No Sky Line and Daylight Distribution

No Sky Line (NSL), sometimes called Daylight Distribution or DD, is the BRE 209 test that maps the part of a room which can still see at least a patch of sky once a development is built. It is measured at working-plane height (0.85m above floor) and reported as a percentage of the room area in front of the NSL contour. The 0.8 ratio applies: the lit area should not fall to less than 80% of its existing value. NSL is the test that catches deep-plan rooms where VSC alone underestimates loss.

What is the No Sky Line?

Imagine standing in a room and slowly walking from the window towards the back wall. At some point, you reach a line beyond which you can no longer see any sky out of the window — the obstructions outside (eaves, opposite buildings, the proposed development) block all of it. That line, projected onto the working plane at 0.85m above the floor, is the No Sky Line.

BRE 209 reports two numbers per room:

• The area of the room in front of the existing NSL (baseline lit area).
• The area of the room in front of the proposed NSL (post-development lit area).

The ratio of these two figures is the daylight-distribution ratio. The 0.8 BRE threshold applies: a ratio below 0.8 (a 20%+ loss of lit area) is a noticeable effect.

How is NSL calculated?

1. Build a 3D model of the room with accurate window-head height, sill height, depth and ceiling height.
2. For every point on the working plane (0.85m above finished floor level), trace a ray from that point through the window head to the sky.
3. If any patch of sky is visible above the obstructions, the point is "in the lit area". If the obstructions (existing or proposed) block the entire visible sky from that point, the point is behind the NSL.
4. The NSL is the boundary between the two regions. The percentage of the room area in front of that boundary is the metric reported.

The window-head height is the most sensitive parameter. A higher window head pushes the NSL further into the room (more sky visible from the back); a low window-head with a tall obstruction outside pulls the NSL towards the window. This is why the test can produce sharply different results from VSC on the same window: VSC only looks at a single point on the outside of the glass, while NSL probes the whole room.

How does NSL differ from VSC?

The two tests are complementary. BRE 209 expects both to be reported together: VSC gives the headline daylight availability at the window, NSL shows whether daylight reaches the back of the room.

How to read NSL plans in a daylight and sunlight report

NSL is reported in two places in a typical BRE 209 daylight and sunlight report:

1. NSL plans. A plan of each affected floor showing the existing NSL contour (usually a solid line) and the proposed NSL contour (usually a dashed line). The shaded area between them is the loss.
2. NSL tables. A table per receptor, with columns for room area, existing lit area, proposed lit area, and the ratio. The ratio column is what you compare to 0.8.

On the plans, look for rooms where the proposed contour sits substantially closer to the window than the existing contour. On the tables, look for ratios printed in the 0.5–0.8 range — these are the receptors most likely to be argued over at committee.

Common pitfalls and challenges

• Wrong working-plane height. Working plane is 0.85m above finished floor level for residential rooms. Some assessments use 0.7m or 1.0m; the difference shifts the NSL noticeably.
• Window-head height assumed not measured. Where survey access was not granted, applicants sometimes assume a generic 2.0m head height. Real Edwardian or Victorian sashes may have heads at 2.4m or higher — understating the head pulls the NSL towards the window and overstates the proposed lit area.
• Room boundary drawn at the inner face of the window. The room area should include the window reveal, not start at the inner glass face. Trimming the area inflates the lit-area percentage.
• Open-plan rooms split. A single open-plan living-dining room is sometimes split into two notional rooms in the assessment, hiding loss at the back. Check the plan for room boundaries that don't match a wall.
• NSL omitted entirely. Some applicant reports only run VSC. BRE 209 expects NSL where there is a meaningful change to the obstruction; if NSL is absent and the obstruction is tall and close, that is a ground for challenge.

How to challenge NSL in an objection

1. Identify the room and floor. Reference the NSL plan page in the applicant's report.
2. State the four numbers: existing lit area (m²), proposed lit area (m²), proposed as a percentage of existing, and the BRE threshold (0.8).
3. Compare to VSC for the same window: where VSC passes but NSL fails, this is the strongest framing — "the developer's own table shows daylight reaches less than half of the room."
4. Cite BRE 209 paragraph 2.2.9 (the daylight-distribution section) and any policy that requires residential amenity to be protected.
5. Conclude. "The NSL ratio of [value] indicates a noticeable loss of daylight distribution in the affected room, contrary to BRE 209 and to [policy reference]."

Frequently asked questions

What is the No Sky Line?
The contour on the working plane (0.85m above floor) that separates the area of a room which can still see at least a patch of sky from the area which cannot. BRE 209 reports the area in front of this contour, before and after the development.

How is NSL different from VSC?
VSC measures sky at the centre of the window from outside; NSL measures the part of the room itself with sky view. A scheme can pass one and fail the other.

What threshold applies?
The 0.8 ratio: the lit area in the proposed condition should be at least 80% of the existing lit area. There is no absolute target, only the ratio.

When does NSL matter more than VSC?
Deep-plan rooms with single-aspect glazing and tall close obstructions — the geometry where a small VSC loss at the window centre can mean a large loss of lit area at the back of the room.

Sources