The no sky line test and how room depth drives it

The no sky line test asks a simple question about an existing room: how much of the floor area can still see a patch of sky after your building goes up. Draw the line on a plan at 0.85 metres above floor level, measure the area on the window side of it, and compare that area with the area before development. If the retained area falls below 0.8 times the former value, the guidance treats the loss of daylight distribution as noticeable. That ratio, applied room by room, is the whole test.

It is a different test from the vertical sky component, which is measured at a single point on the window face. A room can pass on sky component and fail on distribution, and the reason is almost always room depth.

What the no sky line plots

The no sky line is the boundary between the parts of a room that receive direct skylight and the parts that do not. Stand at a point on the working plane and look up and out through the window. If any part of the sky is visible past the window head, the reveals and whatever is outside, that point is on the lit side of the line. If the view is entirely of buildings, ground or the window soffit, it is not.

The line is a plan output, not a number at a window. Which makes it useful to an architect: it shows you where in the room the daylight is lost, so you can see whether the affected area is the back of a kitchen or the whole of a living room.

Guidance on all of this sits in the Building Research Establishment's site layout guide, now in its third edition and aligned with the European daylight standard. The guide is advisory. Weight comes from the local plan amenity policy that adopts it, and the degree of adoption varies, as our note on how local plan amenity policies use BRE 209 sets out.

The working plane and its height

For dwellings the working plane is a horizontal plane 0.85 metres above floor level, which approximates a desk or table top. For other building types a different height may apply, and a modeller should state the height used.

Two things follow from the convention. Because the plane sits well below the window head, the geometry that controls the line is the head height and the depth of any soffit, balcony or overhang above the window rather than the cill. And because the plane is horizontal and extends across the full room, the result is sensitive to the shape of the room as well as its depth.

Set the plane too high and the line retreats, which flatters the proposal. Set it too low and the result is unrealistically generous. Check the stated height in any assessment you are reviewing, because it is one of the easier inputs to get quietly wrong.

The 0.8 retained area criterion

The criterion is a ratio, not an absolute standard. There is no minimum percentage of a room that must see sky. The question is only how much of the previously lit area survives.

Work it through. A room with 20 m2 of working plane, of which 16 m2 can see sky before development, retains 12.8 m2 after. The ratio is 0.8 of the former value, which sits on the boundary. Retain 11 m2 and the ratio is 0.69, which the guidance would describe as a noticeable reduction. Note what the arithmetic does not tell you: whether the remaining lit area is in a useful part of the room, and whether the room has other sources of light. Those are judgements for the assessor and ultimately for the decision maker.

The ratio also means a room that was already poorly lit is hard to harm on this test. A single-aspect room under a deep balcony may have only 4 m2 seeing sky to start with, so the development has little to take away. That is a real finding, and it should be reported as such rather than presented as a good daylight outcome. Those rooms have their own problems, covered in our note on daylight to single aspect flats.

Why deep rooms fail before shallow ones

Geometry decides it. The point at which the line falls is governed by the angle from the working plane point, through the top of the window opening, to the sky past any external obstruction. As you move back into the room, that sight line flattens, and once an opposing building rises above it the point goes dark.

So a 3 metre deep room with a tall window and a modest obstruction keeps most of its area lit. A 7 metre deep open-plan living and dining space with the same window and the same obstruction loses a large fraction of its depth, because the rear third was marginal to begin with. Knock two rooms into one and you have created a room that is more vulnerable on this test than either was separately, which is a point worth making when the existing condition includes a previous conversion.

That gives three levers on a development proposal. Reduce the obstruction angle by stepping the massing back or down at the sensitive elevations. Raise the effective window head on new build. Or change the internal depth, which is available on the proposed side of a scheme and not on the neighbour's.

Window head height is the lever people underuse. Lifting a window head by 300 millimetres pushes the line further into the room across the whole width, and on a new building it costs little.

Rooms with more than one window

Where a room has two or more windows, the lit area is the union of the areas lit by each one. A point counts as lit if it can see sky through any window.

That has a practical consequence for corner rooms and for rooms with a window plus a glazed door. Development opposite one window may take very little of the lit area because the second window continues to serve the back of the room. Assessments that test each window separately and report the worst result overstate the harm, and assessments that only model the largest window understate it.

Rooflights and lightwells are included where they exist, and an atrium or a glazed link needs careful modelling rather than a simplifying assumption. Where a room is served by a window into a covered walkway, the obstruction of the walkway itself goes in the model.

State in the report how many windows serve each assessed room. That one column in the results table prevents most of the arguments about method.

Modelling conventions and what the model ignores

The analysis ignores furniture, internal partitions that are not full height, curtains, blinds and the reflectance of surfaces. It is a geometric test of sky visibility, so a room crowded with fitted units gives the same result as an empty one.

That is a limitation worth stating rather than hiding. The modern illuminance-based approach in the European daylight standard, with its target illuminances for bedrooms, living rooms and kitchens achieved over a proportion of the area for a proportion of the daylight hours, captures surface reflectance and glazing transmittance in a way the no sky line does not. The two methods answer different questions, and an assessment that reports distribution alongside an illuminance calculation is more useful than one that reports either alone.

Other conventions to fix and state: whether existing rooms were surveyed internally or inferred from external observation and planning drawings, how room uses were established, the assumed glazing line, and whether existing consented but unbuilt schemes were included in the baseline. Inferring a layout from the outside is sometimes the only option on a neighbouring property, and the report should say so.

A full daylight and sunlight assessment will also cover sunlight to windows and to amenity areas, and sun on ground analysis where gardens are affected.

Presenting before and after plans

Numbers in a table persuade nobody. Plans do.

Produce, for each affected floor of each affected property, a plan showing the no sky line in the existing condition and the proposed condition on the same drawing, with the two lit areas shaded differently and the retained ratio annotated against each room. Label every room with its established use. Keep the drawing at a readable scale and put the north point on it.

Then write three sentences per property: what the test shows, which rooms fall below the ratio, and what the lost area amounts to in square metres and against the way the room is used. A result of 0.74 in a rear kitchen with a second window is a different matter from 0.74 across a single-aspect bedroom, and saying so is the assessor's job.

Objectors increasingly commission their own analysis, and the points they raise are predictable, as evidence that carries weight in daylight objections describes. Publishing the model assumptions in the report is the best insurance against a late methodological challenge.

Enter your postcode on PlanScope to see which amenity reports your scheme is likely to need, then send the draft for professional review.

This is general guidance, not legal advice, and BRE 209 is advisory rather than a standard with statutory force. Check your authority's adopted amenity policy and take advice from 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. For specialist technical reports, qualified professional input may be required.