Last reviewed: April 2026
Overshadowing is one of the most common reasons neighbours object to planning applications. This guide focuses on the practical geometric tests — the 45-degree rule and the 25-degree rule — that councils use to screen whether a proposed development is likely to cause unacceptable overshadowing, and how you can apply them yourself using the submitted plans.
Overshadowing occurs when a proposed building, extension, or structure blocks direct sunlight from reaching a neighbouring property — its windows, rooms, or garden. It is a material planning consideration, meaning a council can refuse an application or require amendments on this ground. However, to carry weight with planning officers, an overshadowing objection needs to go beyond general complaints about shadow. It must engage with the recognised geometric and technical tests.
This guide focuses on the two most practical screening tests — the 45-degree rule and the 25-degree rule — which you can often apply yourself from the submitted plans, together with the BRE sun-on-ground test for gardens. For the technical daylight tests (Vertical Sky Component, No-Sky Line, and the full BRE methodology), see our separate guide to loss of light planning objections.
The 45-degree rule is the most widely used screening test for overshadowing. It is adopted in supplementary planning documents and residential design guides across the country and is the test that planning officers most frequently apply to householder applications for rear and side extensions.
A line is drawn at 45 degrees from the midpoint of the nearest window of the affected neighbouring property towards the proposed extension. If the extension projects beyond this 45-degree line — that is, if it extends further than the point where the diagonal meets the centre of the window — the proposal is considered likely to cause a noticeable loss of light or overshadowing to that window.
The same test is applied in section view. A line is drawn at 45 degrees upward from the centre of the affected window towards the proposed structure. If the roof or wall of the extension crosses this line, the vertical 45-degree test is breached. A breach in either plane — horizontal or vertical — is significant.
The 45-degree rule is deliberately conservative. It is designed as a quick screening tool. If a proposal passes the 45-degree test, it is generally assumed to have an acceptable impact on daylight and sunlight. If it fails, that does not automatically mean the application should be refused, but it shifts the burden onto the applicant to demonstrate — usually through a full BRE daylight and sunlight assessment — that the actual impact is acceptable.
Many councils treat a 45-degree breach as a strong indicator of harm. Planning committees understand the test intuitively, and we routinely include annotated diagrams showing the 45-degree line in our objection letters.
If the submitted plans are drawn to scale, you can check the 45-degree rule yourself. On the floor plan, identify the midpoint of your nearest affected window. Draw a diagonal line at 45 degrees from that midpoint towards the proposed extension. If the extension falls beyond this line, it breaches the horizontal test. On the elevation or section drawing, repeat the exercise: draw a 45-degree line upward from the centre of your window. If the proposed structure crosses it, the vertical test is breached. Document this with measurements — a clear, measured breach adds considerable weight to an objection letter.
The 25-degree rule is set out in the BRE guidance Site Layout Planning for Daylight and Sunlight: A Guide to Good Practice and serves as a preliminary check before the more detailed daylight tests (VSC, No-Sky Line) need to be applied.
A line is drawn from the centre of the lowest affected window on the neighbouring property to the highest point of the proposed building or extension. If this line makes an angle of less than 25 degrees with the horizontal, the BRE guidance considers it unlikely that the new development will cause a significant loss of daylight, and no further testing is needed.
If the angle exceeds 25 degrees, the development is close enough and tall enough relative to the affected window that it is likely to have a noticeable impact on daylight. At that point, the BRE guidance recommends proceeding to the full quantitative tests — Vertical Sky Component and No-Sky Line — to determine the extent of the impact.
The 25-degree rule is particularly useful for objectors because, like the 45-degree rule, it can be checked from the submitted plans. If the applicant has provided section drawings showing the relationship between the proposed development and your property, you can measure the angle. A breach of the 25-degree line is a strong starting point for an objection because it demonstrates, using the BRE's own screening methodology, that the proposal warrants detailed assessment — and if no daylight report has been submitted, you can argue that one should be required before the application is determined.
Where both the 25-degree and 45-degree rules are breached, the case for refusal or amendment is particularly strong. The two tests measure slightly different things — the 25-degree rule focuses on the vertical relationship (height relative to distance), while the 45-degree rule considers both plan and elevation — but a dual breach indicates that the proposal is too close, too tall, or both.
The BRE guidance includes a specific test for overshadowing of gardens and outdoor amenity areas that is frequently overlooked by applicants but is a powerful tool for objectors.
The test recommends that at least 50% of any garden or outdoor amenity space should receive at least two hours of direct sunlight on 21 March (the spring equinox). If a proposed development would reduce the area receiving two hours of sunlight below 50%, or to less than 0.8 times its former value, the overshadowing of the garden is considered excessive.
