If you are planning a rear extension and a neighbour has a window close to your boundary, daylight is one of the first things your council will scrutinise. The quickest way an officer screens that impact is the BRE 45 degree rule, backed up where needed by the 25 degree rule and, in borderline cases, a full daylight and sunlight report.
Rear extensions are the single most common reason householders are asked for a daylight assessment. Getting ahead of the issue, before you submit, is usually the difference between a smooth approval and a refusal on neighbour-amenity grounds. This guide explains how the rules of thumb work, when they trigger a deeper assessment, and what to do if your design falls short.
Why daylight matters for a rear extension
When you build out at the back of a property, the new wall and roof can block light that previously reached a neighbour's window, particularly habitable rooms such as living rooms, kitchens and bedrooms. Loss of light to a neighbour is a material planning consideration, and local planning authorities routinely refuse applications where the harm is judged unacceptable.
The authoritative technical guidance is BRE Report BR 209, Site Layout Planning for Daylight and Sunlight: A Guide to Good Practice, updated in 2022. It is not statutory, but the overwhelming majority of councils adopt it through their local plan amenity policies and householder design guides. Alongside it sits BS EN 17037, the European daylight standard that now informs internal daylight provision.
The 45 degree rule explained
The 45 degree rule is a simplified screening test. An officer draws a 45 degree line in plan from the midpoint of the neighbour's nearest habitable-room window towards your proposed extension. The same exercise is repeated in section, drawing a 45 degree line upwards from the centre of that window. If the extension crosses both the plan line and the section line, the council will usually consider that daylight loss is significant enough to warrant either a redesign or a full daylight and sunlight report.
The test works best for single-storey and modest two-storey rear extensions next to a window set roughly at 90 degrees to the boundary, which is the most common terraced and semi-detached arrangement. Crucially, crossing the 45 degree line is not an automatic refusal. It is a flag that says the impact needs closer examination using the proper BRE metrics rather than a rule of thumb.
The 25 degree rule and how it differs
The 25 degree rule looks at the same problem from the affected window's point of view. A line is drawn in section at 25 degrees above the horizontal from the centre of the neighbour's window. If the proposed extension sits below that 25 degree line, daylight is generally considered acceptable and no detailed study is needed. If the extension rises above the line, a fuller assessment is advised.
In practice the two rules are complementary. The 45 degree rule is most often applied to side-by-side relationships, where the extension is alongside the neighbour's window. The 25 degree rule is the classic test for facing or directly opposing windows and is the gateway test in the BRE guidance for whether a Vertical Sky Component calculation is required at all. Different councils lean on one or the other in their design guides, so it is worth checking your local validation checklist before you draw anything.
When a full daylight and sunlight report is needed
If your extension crosses the relevant rule-of-thumb line, the council will expect the impact to be quantified with the BRE metrics rather than estimated by eye. The three measures that matter are:
- Vertical Sky Component (VSC) — the proportion of sky visible at the centre of a neighbour's window. The BRE target is 27 per cent, and a window retaining at least 0.8 times its former value is normally considered to keep acceptable daylight.
- No Sky Line (NSL), also called the daylight distribution test — how much of the floor area inside a room can still see the sky. Again, retaining 0.8 of the former area is the working benchmark.
- Annual Probable Sunlight Hours (APSH) — relevant where the affected window faces within 90 degrees of due south, testing winter and annual sunlight.
We explain these three in depth in our guide to VSC, NSL and APSH. A daylight and sunlight report runs these calculations on a 3D model of your scheme and the surrounding buildings, compares the before and after figures against BRE targets, and presents the result in a form a planning officer can rely on. Where targets are not fully met, a good report sets the numbers in context, which is exactly what BRE intends.
What to do if your design falls short
Failing a rule of thumb, or even a BRE target, is not the end of the road. The 2022 guidance is explicit that its figures are advisory, not absolute, and that they should be applied flexibly according to context. Sensible mitigation options include:
- Reducing the depth or height of the extension nearest the shared boundary.
- Introducing a pitched or hipped roof, or a set-back upper storey, to drop below the 25 or 45 degree line.
- Repositioning the most sensitive part of the build away from the neighbour's key window.
- Demonstrating that the affected room is non-habitable, such as a bathroom, hall or utility, where BRE targets carry far less weight.
- Presenting an honest VSC or NSL analysis showing that, although a target is missed, the retained light remains reasonable for a dense urban setting.
The NPPF reinforces this by asking authorities to take a flexible approach to daylight and sunlight where a scheme still delivers acceptable living conditions. A well-argued report that engages with context is often more persuasive than a design that scrapes a target on paper.
Common pitfalls with rear-extension daylight
A few avoidable mistakes account for most neighbour-amenity refusals. Submitting without checking the 45 or 25 degree relationship first leaves you reacting to an objection rather than pre-empting it. Assuming every window counts is another error, as obscure-glazed or non-habitable windows are treated differently. Finally, relying on the rule of thumb alone when the line is crossed rarely satisfies an officer, because at that point only the BRE metrics will resolve the question. If a neighbour has already objected, our guide on the difference between right to light and a daylight report is worth a read, as the two are frequently confused.
How Fortress Associates can help
Fortress Associates prepares BRE 2022 and BS EN 17037 daylight and sunlight reports for rear extensions and householder schemes across the UK. We model your proposal and the neighbouring properties, run the VSC, NSL and APSH tests, and produce a clear report you can submit with your application or use to refine the design before you do. You can see the full scope on our daylight report service page and the wider list of what we offer under services. Typical turnaround is four to five working days, and there is no advance payment. If you would like to talk through whether your extension needs a report, get in touch and we will give you an honest steer.
Sources & further reading
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