The 25-degree rule at the centre of a neighbouring window

The 25 degree rule daylight test is the screening check in BRE 209 that decides whether a full daylight assessment is needed at all. Stand at the centre of the lowest window of the affected room, look at the proposed obstruction in section, and measure the angle it subtends to the horizontal. Below 25 degrees, the guidance says daylight is unlikely to be materially affected and no further calculation is required. Above it, you move on to vertical sky component and daylight distribution.

Architects use it as a design tool rather than as a report. It is the fastest way to know whether a massing option has a daylight problem before anyone commissions a model.

What the test actually measures

The angle is a proxy for how much sky a window can still see. A window's daylight comes overwhelmingly from the sky rather than from reflected ground or wall surfaces, so an obstruction that blocks a low band of sky matters far less than one that fills the upper part of the view.

BRE 209, published by BRE as Site Layout Planning for Daylight and Sunlight, sets the 25-degree figure as the point below which the effect is unlikely to be noticeable. It is a threshold in advisory guidance, not a standard with legal force, and BRE is explicit that its numerical guidelines are to be applied flexibly according to context.

There is a companion check for sunlight. Where a window faces within 90 degrees of due south and the obstruction subtends less than 25 degrees measured from the centre of the lowest window, sunlight is unlikely to be materially affected either.

Measuring from the centre of the lowest window

Three parts of that instruction get lost regularly.

Centre means the centre of the window opening, vertically and horizontally, not the sill, the head or the transom. On a tall window the difference between measuring from the sill and from the centre can be several degrees.

Lowest means the lowest window of the affected room, and where a building has multiple storeys the ground floor window is normally the worst case because the obstruction subtends the largest angle from there. Test the lowest first. If it passes, the floors above will pass too, provided the obstruction does not rise steeply close to the building.

The measurement is taken in a section drawn perpendicular to the window wall, from the window centre to the highest part of the obstruction in that plane. Not perpendicular to the site boundary, not along the shortest line to the new building, and not in a diagonal view. Where the obstruction is oblique to the window, the angle in a true perpendicular section is smaller, which is a legitimate result rather than a trick.

Why passing it ends the assessment

If the test passes, the guidance itself says no detailed calculation is needed. That is worth stating plainly in the submission, with the section drawing attached, because it saves the applicant a report and the officer an argument.

In practice objectors sometimes press for full modelling anyway. The answer is not to refuse but to show the working: the section, the window identified on a plan, the measured angle, and the sentence from the guidance that the check is intended to serve. A case officer who can see the geometry rarely asks for more on a small scheme.

One caution about how far the result travels. The screen is about the room behind the window, so it says nothing about a garden, a balcony or a communal amenity space, and it says nothing about the light reaching a window that does not yet exist in a consented but unbuilt scheme next door. Where a neighbouring permission has been granted and not implemented, most consultants test both the built and the consented situations and report each, because a council asked to weigh harm wants to know which baseline the numbers came from.

The test is a screen, not a defence against every amenity objection. Outlook, sense of enclosure, overshadowing of a garden and privacy are separate considerations, and a scheme that passes at 22 degrees can still be refused for dominance.

When the test is failed

Failing the screen means the detailed work begins. That means vertical sky component measured at the centre of each window on the outside face of the wall, and daylight distribution assessed through the no-sky line inside the room, both compared against the guideline values and against the existing situation.

Two design responses recover more than any other. Setting the upper storeys back from the boundary reduces the subtended angle sharply, because the angle is driven by height at close range. And relocating the mass so the tallest element sits away from the most sensitive windows achieves the same result without losing floorspace.

At that point a full Daylight and Sunlight Assessment is the proportionate document, and councils generally expect an author who works with BRE 209 regularly.

Sloping ground and varied floor levels

The test assumes you know the relative levels of the window and the obstruction, and on a sloping site that assumption fails quietly.

Where the neighbouring building sits below yours, the effective height of your proposal above their window is greater than your elevation drawing suggests. Where it sits above, the reverse applies and a scheme that looks aggressive on plan may screen out easily. Either way, the section must be drawn from a measured survey tied to a common datum, with existing and proposed ground levels shown.

Half-levels, split-level flats and rooms with floors below external ground level all shift the window centre. Basement and lower ground floor windows are the most likely to fail, and they are the ones most often left out of a screening exercise because they do not appear on the front elevation.

Where several buildings of different heights surround a window, take the highest obstruction in the relevant section. Adding the angles of separate obstructions is not the method.

Recording the test for an application

Keep the record short and unambiguous.

1. A plan identifying every affected window with a reference and the room it serves.
2. A section per window, or per representative group, drawn perpendicular to the window wall at a stated scale.
3. Existing and proposed levels on each section, from a measured survey.
4. The measured angle stated on the drawing, with the 25-degree line shown.
5. A short note on the result and, where relevant, why the affected room does not qualify as habitable.

Put the conclusion in the Design and Access Statement in a few sentences rather than as an appendix nobody opens.

The measurement errors that recur

Measuring from the sill. Measuring to the ridge in an elevation rather than in a perpendicular section. Forgetting a parapet, a plant enclosure or a lift overrun that sits above the roof line on the drawing you used. Using existing ground levels where the scheme raises them. Assessing only the windows facing the site while ignoring one in a return elevation. Testing the first floor because it was easier to draw.

Each of these produces an angle that is wrong in a predictable direction, and a consultant reviewing the submission for an objector will find them.

Where the concern is the garden rather than the room, the relevant work is an Overshadowing Study on the equinox rather than an obstruction angle at a window.

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.