The 0.8 times rule is the simple test at the heart of almost every neighbour daylight assessment in the UK: if a window keeps at least 80% of the daylight it had before a development is built, the loss is normally considered acceptable. Drop below that figure — a reduction of more than 20% — and the effect is likely to be noticeable enough to weigh against the scheme.
It sounds straightforward, and in its headline form it is. But the 0.8 factor turns up in several different BRE tests, it is constantly confused with the absolute 27% target, and it is routinely misread by applicants and objectors alike. This guide explains where the figure comes from, how it is actually applied under BRE BR 209 (2022), and the situations where the 20% threshold quietly bends.
Where the 0.8 times rule comes from
The rule sits inside the Building Research Establishment's guidance, Site Layout Planning for Daylight and Sunlight: A Guide to Good Practice (BR 209), now in its 2022 second edition. BR 209 is not law and is not part of national planning policy, but it is the reference that local planning authorities and inspectors lean on when they assess the impact of a proposal on the light reaching existing homes.
The principal daylight measure for a neighbour's window is the Vertical Sky Component (VSC) — the proportion of sky visible from the centre of a window, expressed as a percentage of an unobstructed hemisphere. The maximum possible value is just under 40%. BRE sets out two linked benchmarks:
- An absolute guide value of 27% VSC. A window with a VSC of 27% or more should still receive reasonable daylight.
- A relative guide: if the VSC after development is less than 0.8 times its former value, the reduction is likely to be noticeable to occupants.
That second figure is the 0.8 times rule. It is fundamentally a test of change, not of absolute quality. A window can comfortably exceed 27% VSC and still fail the 0.8 test if the development removes a large slice of its former sky — and, conversely, a window already below 27% may pass if the new proposal barely alters it.
How the rule works in practice
Take a typical rear extension next door to an existing kitchen window. The assessor models the window's VSC in two states: as existing, and with the proposed development in place.
- Before: VSC of 30%.
- 0.8 threshold: 30% × 0.8 = 24%.
- After: if the proposed VSC stays at or above 24%, the loss is within BRE guidance. If it falls to, say, 21%, the window has lost roughly 30% of its former daylight and the effect is reported as a noticeable, potentially significant reduction.
The same logic is applied to the No Sky Line (also called Daylight Distribution) inside a room: BRE looks at whether the area of the working plane that can still see the sky falls below 0.8 times its former extent. For sunlight, the Annual Probable Sunlight Hours (APSH) test uses the same 0.8 multiplier — a window facing within 90° of due south should keep at least 0.8 times its former annual and winter sunlight, subject to minimum absolute hours.
So the “20% rule” is really a family of tests that all share the same 0.8 retained-value threshold. If you would like a fuller breakdown of how the three core metrics fit together, see our explainer on VSC, NSL and APSH.
27% target versus the 0.8 rule: clearing up the confusion
The single most common mistake is to treat 27% VSC as a pass mark and ignore the 0.8 test — or to do the reverse. They answer different questions:
- The 27% absolute target asks: does this window still enjoy good daylight in its own right?
- The 0.8 relative test asks: how much daylight has this particular development taken away?
In planning terms the relative loss usually carries more weight, because the question before the authority is the impact of the new proposal, not whether a tightly packed Victorian terrace met a modern target it was never designed for. A window that was always at 18% VSC because of the building opposite is not failing because of your extension; what matters is whether your extension pushes it below 0.8 of that 18%. This is exactly the kind of nuance that decides contested cases, as we discuss in our guide to a neighbour's loss-of-light objection.
When the 0.8 times rule bends
BRE is explicit that its numerical guidelines should be applied flexibly, and the 2022 edition reinforces this. The 0.8 threshold is a starting point for judgement, not a legal line. It commonly gives way in these situations:
- Dense urban context. In town and city centres, existing daylight levels are already low and strict application of the 0.8 rule would block almost any development. BRE's Appendix F allows assessors to set alternative, locally appropriate targets based on the prevailing context — for example, the daylight typical of comparable nearby streets. We cover this in detail in BR209 Appendix F: setting alternative targets in cities.
- Low starting values. Where a window already has a very low VSC, even a large proportional loss can represent a tiny absolute change that occupants would never perceive. BRE advises caution before treating such results as significant.
- Adopted local policy. Boroughs such as those operating under the London Plan may apply their own design standards and density priorities, which can outweigh a marginal BRE breach where housing delivery is at stake.
None of this means the 0.8 rule can be waved away. Departing from it requires a reasoned, evidenced case — not an assertion. An assessment that simply declares the context “urban” without demonstrating appropriate comparator values is unlikely to survive scrutiny at appeal.
Common misunderstandings
- “A 20% loss is automatically a refusal.” No. It is a flag for a potentially noticeable effect that must be weighed in the planning balance against the benefits of the scheme.
- “The rule applies to my own new rooms.” The 0.8 retained-value tests are for existing neighbouring windows. Daylight within a proposed dwelling is assessed differently, against target illuminance values aligned with BS EN 17037.
- “Passing 27% means I am fine.” Only if you also satisfy — or reasonably address — the relative loss test for each affected window.
- “The rule covers gardens.” Sunlight to open amenity space is a separate overshadowing test, measured against the 21 March condition rather than window VSC.
How Fortress Associates can help
If you are weighing up an extension, an infill plot or a larger residential scheme, an early daylight and sunlight appraisal tells you exactly where you stand against the 0.8 times rule before you commit to a design — and where there is room to negotiate. We prepare BRE-compliant daylight and sunlight reports to BR 209 (2022) and BS EN 17037, typically within 4–5 working days, with no advance payment required. Where a scheme is tight, we set out the mitigation and contextual arguments that give it the best prospect of approval. To discuss a specific site, see our services or get in touch.
Sources & further reading
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