This test is particularly relevant where a proposed two-storey extension to the south of your property would cast long shadows across your garden, or where a new building would shade a significant portion of your outdoor space during the spring and summer months. Even where the impact on your windows may be acceptable, the impact on your garden can be a separate and sufficient ground for objection.
While the 45-degree and 25-degree rules are screening tests, Annual Probable Sunlight Hours (APSH) is the BRE's detailed measure of direct sunlight reaching a window. It applies to any window facing within 90 degrees of due south.
The BRE guidance recommends that such windows should receive at least 25% of annual probable sunlight hours in total, with at least 5% falling in the winter months (21 September to 21 March). If a proposed development would reduce the APSH below these thresholds, and the reduction is to less than 0.8 times its former value, the loss of sunlight is considered noticeable.
The winter threshold is particularly important. A living room that receives adequate sunlight in summer but is left entirely in shadow from October to March will feel noticeably darker during the months when artificial light use and heating costs are already at their highest.
In everyday language, people use "overshadowing" and "loss of light" interchangeably, but in planning terms they refer to different things. Overshadowing refers specifically to the blocking of direct sunlight — the sun itself — measured by APSH and the sun-on-ground test. Loss of light typically refers to reduced daylight — the diffuse light from the sky — measured by Vertical Sky Component (VSC) and No-Sky Line (NSL).
A development can cause one without the other. A north-facing extension is unlikely to cause meaningful overshadowing because the sun does not come from the north, but it may still cause significant loss of daylight by reducing the VSC to nearby windows. Conversely, a low structure to the south may cast long shadows across your garden without substantially affecting the daylight reaching your windows.
For the full technical methodology on loss of daylight — VSC, No-Sky Line, the 0.8 times threshold, and BRE guidance in detail — see our guide to loss of light planning objections.
Planning Voice has extensive experience raising overshadowing effectively. In Bradford, our objection to multi-level extensions detailed how shadows would affect neighbouring gardens — the applicant withdrew. In Sefton, we demonstrated unacceptable loss of sunlight, contributing to refusal. In Southwark, we engaged with BRE guidance to show significant daylight reduction. In Adur & Worthing, we addressed overshadowing alongside coastal landscape impact.
We routinely apply the 45-degree rule, 25-degree rule, and BRE sun-on-ground methodology, and we know how to present the results in terms that carry weight with planning officers and committees. View our case studies →
Almost every local planning authority in England, Wales, Scotland and Northern Ireland has an adopted policy addressing the impact of development on the amenity of neighbouring properties. These amenity policies typically require that development should not result in an unacceptable loss of daylight, sunlight, or outlook.
At national level, the NPPF 2024, Chapter 12, establishes the general framework for good design, requiring development to provide a high standard of amenity for existing and future users. The London Plan 2021 Policy D6 is more specific, requiring development in London boroughs to provide adequate daylight and sunlight to surrounding properties.
Many councils also publish supplementary planning documents (SPDs) or residential design guides that set out how they apply the 45-degree rule and BRE methodology in practice. Before drafting an objection, check whether your local authority has published specific guidance on daylight, sunlight, and overshadowing — this will inform the standard against which the application is assessed.
The 45-degree rule is a geometric screening test used by councils to assess whether a proposed extension or building is likely to cause overshadowing to a neighbouring window. A line is drawn at 45 degrees from the midpoint of the affected window, both in plan and elevation. If the proposal crosses this line, it is considered likely to cause a noticeable impact. You can measure it yourself from scaled plans.
The 25-degree rule is a preliminary check from the BRE guidance. A line is drawn from the centre of the lowest affected window to the top of the proposed building. If the angle exceeds 25 degrees, the development is likely to have a noticeable impact on daylight and the full BRE tests should be applied.
Yes. The BRE guidance recommends that at least 50% of a garden should receive at least two hours of sunlight on 21 March. If a proposed development would reduce the sunlit area below 50%, or to less than 0.8 times its former value, this is a valid ground for objection.
Overshadowing refers to the blocking of direct sunlight, measured by APSH and sun-on-ground tests. Loss of light refers to reduced daylight (diffuse sky light), measured by VSC and No-Sky Line tests. Different BRE tests apply to each, and a development may cause one without the other.
Not necessarily. The 45-degree and 25-degree rules can be checked from the submitted plans. However, a professionally prepared objection that references the correct methodology and policies carries significantly more weight. Planning Voice prepares objection letters from £250 that apply the appropriate technical framework. Over 61% of our objections to date have resulted in refusals, revisions or withdrawals.
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