The 45 degree rule: how councils test loss of light

The 45 degree rule planning test is a rule of thumb councils use to judge whether a rear extension will take too much light and outlook from a neighbour's window. A line is drawn at 45 degrees from the centre of the neighbour's nearest habitable room window towards the proposed extension. If the extension crosses that line, the council treats it as a warning sign and looks harder.

It is guidance rather than law. You will not find it in national policy, in the Town and Country Planning Act or in the BRE daylight guide. It lives in householder design guides adopted by individual authorities, which means the version that applies to your house is the one your council published.

What the rule is measuring

The test is a proxy for two things at once: the amount of sky a neighbouring window can still see, and the sense of enclosure created by a wall close to a boundary.

It is crude by design. A single line cannot capture orientation, window size, room depth, ceiling height or the presence of a second window in the same room. What it does is give an officer, a neighbour and an applicant the same starting point, and that consistency is why it has survived for decades.

Failing the test does not mean refusal. Passing it does not mean approval. It shifts the burden of explanation, which in a delegated decision made in six weeks is worth a great deal.

Drawing the line in plan and in section

Most design guides apply the test in plan. Take the nearest habitable room window in the neighbouring property, find its centre point, and draw a line at 45 degrees from the plane of that window wall towards the boundary and beyond. A proposed extension that breaks through the line is judged to have a potential effect.

Some authorities also apply it in section, taking a 45 degree line from the centre of the window upwards towards the extension. That version catches the two storey extension that would pass in plan but loom in elevation.

A handful of guides measure from the window jamb nearest the boundary rather than the centre, which is a stricter test. Others measure from the midpoint of the window and require the line to be drawn from the window head. Read your council's diagram, because a difference of half a metre in the starting point can change the answer.

Note: the same document usually sets depth limits for single storey rear extensions, often 3 metres for a terraced or semi detached house and 4 metres for a detached house. Those figures echo the permitted development limits in Class A of Part 1 of the GPDO, and where the extension is permitted development the 45 degree test does not arise at all.

Which windows it applies to

Habitable room windows are the target. That normally means living rooms, dining rooms and bedrooms. Kitchens are treated as habitable by some authorities and not by others, and bathrooms, halls, landings and utility rooms are almost always outside the test.

Secondary windows serving a room that has another principal window are usually discounted. So are windows in outbuildings, conservatories and garages, though a conservatory that forms the main living space of a bungalow is an argument worth having.

Where the neighbouring window is on a side elevation facing the boundary, the test is applied differently or not at all, and officers fall back on general amenity judgement. That is also where the 25 degree screening test tends to be used instead.

Why it is guidance rather than policy

The decision is made under the development plan, and the relevant policy is usually a general amenity policy requiring development to avoid unacceptable harm to the living conditions of neighbours. The 45 degree rule sits beneath that policy as a tool for applying it.

Weight follows status. A test set out in an adopted supplementary planning document, consulted on and referenced by the local plan, carries real weight. A test in an unadopted officer guidance note carries less. Either way it informs the judgement rather than making it.

Inspectors treat it the same way on appeal. Breaching the line invites scrutiny, and appeals are allowed where the applicant shows that the actual effect on the neighbouring room is modest, particularly where orientation means the window never received direct sun from that direction anyway.

The 25 degree variant and the numerical tests

The 25 degree rule comes from a different place. The BRE guidance on site layout planning for daylight and sunlight uses a 25 degree screening test: if a new obstruction subtends an angle of 25 degrees or less to the horizontal, measured from the centre of the lowest window of the affected building, diffuse daylight is unlikely to be materially affected.

Pass that test and no further analysis is usually needed. Fail it and the numerical tests follow, chiefly the vertical sky component, where a value of at least 27 per cent, or at least 0.8 times the former value, indicates that daylight will remain adequate.

The two rules answer different questions. The 25 degree test is a daylight screening tool used mainly on larger schemes. The 45 degree rule is an amenity tool used on householder applications. Where a scheme is large enough or the relationship tight enough, a full daylight and sunlight assessment replaces both, and councils usually expect a suitably qualified author for that work.

Failing the test, and designing your way back

An extension that breaks the line is not finished. These are the changes that recover the position, roughly in order of how often they work.

1. Reduce the depth at the boundary while keeping the depth at the party wall or house wall, so the rear wall steps back.
2. Move the extension off the boundary by half a metre or a metre, which moves the 45 degree intersection significantly.
3. Drop the eaves height on the boundary side and pitch the roof away from the neighbour.
4. Take the rear element down to single storey where the first floor is the problem.
5. Chamfer or splay the rear corner nearest the neighbouring window.
6. Relocate the neighbour facing glazing and take light from a rooflight or a side light instead.

Each of those changes costs almost nothing on paper and a great deal once the drawings are done, which is the argument for testing the line at the first sketch.

Where the effect is on a garden rather than a window, the relevant analysis is shading rather than daylight, and an overshadowing study is the document that addresses it.

One honest uncertainty: the rule performs badly on sloping sites and on properties at an angle to one another, and different officers reach different answers on the same facts. Where the relationship is awkward, a pre-application enquiry is worth the fee.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and remember that a lawful development certificate is the only formal way to confirm that works are permitted development.

To see which documents your extension application is likely to need, enter the postcode on PlanScope. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.