Obstruction angles and the early massing test
Drawing the 25 degree obstruction angle from a neighbour's lowest window tells a design team which windows need vertical sky component calculations and which do not, while the massing can still move.
The obstruction angle is the cheapest daylight test in the book, and it is the one that should be drawn before a massing option gets as far as a render. If the angle from the centre of a neighbour's lowest window up to the top of your proposal is less than 25 degrees, BRE 209 treats diffuse daylight to that window as unlikely to be adversely affected, and no further calculation is needed. Above 25 degrees you are into vertical sky component territory and the cost of the evidence rises sharply.
Used properly, the obstruction angle daylight test is a filter rather than an answer. It tells a design team which neighbours are in play and which are not, on a day when the scheme can still move.
What the check actually tests
BRE 209, Site Layout Planning for Daylight and Sunlight, sets out a staged approach. The first stage is a simple geometric screening test on the angle subtended by the obstruction.
The logic is that a low obstruction leaves most of the sky visible from the window, so the diffuse light reaching it changes little. Once the obstruction rises above roughly a quarter of the way to the zenith, seen from the window, the amount of visible sky starts to fall away quickly and the effect becomes measurable.
The same 25 degree threshold appears in the guidance for a first check on sunlight, applied to windows within 90 degrees of due south. Windows facing within that arc and obstructed by more than 25 degrees need the annual probable sunlight hours calculation. Windows outside it are treated as receiving little direct sun in any case.
Measuring it from the neighbour's window
Three details decide whether your section is usable.
- Measure from the centre of the lowest window in the affected elevation, not from ground level and not from the middle of the facade.
- Take the section in a vertical plane running from that window towards the proposal, and draw the existing situation on the same sheet.
- Take the top of the obstruction as built, including parapets, plant enclosures, lift overruns and balustrades, because those are what cast the shadow.
Existing obstructions matter as much as the new one. Where a neighbour already faces a four storey wall at eight metres, the relevant question is what your scheme adds above the current profile, and the baseline section is the only way to show it. That is the situation dealt with more fully where existing daylight conditions are already poor.
The lowest window is the worst case, so if it passes, the windows above it pass too. That single property is what makes the test fast.
Using it before any modelling
On a typical urban infill the useful sequence takes an afternoon. Identify every habitable room window facing the site within a sensible radius. Draw one section per affected elevation. Mark the windows that clear 25 degrees and the ones that do not.
What you get is a plan of the site with a ring of neighbours colour coded by exposure, and that plan drives the massing. Set backs, shoulders, a lower element along one boundary, a rotated block: each move can be tested against the same sections in minutes rather than waiting a fortnight for a consultant to remodel.
It also gives you a defensible answer to the question a planning officer asks at pre-application, which is whether the neighbours have been thought about at all. A dated set of sections answers that better than a paragraph.
Where the check overstates or understates
The test assumes a continuous obstruction across the field of view. Real buildings are not always continuous, and the screening test is deliberately cautious.
A narrow tower set at an angle to a neighbour can breach 25 degrees in section while leaving most of the sky visible either side of it. The full vertical sky component calculation will often show a modest change where the angle test suggested a problem. The opposite case is rarer but real. A long, low terrace that clears the angle everywhere can still cut sunlight to a garden, and a courtyard form can meet the angle test at every window while producing rooms that are dark at the back because of their depth rather than their view of the sky.
So treat a failure at 25 degrees as a prompt to calculate, not as a finding. And treat a pass as covering diffuse daylight only. Sunlight to gardens and amenity areas is a separate test on a separate measure, and transient shading is different again, as set out in transient overshadowing and permanent loss of sunlight.
It is not the 45 degree rule
These two get confused constantly, and they come from different places.
| Obstruction angle | The 45 degree rule | |
|---|---|---|
| Source | BRE 209, national technical guidance | Council householder design guides and SPDs |
| Measured | In section, from the centre of the lowest window | In plan and in elevation, from the nearest edge of the neighbour's window |
| Tests | Whether diffuse daylight needs calculating | Whether an extension is acceptable in amenity terms |
| Status | Screening stage of a technical method | Local policy guidance, applied as a rule of thumb |
An extension can pass the 45 degree rule in a council guide and still fail a vertical sky component test, and the reverse happens too. Quote whichever the decision maker actually uses, and if the council's own guide sets the test, cite the guide rather than the British standard.
When to move to a full assessment
Four situations justify commissioning the calculations rather than relying on the screening test.
Where any affected window breaches 25 degrees and the neighbour is residential. Where the council's validation list or an adopted supplementary planning document requires a Daylight and Sunlight Assessment for schemes of your scale. Where a sensitive receptor such as a school, nursery or care home is affected, because amenity space sunlight will be examined as well. And where you expect objection, since a neighbour's own consultant will run the numbers whether you do or not.
Amenity areas and gardens need their own work. An Overshadowing Study tests the proportion of a garden or courtyard receiving at least two hours of sun on 21 March, which is a different question from the one the obstruction angle answers.
Recording the check in the design record
Keep the sections, date them, and name the drawing revision they were taken from. When the scheme changes, redraw rather than re-use, because the whole value of the method is that it tracks the design.
Put a short account of the screening exercise in the Design and Access Statement, with one representative section reproduced. Two paragraphs saying which windows were screened, which cleared 25 degrees, and what the massing did in response, will save an officer a request for information and gives a later reviewer something to check.
Daylight and sunlight work is a field where councils expect a suitably qualified author for the full assessment, and professional review is strongly advised before anything is submitted. An AI-assisted draft can set out the methodology, the policy context and the structure of the chapter, with the measured results supplied by the assessor. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it.
Frequently asked questions
What is the 25 degree rule in BRE 209?
It is the screening test at the start of the staged method. If the angle from the centre of the lowest affected window to the top of the obstruction is less than 25 degrees above the horizontal, diffuse daylight to that window is unlikely to be adversely affected and no vertical sky component calculation is needed.
Is the obstruction angle the same as the 45 degree rule?
No. The obstruction angle comes from BRE 209 and screens whether daylight needs calculating. The 45 degree rule comes from council householder design guides, is measured in plan and elevation from the edge of a neighbour's window, and is applied as an amenity rule of thumb. A scheme can pass one and fail the other.
Why measure from the lowest window?
Because it is the worst case. The obstruction subtends the largest angle at the lowest window in an elevation, so if that window clears 25 degrees every window above it clears it too. That property is what makes the test quick enough to run across a whole ring of neighbours in an afternoon.
Does passing the obstruction angle test mean there is no impact?
It means diffuse daylight to that window is unlikely to be affected. Sunlight to gardens and amenity areas, transient overshadowing, and internal daylight in deep rooms are separate questions on separate measures. A long low building can clear the angle at every window and still shade a garden through the afternoon.
When should a full daylight assessment be commissioned?
Where an affected residential window breaches 25 degrees, where the council's validation list or an adopted document requires one, where a school, nursery or care home is affected, and where objection is likely. In the last case a neighbour's consultant will run the calculations whether or not you do.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.