No-sky line: assessing daylight distribution inside a room

A no sky line assessment answers a question that vertical sky component cannot. VSC measures how much sky reaches the outside face of a window. The no-sky line measures how far that daylight gets into the room. Draw the contour on a plan of the working plane, and it divides the points that can see a patch of sky through the window from the points that cannot.

BRE 209 treats a reduction in the sky-visible area to less than 0.8 times its former value as the point at which the loss becomes noticeable to the occupants. That single ratio decides a great many daylight objections.

What the contour shows

Imagine lying on the working plane in a rear living room and looking up at the window. From a point near the glazing you see sky. Walk back into the room and at some point the head of the window, or an obstruction beyond it, cuts the sky off entirely. Join all those points and you have the no-sky line.

Everything in front of the line receives direct skylight. Everything behind it is lit only by light reflected from surfaces, which is far weaker and changes the character of the space rather than only its brightness.

Two variables move the contour. The geometry of the window, meaning its head height, width and reveal depth. And the geometry of whatever sits outside it. A neighbouring extension does not have to block the window to push the line back several metres.

That is why a room can pass on vertical sky component and fail here. VSC is a measurement at a point on the wall. The no-sky line is a measurement of the room.

Working plane heights

The working plane is the horizontal surface at which daylight is assessed, and its height depends on the use of the room.

For dwellings BRE 209 uses a working plane 0.85 m above floor level, which corresponds roughly to table height. For offices and similar workplaces the plane is taken at 0.7 m, the height of a desk. Where a room has a different use again, say which height you used and why.

Get this wrong and every number in the assessment is wrong in the same direction. A plane set at 0.7 m in a house produces a slightly more generous result than the guidance intends, and a reviewer checking the model will find it in the first five minutes.

The plane is normally taken across the whole room excluding a border adjacent to the walls. State the convention adopted in the methodology so that two assessments of the same room can be compared.

Calculating the area that loses sky visibility

The calculation itself is geometric and is normally done in software, with the manual method in the guidance available as a check.

What the model needs is unglamorous:

• Room dimensions and floor-to-ceiling height, per room.
• Window position, size, head and sill height, reveal depth and any transoms or heavy glazing bars.
• Balconies, overhangs, canopies and brise soleil above the window, which are routinely omitted and matter enormously.
• The existing obstruction geometry as built, from a measured survey rather than from mapping.
• The proposed obstruction geometry, including parapets, plant enclosures and lift overruns.

Run the model twice, once for the existing situation and once with the proposal in place, and report the sky-visible area in square metres for each along with the ratio between them. Where a neighbouring permission has been granted and not implemented, report both baselines and say which you rely on.

The 0.8 ratio and how it is applied

The test is proportional rather than absolute. If the area of the working plane that can see the sky after the development is at least 0.8 times the area before, the guidance treats the change as unlikely to be noticeable. Below that, it is likely to be noticed by the occupants.

Three points of interpretation come up constantly.

The ratio says nothing about whether the room was adequately lit to begin with. A deep room with a small window can fail the ratio while remaining as poor as it always was, and it can equally pass the ratio while being unpleasantly dark. Say which situation you are describing.

BRE is explicit that its numerical guidelines are advisory and should be applied flexibly according to context. A dense urban site is not judged against the same expectations as a suburban street, and decision-makers do accept departures where the context justifies them.

And the test applies to rooms with a reasonable expectation of daylight. Bathrooms, halls, landings, stores and garages are outside it, as are, in most assessments, rooms lit solely through a conservatory or a covered area where the outer opening is taken as the relevant aperture.

Deep rooms and rear extensions

The classic failure is a Victorian or Edwardian terrace where a ground-floor rear room has been knocked through into a kitchen, giving a room 7 or 8 m deep lit from one end.

In that geometry the no-sky line already sits close to the window before anything is built. A neighbour's single-storey rear extension along the boundary then removes a large proportion of the remaining sky-visible area, and the ratio falls below 0.8 even though the extension is modest and complies with permitted development limits for its own site.

This is where the 45-degree rule of thumb in council design guides and the numerical test diverge, and where an objector's consultant will concentrate. It is also the point at which a scheme that would sail through on a plan test needs the room modelled, and the 66 report types index shows which daylight and amenity documents a council usually asks for. If your scheme sits alongside a knocked-through rear room, model it before you submit rather than after.

The design responses are limited and known. Reduce the depth against the boundary, step the rear wall so the deepest part sits away from the affected window, or drop the height at the boundary. A full Daylight and Sunlight Assessment tests those options quickly once the model exists.

Where internal layout assumptions matter

Assessing a neighbouring property means assuming its internal layout, because you cannot survey inside it.

State every assumption. Which rooms you assumed, their depths, the use you assigned to each, and the source, whether that is a historic planning application drawing, a sales particulars floor plan, an external survey or a typical arrangement for the house type. Where an assumption is doing a lot of work in the conclusion, test the sensitivity of the result to it.

Where the neighbour has extended under permitted development, the room behind the window may be much deeper than the original plan shows. Aerial imagery and the council's own planning history are the practical sources, and a polite letter asking for a floor plan is sometimes answered.

Presenting the contour

Show the work rather than only the ratio.

Give a plan per affected room at a stated scale, with the no-sky line drawn for the existing situation and for the proposal, the sky-visible areas hatched, and the areas and ratio annotated. Add a table listing every room assessed with its use, the two areas, the ratio and the result. Keep the assumptions on the same sheet as the drawings.

A case officer weighing amenity harm needs to see which rooms fail and by how much, not a paragraph saying the scheme broadly accords with the guidance. Where the concern is a garden rather than a room, the relevant study is an Overshadowing Study on the equinox, and the two are often needed together.

This is general guidance and not 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.