45-degree rule for extensions: what it means in planning

The 45-degree rule is a simple design test used by some councils when considering whether a house extension could have an unacceptable effect on a neighbour’s daylight or outlook. You may see it in a local design guide, householder extensions guide or supplementary planning document.

It is useful, but it is also regularly misunderstood. There is no single national 45-degree planning rule, it is not written into the GPDO and it does not automatically decide whether an extension should be approved or refused.

The exact test depends on the council’s own adopted guidance. That local wording and diagram matter.

What is the 45-degree rule?

At its simplest, a council draws an imaginary line at 45 degrees from a specified point on a neighbouring habitable-room window. If a proposed extension crosses that line, it may indicate a greater risk of loss of daylight or an overbearing effect.

That sounds straightforward, but the important words are “specified point”. Different councils use different versions of the test. One may measure from the centre of a window, another from its nearest edge. Some apply the test in plan, some in elevation, and some use both. The rooms covered can also differ.

So a diagram taken from another council’s website is not necessarily the rule that applies to your property.

Is the 45-degree rule planning law?

No. There is no national statute or regulation saying that every domestic extension must pass a 45-degree test.

Where a council has adopted the rule in a supplementary planning document or other design guidance, that guidance can be a material consideration when a planning application is decided. The weight given to it will depend on the wording of the development plan, the status and age of the guidance and the facts of the site.

That is different from saying the guidance has the same legal status as the development plan. An SPD cannot create a new development-plan policy by itself.

Passing the test does not guarantee planning permission

An extension which sits comfortably inside a council’s 45-degree line can still cause other planning problems. Its depth, height, bulk, proximity to boundaries, overlooking, design, effect on outlook or cumulative relationship with existing extensions may still be relevant.

Likewise, passing the test does not override development-plan policy or make an otherwise unacceptable proposal acceptable.

The 45-degree rule is normally a screening or assessment tool, not a magic approval button.

Failing the test does not always mean refusal

The reverse is also true. Unless the local policy or guidance says otherwise, crossing a 45-degree line is not usually an automatic statutory bar to permission.

The decision-maker still has to exercise planning judgment. The amount of infringement, orientation, existing buildings, window position, room use, boundary treatment, levels and overall effect on living conditions may all matter.

A modest infringement in an unusual site relationship may be judged acceptable. A much deeper extension which technically passes a simplified diagram could still be harmful for other reasons.

Does the 45-degree rule apply to permitted development?

Not as a separate national test.

If an extension is genuinely permitted development under Class A householder permitted development rights, the local council cannot add its ordinary 45-degree design-guide test to the GPDO limitations and conditions.

The GPDO has its own rules on matters such as depth, height, eaves and proximity to boundaries. Larger home extensions using the prior approval route for extensions up to 6 or 8 metres also have a specific neighbour-consultation procedure.

However, do not assume that a drawing labelled “permitted development” actually qualifies. If planning permission is required, the council’s local amenity guidance can become directly relevant.

Which window is used?

This is precisely why you need the local guidance. A common approach is to use the nearest window serving a habitable room in the neighbouring property, but the definition of the starting point varies.

Kitchens may be treated differently between authorities, particularly where they are also dining or living spaces. Bathrooms, halls and landings are commonly given less protection because they are not normally treated as principal habitable rooms.

Bay windows, corner windows, secondary windows and rooms served by more than one window can make a simple 45-degree diagram much less simple.

What happened in Manby v Hackney?

The importance of actually applying the local test was underlined by R (Bronwen Manby) v Mayor and Burgesses of the London Borough of Hackney [2026] EWHC 1543 (Admin).

The case concerned planning permission for extensions to a neighbouring house. Hackney’s Residential Extensions and Alterations SPD contained a 45-degree rule, and the objection to the proposal expressly raised it.

The officer report referred to the rule, but the High Court found that the council had never actually reached a conclusion on whether the proposal complied with it or what any non-compliance meant. Merely naming the relevant guidance was not enough.

The permission was quashed.

The judgment is particularly useful because it cuts both ways. The court did not say the 45-degree rule was an inflexible legal requirement. Applying it involved planning judgment. But once it was a material consideration which the council needed to address, the council had to do the assessment rather than simply mention the guidance.

This same principle is explained in more detail on our page about planning officer reports.

How should an applicant deal with the rule?

If the council uses a 45-degree test, deal with it on the drawings rather than hoping nobody notices it.

• Find the current local guidance and check exactly how its line is drawn
• Identify the relevant neighbouring habitable-room windows
• Show the test clearly on an existing and proposed plan or elevation where appropriate
• Explain any infringement rather than pretending it is not there
• Consider whether a small reduction in depth or height removes the issue altogether
• Remember that daylight, outlook, privacy and overbearing impact still need to be considered as a whole

What if you are the neighbour?

If a neighbouring application appears to breach the council’s own 45-degree guidance, identify the exact document and paragraph when commenting. A focused planning objection is much more useful than simply saying the extension will block light.

Explain which window is affected, what room it serves and how the proposal relates to the council’s test. If the guidance uses a diagram, a simple marked-up copy can help.

Our guide to neighbour consultation and planning objections explains what councils can and cannot take into account.

The 45-degree rule is not the same as a right to light

Planning amenity and the private-law right to light are different things. A council may assess the effect of a proposal on daylight as a planning matter, but planning permission does not determine whether a private right to light exists or whether it would be infringed.

Equally, a private right-to-light dispute does not by itself determine whether planning permission should be granted.

Relevant legislation and case law

Section 70(2) of the Town and Country Planning Act 1990 requires the decision-maker to have regard to the development plan and any other material considerations when determining a planning application.

Section 38(6) of the Planning and Compulsory Purchase Act 2004 provides the familiar starting point that determination must be made in accordance with the development plan unless material considerations indicate otherwise.

The key recent authority is R (Bronwen Manby) v Mayor and Burgesses of the London Borough of Hackney [2026] EWHC 1543 (Admin).

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Created: 29th August 2026 | 45-degree rule