The 45-degree and 25-degree rules

The 25-degree rule is a daylight screening test set out in BRE 209: drawn in section from the centre of a neighbouring window at 2.0m above ground, an obstruction whose top falls below a 25-degree line above horizontal is unlikely to materially reduce daylight, and full VSC analysis is not strictly required. The 45-degree rule is a separate proximity test, no longer in BRE 209, but kept alive in many borough Supplementary Planning Documents: an extension or new building should not project beyond a 45-degree line in plan or section from the nearest neighbouring window. Both are quick checks, both have limitations, and both are still cited by case officers and inspectors.

The 25-degree rule

The 25-degree rule is the BRE 209 first-pass test for whether a proposed obstruction is likely to harm daylight. The geometry:

• Take the centre of the affected window, on the outside face, at 2.0m above the ground level outside.
• From that point, draw a line at 25 degrees above horizontal towards the obstruction.
• If the entire obstruction falls below that line, BRE 209 treats it as having no material impact on daylight, and detailed VSC / NSL analysis is not strictly required.
• If any part of the obstruction rises above the line, full daylight analysis is needed.

The 25-degree line corresponds approximately to the angle at which a tall obstruction begins to bite into the sky visible from a typical window, given a typical urban street width and storey height. It is the planner's equivalent of a triage tool: a sub-25-degree obstruction can be screened out; an above-25-degree obstruction triggers the full BRE methodology.

Note: passing the 25-degree screen is not the same as passing BRE 209. It only excuses the applicant from doing the full VSC, NSL and APSH analysis. A modest obstruction that sits below the 25-degree line is presumed not to cause noticeable harm; a careful objector will still ask for the analysis if there are unusual factors (deep room plans, tall windows, narrow streets).

The 45-degree rule

The 45-degree rule is older, simpler and more contested. It comes from the predecessors of BRE 209 (the original 1971 BRE digest) and survives in numerous local planning documents. There are two variants:

45-degree in plan

Looking down on the site, draw a line at 45 degrees in plan from the midpoint of the affected neighbour's window. The proposed building (or extension) should not extend across that line. The intent is to limit the sense of enclosure on a window viewed obliquely.

45-degree in section

Looking at the side of the building, draw a line at 45 degrees from the head of the affected neighbour's window upwards and outwards. The new building should not rise above that line. The intent is to limit the apparent height of the obstruction relative to the window.

Many borough Supplementary Planning Documents apply both lines simultaneously: an extension passes only if it sits inside both the plan and the section 45-degree envelopes, taken from each affected neighbouring window. The geometry is sensitive to the window-head height and to whether the test is taken from the window centre or edge.

Where they are still used

For larger schemes (more than a few units), case officers will normally rely on full BRE 209 analysis rather than the 45-degree rule, but the rule is still useful to objectors as a quick visual check. If a proposal breaches the 45-degree line on a section drawing, that is something a councillor can see at committee without any specialist knowledge.

Limitations

• Geometric, not visual. The 25-degree rule looks at the angle to the top of the obstruction, not at how much sky is left visible. A long, low obstruction filling 270 degrees of sky in plan can pass the 25-degree section test while still cutting daylight materially.
• Window-centric. Both rules are taken from a specific window. Rooms with shallow secondary glazing or rooflights are not assessed.
• Single window assumption. A bay window or a window with returns is normally tested at the centre, missing asymmetric loss at the edges.
• Local variation. The 45-degree rule is in some borough SPDs but not others. In Westminster, for example, the rule is invoked rarely; in Lewisham or Wandsworth it is routine. Always cite the specific local document.
• No account of room use. Both rules treat all windows the same. A bedroom and a living room get the same line, despite BRE 209 weighting them differently.

How to use these rules in an objection

1. Find the rule in the local document. Search the borough's Supplementary Planning Document on residential amenity (titles vary: "Residential design guide", "Householder SPD", "Daylight and sunlight SPD"). Quote the paragraph and, if there is a diagram, reproduce it.
2. Lift the geometry from the applicant's drawings. Use the side elevation for the section test; use the proposed plan for the plan test. Draw the 45-degree (or 25-degree) line on a print and measure where the proposed building sits relative to it.
3. State the breach in metres. "The proposed extension projects 1.8m above the 45-degree line taken from the head of the kitchen window at 22 local road."
4. Combine with BRE 209 where the scheme is large enough to warrant it. The two are complementary: the 45-degree rule is the visual headline, BRE 209 is the technical backstop.

Frequently asked questions

What is the 25-degree rule?
A section test from BRE 209: an obstruction whose top falls below a 25-degree line drawn from 2.0m above ground outside the affected window is unlikely to materially harm daylight, and full VSC analysis is not strictly required.

What is the 45-degree rule?
A proximity test, applied in plan and in section from the nearest neighbouring window. It is no longer in BRE 209 but survives in many borough SPDs as a quick presumption.

Are these rules still relevant?
Yes. The 25-degree rule is the BRE 209 first-pass screen; the 45-degree rule is widely cited in householder amenity policy.

Can a scheme pass the 25-degree rule and still cause harm?
Yes — the rule is a screen, not a ceiling. Where there are unusual factors (deep rooms, very tall windows, narrow streets) the full BRE 209 analysis can still find a noticeable effect.

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