level in a dwelling. At every point on that plane, ask whether a direct view of the sky exists through the window.
The boundary between the points that can and cannot see sky is the no sky line. Everything in front of it receives direct skylight. Everything behind it is lit only by light reflected off surfaces outside and inside.
The contour moves when an obstruction rises outside the window, because raising the obstruction lifts the lowest visible sky angle and pushes the line back towards the window wall. That is the whole mechanism, and understanding it tells you immediately which design changes will move the number and which will not.
A Daylight and Sunlight Assessment reports the result as the ratio of the lit area after development to the lit area before, room by room.
Why room depth drives the result
Two geometric properties control where the line falls: the height of the window head above the working plane, and the depth of the room.
A high window head pushes the contour deeper into the room. A deep room puts more floor area behind the contour in the first place, so a small movement of the line strips out a larger proportion of what was lit.
This is why the same external obstruction produces very different results across a block. In a shallow room with a tall window the line may already sit at the back wall, and the whole working plane stays lit even after a substantial new building appears opposite. In a deep open plan flat with a standard window head, the line sits perhaps a third of the way in, and modest new massing can take the ratio below 0.8 on its own.
When you are assessing existing neighbouring properties, the room depths are not yours to change, which is exactly why the test can be the binding constraint on massing. Survey the internal layouts where access allows, because assumed room depths taken from an external survey are the weakest part of most reports.
The 0.8 area comparison
The comparison is between the area of the working plane receiving direct skylight before the development and the same area after it. Fall below 0.8 of the former value and BRE treats the loss as noticeable.
Three points about the ratio are worth keeping in mind.
It is a relative test, so a room that was already poorly lit can pass while a well lit room fails, even though the second room is still better lit after development than the first was before. That asymmetry is the main criticism of the method, and it is worth addressing head on in the report rather than waiting for an objector to raise it. Our note on low baseline conditions covers how to present those cases.
The guidance is advisory. BRE 209 says its numerical guidelines should be applied flexibly according to context, and the 2022 edition is more explicit than its predecessors about urban density. A borough with an adopted policy that references the BRE targets will still expect an explanation for each departure.
Percentages have to be read with absolute areas. A room losing 25 per cent of a 12 square metre lit area is a different proposition from one losing 25 per cent of 3 square metres, and reporting both keeps the discussion honest.
Rooms with more than one window
Where a room has two or more windows, the no sky line is computed from all of them together. A point on the working plane is lit if it can see sky through any window.
Corner rooms often perform well for that reason, and a scheme that obstructs only one aspect may barely move the contour. The trap is the room with a main window and a small secondary opening, where the secondary window contributes a narrow wedge of lit area that disappears entirely when the obstruction rises.
Model the room as built. Internal partitions, deep reveals, balconies above windows and projecting bays all change the sky visibility from points inside, and an assessment that treats every room as a simple rectangle with a flush window will overstate performance.
Living rooms, kitchens and bedrooms
BRE ranks rooms by how much daylight matters to their use. Main living rooms carry the most weight, kitchens next, and bedrooms least.
Open plan living, kitchen and dining rooms are now the common case in flats, and they should be assessed as living space rather than as a kitchen. Deep open plan rooms are also the hardest to pass on distribution, which is one reason the test bites harder on recent stock than on Victorian terraces with separate, shallow rooms.
Hallways, bathrooms, utility rooms and stores are not assessed. Do not pad a report with them to improve the overall pass rate, because it is transparent and it costs credibility on the rooms that matter. Where a scheme causes real loss to a habitable room, say so plainly and make the planning balance argument.
Where the result contradicts the vertical sky component
The two tests measure different things, so disagreement is normal rather than an error.
| Situation | Vertical sky component | Daylight distribution |
|---|---|---|
| Deep room, modest new obstruction | Often passes | Can fail |
| Shallow room, tall new obstruction | Can fail | Often passes |
| Room with a balcony above the window | Already low, small change | Line already close to window |
| Room with two aspects, one obstructed | Reduced on that face | Often little change |
When the results diverge, explain the geometry that causes it. An officer reading a vertical sky component failure alongside a distribution pass needs a sentence telling them the room is shallow and the window head is high, not a table with no commentary. The same applies in reverse when a neighbour objects on daylight grounds and the numbers appear to contradict their experience of the room.
Drawing the contour so it can be checked
An assessment that cannot be verified is an assessment that gets challenged. Include a plan for each assessed room showing the room outline, the window positions, the working plane height, the no sky line in the existing condition and the line in the proposed condition, with the lit areas hatched differently.
State the assumptions in the same place: the source of the internal layouts, the assumed window head heights, whether balconies and reveals are modelled, and the version of the guidance used. Quote both the ratio and the two absolute areas for every room.
Then give the reader the summary they need: how many assessed rooms meet the 0.8 ratio, how many fall below it, by how much, and what the affected rooms are used for. That is the paragraph the committee report will lift.
Browse all 66 report types to see how a daylight assessment sits alongside the rest of an application's evidence base.
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.
Frequently asked questions
What is the no sky line?
The boundary on a room's working plane between points that have a direct view of sky through a window and points that do not. BRE 209 takes the working plane as 0.85 metres above floor level in a dwelling. Raising an obstruction outside the window pushes the line back towards the window wall, reducing the lit area.
What does the 0.8 ratio mean?
It is the comparison between the lit working plane area after development and the same area before it. A reduction below 0.8 of the former value is treated in BRE 209 as noticeable to occupants. The guidance is advisory and intended to be applied flexibly according to context, so departures need explanation rather than automatic refusal.
Why does a room pass on vertical sky component but fail on distribution?
Because the two tests measure different things. Vertical sky component measures light reaching the outside face of the window, while distribution measures how far that light reaches into the room. A deep room with a standard window head can pass at the facade and still lose a large share of its lit floor area.
How are rooms with two windows assessed?
The no sky line is calculated from all windows together, so a point is lit if it can see sky through any of them. Corner rooms often perform well as a result. Watch rooms with one main window and a small secondary opening, where the narrow wedge of lit area from the second window can disappear entirely.
Which rooms need to be assessed?
Habitable rooms where daylight matters to the use. BRE gives most weight to main living rooms, then kitchens, then bedrooms. Open plan living, kitchen and dining spaces are assessed as living space. Halls, bathrooms, utility rooms and stores are not assessed, and including them to lift a pass rate is easily spotted.
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.