The 25 degree rule and when councils apply it

The 25 degree rule is a screening test, not a decision. If you draw a section through your neighbour's window and no part of your proposed extension rises above a line at 25 degrees from the horizontal, taken from the centre of the lowest window, the guidance treats enough skylight as still reaching that window and no further daylight calculation is needed. Pass it and the daylight question is usually over. Fail it and nothing has been decided yet, except that someone now has to do the proper calculations.

The test comes from the Building Research Establishment's guide to site layout planning for daylight and sunlight, usually cited as BRE 209. That document is guidance, not planning policy, and it says in terms that its numerical guidelines should be applied flexibly. Officers who treat the 25 degree line as a pass or fail rule are misapplying it, and so are objectors.

What the 25 degree line screens for

The concern is skylight: the diffuse light from the sky that reaches a window and lights a room on an overcast day. An obstruction opposite a window blocks part of the visible sky, and the higher it rises in the view from that window, the more sky it takes.

Twenty-five degrees is the angle at which, for a typical window in a typical room, the remaining sky is likely to be sufficient. It is a proxy, chosen because it is quick to draw and errs on the side of caution. The guidance frames it as a check that avoids unnecessary calculation rather than as a standard to be met.

Two points follow. The test is about the existing window, not about the new building's own daylight. And it concerns daylight rather than sunlight, privacy or the sense of enclosure, each of which is assessed separately.

Drawing the line in section from the window

Precision matters here, because the way the line is drawn decides the answer.

1. Identify the affected window. It is the lowest window serving a habitable room on the elevation facing the proposal, usually a ground floor living room or kitchen.
2. Take the measurement point at the centre of that window, in section.
3. Draw a section perpendicular to the plane of the window, through the proposed development, at the same scale and with real levels.
4. Project a line at 25 degrees above the horizontal from the measurement point.
5. Check whether any part of the proposal, including the roof, rises above that line.

Three mistakes recur. Measuring from the ground rather than the window centre, which flatters the applicant. Drawing the section at a convenient angle rather than perpendicular to the window. And ignoring a level difference between the two properties, which on a sloping street can change the result completely.

Where the affected window is in a bay, a conservatory or a non-habitable room such as a bathroom or a hallway, the picture is less clear and judgement enters. A window serving only a bathroom attracts little protection in practice.

Passing the check and what follows

If the proposal stays below the line for every affected window, write that down with the section drawing and move on. An applicant who submits a labelled section showing the line, the window centre and the proposal below it gives the case officer a one-page answer to the most common neighbour objection.

That is worth doing even where you expect no objection. Officers write reports under time pressure, and an unanswered daylight objection is easier to resolve by refusal than by calculation. The drawing costs almost nothing to produce if the architect is already drawing sections.

Do not overstate what a pass means. It says the proposal is unlikely to have a noticeable effect on skylight to that window. It says nothing about overlooking, outlook, noise or the effect on a garden.

Failing the check and the detailed tests that follow

Breaching the line triggers the quantitative tests in the same guidance, and these are the ones that decide contested cases.

The vertical sky component measures the proportion of sky visible from the centre of the window as a percentage. The guidance uses a value of 27 per cent as the point above which a window retains good skylight, and treats a reduction to below 27 per cent, combined with a fall to less than 0.8 times the former value, as noticeable to the occupants.

Daylight distribution within the room is tested by the no-sky line, which maps the part of the working plane that can see the sky. A reduction of the lit area to less than 0.8 times its former extent is the threshold for a noticeable loss.

Sunlight is assessed separately, using annual probable sunlight hours for windows facing within 90 degrees of due south, with a winter component covering the period from late September to late March. Amenity space has its own check, based on the proportion of the area receiving at least two hours of sunlight in late March, which is where an overshadowing study does its work.

A daylight and sunlight assessment covering these tests is the appropriate response to a breach. Officers weigh the results alongside everything else, and a modest numerical shortfall in a dense urban context is routinely accepted, as how officers weigh neighbour daylight objections explains.

Where local design guides use 30 or 45 degrees instead

Many councils publish a householder design guide containing a 45 degree test, and that is a different animal from the 25 degree line.

The 45 degree tests are usually drawn in plan, from the centre of the nearest habitable room window on the neighbouring property, to the nearest corner of the proposed extension, with a parallel test in elevation. Some guides use 30 degrees for single storey rear extensions, or apply the test only where the window is within a stated distance of the boundary. The thresholds and the method vary between authorities, and a design guide adopted as a supplementary planning document carries weight in the decision.

The practical consequence is that you have to read the local guide. A scheme that passes the 25 degree check in section can still fail a local 45 degree test in plan, because the two measure different geometry. Where the guide and the national guidance conflict, officers generally apply the local guide and use the quantitative tests to decide whether the breach matters.

Application to rear extensions on terraces

Terraces and semi-detached pairs are where this is argued most often, because the neighbouring window is close to the boundary and the extension runs along it.

An illustrative example. A two storey semi with a ground floor kitchen window 1.5 metres from the party boundary, and a proposed single storey rear extension three metres deep with a flat roof at 3 metres. Drawn in section from the centre of that window, the extension is likely to rise above the 25 degree line, and a local 45 degree test in plan may also be breached. The usual design responses are to reduce the depth, to pull the extension away from the boundary, to drop the eaves height, or to use a pitched roof sloping away from the neighbour. The figures here are illustrative and the result depends on the actual levels and distances.

What rarely works is arguing that the neighbour's window is only a kitchen, or that they have a second window elsewhere. Officers do consider room use, but the argument is better made alongside a design change than instead of one.

Limits of a single line test

Three honest limitations are worth holding in mind.

The test is two dimensional. It takes no account of how wide the obstruction is, so a narrow extension and a full width one can produce the same section while having very different effects on the sky visible from the room.

It takes no account of the existing obstruction. A window that already looks at a boundary wall two metres away has little skylight to lose, and the percentage tests capture that while the line does not.

And it is not a legal right. Planning daylight assessment is separate from the private law of rights of light, which has its own tests and its own remedies, as rights of light contrasted with planning amenity sets out. Satisfying the council does not protect you from an injunction, and vice versa.

For a householder extension, the sensible sequence is to draw the section early, test it against both the national guidance and the local design guide, and change the design rather than argue about the geometry. Enter your postcode on PlanScope to see which documents a householder application is likely to need.

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.