The 25 degree line and its role in screening a scheme

The 25 degree line daylight check is the first test in the British Research Establishment guidance on site layout planning for daylight and sunlight, and it is the only one that can end an assessment before it starts. Draw a section through the proposal and the affected window. If the new obstruction subtends an angle of less than 25 degrees to the horizontal, measured from the centre of the lowest affected window, the guidance indicates that enough skylight should still reach that window and no further daylight testing is needed.

That is a useful result when it applies. It applies less often than people hope.

What the line screens for

The check is about skylight reaching an existing window, and it works because of a relationship between obstruction angle and the amount of sky visible from a point on a vertical facade.

An obstruction that sits low in the field of view removes a small part of the sky. As it rises, it removes progressively more, and the rate of loss is not linear. The 25 degree threshold is the point below which the guidance treats the residual skylight as sufficient without further calculation.

Two things the line does not do. It says nothing about sunlight, which is tested separately and depends on orientation. And it says nothing about the daylight inside the new building, which is a different question governed by the internal standards rather than by an obstruction angle.

There is a parallel check for sunlight in the same guidance: where an obstruction lies more than 90 degrees from due south of a window, or subtends less than 25 degrees in the vertical plane, no further sunlight testing is needed. Architects frequently use one and forget the other.

Measuring it from the neighbouring window

Accuracy here is worth more than argument later.

The measurement is taken in a vertical plane perpendicular to the affected window, from the centre of the lowest window serving the affected room, to the top of the new obstruction. The lowest window matters because a ground floor window has the highest obstruction angle for any given building opposite.

Practical steps that avoid the usual disputes:

1. Establish existing levels by survey, at both the site and the neighbouring facade, to a common datum.
2. Locate the neighbouring windows by measurement, including cill and head heights, rather than scaling from photographs.
3. Identify which windows serve habitable rooms, and which rooms they serve.
4. Draw the section at the point of greatest obstruction, not at a convenient location.
5. Include parapets, plant enclosures, lift overruns and balustrades in the obstruction outline.

Roof-level additions are the item most often left off. A scheme that passes the check against the main roof and fails it against the plant screen has not passed anything.

When it removes the need for further testing

Where the check is passed, the guidance indicates that further daylight analysis is unnecessary for that window. In practice that conclusion is accepted by most authorities where the section drawings are clear and the levels are surveyed.

It saves real money on the right kind of scheme. A two storey infill opposite a wide street, a modest rear extension where the neighbouring window is well set back, a scheme on the north side of an existing building with generous separation. On those, a set of sections and a short note is a proportionate answer.

Where the check fails, it has told you something useful rather than nothing. It has identified which windows need the detailed work, and the assessment can then be scoped to those rather than to every opening on the facade.

Where it under-reports harm

The check is a screening device built on a simplifying assumption, and it can give a comfortable answer on a scheme that will still cause a problem.

It assumes a continuous obstruction of effectively infinite length. Where the new building is wide and flanked by open ground, the check is conservative and the real position is better. Where the obstruction wraps around a window on more than one side, a section taken perpendicular to the window can pass while the cumulative loss of sky is substantial.

It is also a test of a single window at a single point, so it says nothing about distribution of daylight within the room behind. A room can retain adequate sky visibility at the window and still have its rear half fall behind the no-sky line, which is why the guidance includes a separate distribution measure. The effect of assumptions about how rooms are used is covered in room use assumptions and how they change results.

And it deals only with the affected building's windows. Loss of sunlight to gardens and amenity space is a separate analysis, usually addressed through an Overshadowing Study, and a scheme that screens out on the 25 degree check can still shade a neighbour's garden through the winter afternoon.

Combining it with other measures

Use the check to scope, then use the right measure for what remains.

Vertical sky component measures the proportion of available skylight reaching a point at the centre of a window, and the guidance uses a benchmark figure alongside a proportional comparison against the existing value. On dense urban sites the proportional comparison usually does the work, because existing values are often already low, and the baseline question of what the existing condition actually is matters more than the absolute number. That baseline question is explored in the mirror image baseline and existing obstructions.

Daylight distribution measures how much of the working plane can see the sky, and it catches deep rooms that a window-based measure does not.

Annual probable sunlight hours measures sunlight availability across the year and across the winter months separately, and applies to windows within 90 degrees of due south.

Run them in that order and report each separately. A report that gives a single pass or fail conclusion without showing which measure drove it is of little use to a committee, and the presentation problem is discussed in the wider approach to obstruction angles and the early massing test.

Terraces, side returns and awkward geometry

Standard suburban geometry is rare in the cases that reach an assessment. What turns up instead is a side return extension against a neighbouring rear window at an oblique angle, a rear addition on a stepped terrace where levels change between plots, and windows in a return wall facing a boundary a metre away.

Oblique relationships defeat a simple section. Where the new structure is not parallel to the affected window, the perpendicular section can miss the part of the obstruction that matters, and the honest response is to model rather than to draw one section and call it a screening exercise.

Windows in a return wall close to a boundary often have very low existing values, sometimes so low that any proportional test produces a large percentage change from a small absolute one. Report both the absolute and the relative figures in those cases and explain the arithmetic, rather than leading with whichever flatters the scheme.

Where the neighbouring property has been extended already, the existing condition includes that extension, and the baseline has to reflect what is there rather than what the original house had.

Recording the screening conclusion

A screening conclusion needs to be as checkable as a full assessment, and it usually is not.

Include a plan showing the affected windows, numbered, with the rooms they serve identified. Include a section for each relationship tested, drawn to a stated scale, with the 25 degree line drawn and the measured angle annotated. Include the survey source and date for the levels. State which windows passed, which failed, and what was done about those that failed.

Then state the limitations plainly. Say that the check assumes a continuous obstruction, say where that assumption is conservative and where it is not, and say what has been excluded from the scope. A Daylight and Sunlight Assessment that is candid about the limits of its screening is far harder to attack than one that presents the check as a clean pass.

Councils usually expect a suitably qualified author for daylight work, and professional review before submission is strongly advised where the result is finely balanced.

This is general guidance, 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. To see which amenity documents your scheme is likely to need, enter your postcode on PlanScope.