BRE 45 Degree Rule: Quick Check for Domestic Extensions and Daylight

One of the most widely used and straightforward tools for assessing the daylight impact of domestic extensions is the BRE 45 Degree Rule. Often described as a “rule of thumb,” it offers a quick, visual way to assess whether an extension is likely to cause material harm to the light received by a neighbour’s window.

Domestic extensions are a common feature of residential development in the UK. Whether it's a rear kitchen extension, a side infill, or a wraparound addition, homeowners often seek to maximise internal space. However, ensuring that such developments do not significantly impact the daylight enjoyed by neighbouring properties is a critical part of the planning process.

This article explores the BRE 45 Degree Rule in detail, its purpose, how it works, when it should be used, and how it fits within the wider context of daylight and sunlight assessments under the BRE 209 (2022) guidelines.

What is the BRE 45 Degree Rule?

The BRE 45 Degree Rule is a simplified method for assessing the potential daylight loss to a neighbouring window caused by a proposed extension. It involves drawing 45-degree lines from a reference point on a neighbour’s window in both elevation and plan. If the proposed extension intrudes within both of these 45-degree angles, it is likely to result in a significant reduction in daylight.

The method is specifically intended for use in domestic situations, typically involving rear or side extensions to houses, and where the nearest window on the neighbouring property is set at 90 degrees (perpendicular) to the proposed development.

The 45-degree rule is also referred to as the “45-degree line of sight test” or the “45-degree angle rule.”

Purpose of the 45-Degree Rule

The rule is designed to provide a quick and conservative test to highlight whether a full daylight and sunlight assessment may be necessary. It offers a screening tool rather than a definitive measurement of daylight loss.

If an extension does not breach the 45-degree angles drawn from the centre of a neighbouring window, it is unlikely to cause a significant reduction in skylight to that window. If it does breach the line in both plan and elevation, a more detailed Vertical Sky Component (VSC) or No Sky Line (NSL) analysis should follow.

When and Where to Use the 45 Degree Rule

The BRE 45 Degree Rule is particularly useful where:

The proposed extension is adjacent to a neighbouring house.

The neighbouring property has a main window (serving a habitable room) close to the boundary.

The window is set at 90 degrees to the proposed development – i.e., it is not directly facing the extension.

The extension is located at the rear or side of a house.

It is not suitable for assessing impacts on:

Windows that directly face the extension.

Situations involving windows opposite the proposed development.

Non-domestic buildings or complex urban situations where other assessment methods such as VSC or NSL are more appropriate.

How the 45-Degree Rule is Applied

To apply the rule, two checks must be made: one in elevation, and one in plan.

1. Elevation Check

Draw the elevation of the window wall of the neighbouring property.

From the centre point of the affected window, draw a line at 45 degrees downward towards the site of the proposed extension.

If the extension remains below this line, it passes the elevation check.

2. Plan Check

Draw the site plan including the affected window and the proposed extension.

From the centre of the window, draw a 45-degree angle inward toward the extension.

If the extension stays outside of this line, it passes the plan check.

Combined Test

For a proposed extension to pass the 45-degree rule:

It must not intrude within the 45-degree angle in both elevation and plan.

If it does intrude in both dimensions, it indicates a potential material loss of daylight, and further testing (such as VSC or NSL) is recommended.

Worked Example

Imagine a neighbour’s kitchen window is 1.6m high and located 2.5m from the boundary. A proposed rear extension would be 3.2m deep and 3m high at the eaves.

In elevation, draw a 45-degree line downward from the window centre (1.6m above ground) towards the extension.

In plan, draw a 45-degree line diagonally from the window’s centre point toward the rear extension.

If the extension cuts into both lines, then a more thorough daylight and sunlight analysis should be carried out using Vertical Sky Component (VSC) or No Sky Line (NSL) methods, as set out in BRE 209 (2022).

BRE 209 (2022) and the 45 Degree Rule

BRE 209 (2022) – Site Layout Planning for Daylight and Sunlight – remains the UK’s most authoritative guidance for assessing natural light in planning applications. While the document introduces new methodologies and flexible targets, the 45-degree rule remains a valid and practical early-stage tool.

The guidance notes that this test applies best to domestic rear extensions, particularly those next to windows set at 90 degrees to the extension.

The BRE also warns that:

“Special care needs to be taken in cases where an extension already exists on the other side of the window, to avoid a ‘tunnel effect’.”

In such scenarios, where multiple obstructions exist or are proposed, a more detailed analysis is warranted.

Strengths and Limitations of the 45 Degree Rule

Strengths

Quick and easy to apply.

Useful during early design and pre-app stages.

Helps identify where further analysis is needed.

Supports decision-making for small-scale domestic extensions.

Limitations

Does not quantify the actual amount of daylight loss.

Cannot be used for windows directly facing the development.

Less applicable in urban and complex layouts.

Does not account for internal room layouts, room depth, or window glazing area.

Integrating the 45 Degree Rule in Planning Submissions

While the 45-degree rule is not a statutory test, many local planning authorities refer to it in local validation checklists or design guides. It is a useful inclusion in supporting statements for householder planning applications.

However, where the rule is breached, applicants should be prepared to submit:

Daylight and sunlight reports using BRE 209 methodologies.

Vertical Sky Component VSC/No Sky Line NSL diagrams.

Commentary on room use and internal layout.

Frequently Asked Questions

What is the 45 Degree Rule in Planning?

The BRE 45 Degree Rule is a quick test used to assess whether a proposed extension might adversely affect daylight to a neighbouring window. It involves drawing 45-degree lines from the affected window in elevation and plan to see if the extension blocks the view of the sky.

Is the 45 Degree Rule legally binding?

No. It is advisory, not mandatory. However, many local planning authorities use it as part of their design guidance and planning assessments.

Can an extension breach the 45 Degree Rule and still be approved?

Yes. If a detailed daylight and sunlight assessment shows that the impact on the neighbouring property is acceptable, then a breach of the rule does not automatically mean refusal.

What happens if my neighbour already has an extension on the other side of the window?

This is known as the ‘tunnel effect’. If your extension results in both sides of your neighbour’s window being blocked, it’s more likely to cause a significant impact. A detailed daylight analysis is recommended.

Should I still use the 45 Degree Rule if I plan a loft extension?

Not usually. The rule applies to ground floor or lower floor extensions and perpendicular windows. Loft extensions may require different assessment methods, such as VSC or shadow plotting.

Conclusion

The BRE 45 Degree Rule is a trusted, simple method for early evaluation of daylight impacts from domestic extensions. While it does not replace more rigorous daylight and sunlight assessments, it plays a vital role in guiding design decisions and preventing planning conflicts.

Used correctly, it helps ensure that extensions respect the light enjoyed by neighbouring properties, supporting better outcomes for both applicants and neighbours alike.

Need Help with BRE 45 Degree Rule?

At Anstey Horne, we specialise in delivering clear, accurate, and robust daylight & sunlight assessments for planning applications and appeals in London. If you're planning an extension or want to understand how the BRE 45 degree rule could help with the assessment of daylight & sunlight, get in touch with us today.

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