Worked example: daylight distribution and the no-sky line
An illustrative worked example of the no-sky line, the 0.8 times former area test, and why room geometry rather than the proposal often drives the result.
The no-sky line is the boundary on a room's working plane between the part of the floor that can see sky through the window and the part that cannot. A no sky line worked example makes the point faster than any description: in a 5 m deep room with a window head at 2.1 m, the sky-visible area shrinks when an opposing building rises, and the measure of harm is how much of that area is lost rather than how much light reaches the window.
Daylight distribution is the test that catches deep plan rooms, and it is distinct from the vertical sky component, which is measured at the window face. A room can pass at the window and fail inside it.
What the no-sky line represents
Stand at a point on the floor of a room, at desk height, and look up at the window. If any part of the sky is visible, that point is on the sky side of the line. If the view through the window is entirely of the opposing building, that point is behind the line.
The published guidance treats the area of the working plane receiving direct skylight as the measure of daylight distribution, and the relevant comparison is between the existing and proposed situations. Where the sky-visible area after development is less than 0.8 times its former area, the guidance treats the loss as likely to be noticeable to the occupants.
That multiplier is the number that matters in practice. It is a proportional test, so a room with a small sky-visible area to begin with loses its protection quickly, and a room with a generous one can absorb a substantial reduction and still pass.
Setting the working plane and the geometry
Four inputs define the calculation, and all four should be stated.
The working plane is a horizontal plane at a stated height above floor level, conventionally 0.85 m in residential rooms, which is roughly table height. Change the height and the line moves.
The room geometry: internal length, width and floor-to-ceiling height, with any projections, chimney breasts or internal partitions that obstruct the view.
The window geometry: width, head height, sill height and reveal depth. Head height does most of the work. A window head at 2.4 m pushes the no-sky line much deeper into the room than one at 1.9 m, which is why the simplest improvement to a daylight result is often raising the head rather than widening the opening.
The external obstruction: the height and distance of whatever is opposite, measured from the window. On an urban site that means the proposed building, and where relevant the existing building being replaced, since the comparison is against the existing condition.
A worked plan before and after
Illustrative example, and the figures are illustrative only.
Take a living room 3.6 m wide and 5.0 m deep, with a 2.0 m wide window centred in the end wall, head at 2.1 m and sill at 0.9 m. Floor to ceiling is 2.5 m, working plane at 0.85 m. Opposite the window, across a 12 m gap, sits an existing two-storey building 6.0 m high.
In the existing condition, the obstruction subtends a modest angle from the window, and the no-sky line falls roughly 3.6 m into the room. The sky-visible area is therefore about 3.6 multiplied by 3.6, which is 12.96 m² of a 18.0 m² room.
The proposal replaces the two-storey building with a five-storey block 15.0 m high at the same 12 m distance. The obstruction angle roughly doubles, and the no-sky line moves closer to the window, falling at around 1.9 m. The sky-visible area becomes about 1.9 multiplied by 3.6, which is 6.84 m².
Apply the test. 6.84 divided by 12.96 is 0.53, well below the 0.8 threshold, so the guidance would treat this as a noticeable loss of daylight distribution. The result also shows where the design response lies: reducing the height at the frontage, setting the upper storeys back, or increasing the gap are all more effective than anything done inside the neighbour's room.
In real work the line is plotted by software from a three-dimensional model rather than estimated from a section, and the plotted contour is rarely parallel to the window wall. The approximation above is good enough to understand the mechanism and nowhere near good enough for a report.
Deep plan rooms and why they fail
Three room types fail daylight distribution routinely.
Rooms deeper than about twice the window head height have a large area behind the no-sky line before any development happens. Their sky-visible proportion is small, so a modest absolute loss is a large proportional one.
Rooms served by a window under a balcony or a deep reveal start with a reduced view of sky, and the overhang acts as an obstruction before the external one is considered.
Through-rooms created by removing a dividing wall are the awkward case, because the combined room is deep and the rear portion may have been behind the line already. The treatment of a room with two windows is a separate question, and the baseline question of what the room looked like before is dealt with in the article on the mirror image baseline in a daylight assessment.
The honest point about all three is that the host room's geometry, not the proposal, is doing most of the damage. A report that says so, with the existing sky-visible proportion stated, gives the decision maker the context to weigh the result properly.
Reporting alongside the vertical sky component
The two measures answer different questions and should be presented together.
| Measure | Where it is taken | What it tells you |
|---|---|---|
| Vertical sky component | At the centre of the window on the outside face | How much sky the window can see |
| Daylight distribution (no-sky line) | On the working plane inside the room | How far into the room daylight reaches |
A window can retain a reasonable vertical sky component while the room behind it loses distribution, and the reverse happens where a room is shallow. Report both, per room, with the existing and proposed figures and the ratio.
Where the room is also assessed for sunlight, the annual probable sunlight hours measure is a third and separate test, applied only to windows within a certain orientation band.
Keep the presentation honest. A table listing only the rooms that pass is a presentational choice an officer's consultant will notice. List every assessed room, mark the failures, and discuss them in the text. The same discipline applies on the smallest schemes, where a single storey extension can take distribution from one neighbouring room while leaving every other result untouched.
Assumptions to state
Every no-sky line result rests on assumptions about a room the assessor may not have entered. State them explicitly:
- the working plane height used, and the source of that convention
- room dimensions, and whether they were measured internally or derived
- ceiling height, and the floor and ceiling build-up assumed if derived
- window head and sill heights, and how they were obtained
- whether internal partitions or projections were modelled
- the assumed room use, since the guidance distinguishes between room types
- the existing condition modelled, and its basis
Then say which way each assumption errs. An assessor who has assumed a shallower room than is likely has produced a more favourable result, and should say so.
A Daylight and Sunlight Assessment is a report type where councils expect a suitably qualified author and professional review is strongly advised. Where the question is loss of sun to gardens and amenity space rather than to interiors, that work belongs in an Overshadowing Study.
If you are assembling the design evidence around a scheme with daylight issues, the Design and Access Statement is where the design response to the result should be explained. 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.
Frequently asked questions
What is the no-sky line?
The boundary on a room's working plane between the floor area that can see sky through the window and the area that cannot. It measures how far daylight penetrates into a room, which is a different question from how much sky the window itself can see, and it is plotted from a three-dimensional model.
What is the 0.8 times former area test?
Where the area of the working plane receiving direct skylight after development is less than 0.8 times its former area, the published guidance treats the reduction as likely to be noticeable to occupants. It is a proportional test, so rooms with a small sky-visible area to begin with lose protection quickly.
What working plane height should be used?
Conventionally 0.85 metres above floor level in residential rooms, roughly table height. The height changes where the result sits, so the assessment should state the plane used rather than leaving it implicit. Non-residential rooms may use a different height depending on the assessed activity.
Why do deep rooms fail daylight distribution?
Rooms deeper than roughly twice the window head height already have a large area behind the no-sky line before any development. Their sky-visible proportion is small, so a modest absolute loss becomes a large proportional one. Balconies, deep reveals and removed dividing walls produce the same effect.
How is this different from the vertical sky component?
The vertical sky component is measured at the centre of the window on its outside face and describes how much sky the window can see. Daylight distribution is measured inside the room on the working plane. A window can retain a reasonable vertical sky component while the room behind it loses distribution.
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.