The 45 and 25 Degree Rules, Explained Properly

Somebody will tell you your extension breaks the 45 degree rule. It will be said with the confidence people reserve for things they believe are law, and it is not law. Neither of these rules appears in the permitted development Order, in national planning policy, or in the Building Regulations. They are lines drawn on a drawing to test one thing: whether the extension takes an unreasonable amount of light or sky away from the room next door. They matter a great deal, and they matter for reasons that have nothing to do with statute.

What these rules are, and what they are not

Guidance applied through local policy, not a national threshold you either meet or breach.

Loss of daylight and sunlight to a neighbour’s habitable rooms is a material planning consideration. It is one of the things a case officer is entitled to weigh, and on the prior approval route for a deep rear extension it is very nearly the only thing. What the 45 and 25 degree rules do is give that judgement a shape. They convert a vague worry into a line an officer can draw on a section and a plan.

Where they come from is a mixture of two sources. The technical basis is the BRE’s report on site layout planning for daylight and sunlight, now in its third edition, which sets out how obstruction of light is assessed. The version most homeowners meet is a simplification of that, published by individual councils in householder design guidance or a supplementary planning document, because running a full daylight assessment on every rear extension would be disproportionate.

Two consequences follow, and both are practical. The rules are not uniform, because each council writes its own. And breaching one is not a refusal, because they are indicative tests rather than standards.

The 25 degree test, properly

A section, a single point, and an angle above the horizontal.

Take the centre of the lowest window of the neighbouring habitable room. Draw a horizontal line from it, then a second line rising at 25 degrees towards your proposal, in vertical section. If no part of what you are building rises above that 25 degree line, enough skylight will still reach the window and no further assessment is needed.

That is the whole of the test, and its scope is narrower than people assume. It is an initial check, not a verdict. Where the obstruction does rise above 25 degrees, the guidance does not say the scheme is unacceptable; it says a more detailed assessment is now appropriate.

Detailed assessment means the numerical tests. Vertical sky component measures the proportion of sky visible at the centre of the window: a value of 27 per cent is treated as the point below which a reduction becomes noticeable, and the guidance pairs that with a proportional test, so a window losing more than a fifth of its former value is the one to look at. Alongside it sits a daylight distribution test, which maps the part of the room that can still see sky and asks how much of the floor loses it. Neither of those calculations is something a homeowner needs to do. Knowing they exist tells you what happens after a 25 degree breach, which is the useful part.

The 45 degree test, properly

Two drawings, not one, and the version that applies depends on which council you are in.

The 45 degree test is applied on plan and, for taller extensions, in section as well. On plan, a line is taken at 45 degrees from the neighbour’s nearest habitable room window towards your boundary. If the extension crosses that line, the council will look harder at what it does to the room behind the window. In section, the same 45 degree line is taken upwards from the same point, which is the test that catches height rather than depth.

Common practice is that a single storey extension is assessed on plan alone and a two storey extension on plan and section together, but that is a convention rather than a rule, and it is one of the things that varies.

What counts as a habitable room varies less. Living rooms, dining rooms, kitchens where the household eats, and bedrooms are in. Bathrooms, cloakrooms, halls, landings, utility rooms, garages and conservatories are generally out, on the basis that they are not rooms where daylight is required for ordinary use. A neighbour objecting about light to a downstairs toilet is objecting about something the officer is unlikely to weigh.

Why the answer changes at the district boundary

There is no national version of the 45 degree rule, which is exactly why people argue about it.

Councils differ on where the line is measured from, and the choice materially changes the result. Some take it from the centre of the window, some from its nearest edge, some from the nearest edge of the window opening and some from the flank wall. They differ on whether the test is applied in plan only or in plan and section. They differ on which extensions it applies to at all, and some publish variant angles for particular relationships rather than a single figure.

This part of Essex is a patchwork of authorities, and a scheme in one village is not being judged against the same document as a scheme two miles away in the next district. The Essex Design Guide, maintained by the county with the district authorities, sits behind a good deal of local practice, and most councils also publish their own householder or design guidance on top of it. Which document governs your address is worth establishing before anybody draws, because the numbers in them are not identical and neither are the diagrams.

The useful instruction is short. Find your council’s own published householder guidance, look at the diagram in it, and measure from the point that diagram uses. What a friend was told about their extension in another district is not evidence about yours.

What breaking the line actually means

A trigger for a closer look, in both directions.

Failing the test is not an automatic refusal. Officers routinely grant permission for schemes that cross the line, because the test is a screening tool and the real question is the effect on the room. A window that already faces a two metre boundary wall and a mature hedge is not losing much when a single storey extension appears beside it. A north facing kitchen window that currently sees open sky is.

Passing it is not a right to permission either, and that is the half people forget. Daylight is one material consideration among several. An extension that comfortably clears the 45 degree line can still be refused as overbearing, or for overlooking, or on its effect on the character of a conservation area.

What the tests genuinely give you is a way of having the argument in geometry rather than in adjectives. An objection that says the extension will make the kitchen dark is difficult to answer. A section showing the 25 degree line clearing the window head by 400 millimetres is not.

Sunlight is a separate test again

Daylight is about sky. Sunlight is about the sun, and only some windows are entitled to worry about it.

The BRE guidance assesses sunlight through annual probable sunlight hours, and it applies only to windows within 90 degrees of due south. A north facing window never receives direct sun, so there is nothing for your extension to take from it. The guidance looks for at least a quarter of annual probable sunlight hours to be retained, including a proportion of them in the winter months, when the sun is low and an extension does most of its blocking.

Overshadowing of a neighbour’s garden is assessed differently again, by looking at how much of the amenity space still receives sun on a given day. On a small rear plot with a two storey extension to the south of it, that is often the objection that carries more weight than the window does.

Overbearing is not a daylight test at all

The third consideration, and the one no angle captures.

A neighbour can lose very little measurable light and still be seriously affected. An enclosing or overbearing effect is about the presence of a large mass close to a boundary, seen from a window or from the part of the garden immediately behind the house. It is judged rather than calculated, which frustrates everybody, and it is the reason a two storey rear extension on a semi attracts objections that the same footprint at single storey would not.

The design responses are well understood. Set the upper storey back from the boundary even where the ground floor comes closer. Reduce the depth at the boundary end and let it run further out on the side away from the neighbour. Pitch the roof so it falls away from the shared boundary rather than presenting a flat parapet to it. Drop the eaves. Splay or step the corner nearest the neighbour’s window. All of these cost less at drawing stage than a refusal costs afterwards.

Two other things measured at 45 degrees

The number turns up three times in one project and means something different each time.

Under the Party Wall etc. Act 1996, excavating within 6 metres of a neighbouring structure and deeper than a line drawn downwards at 45 degrees from the bottom of their foundations triggers a notice. That is a structural provision about undermining, entirely unconnected to light, and it is set out in the Party Wall Act explained. Victorian and Edwardian roofs were also commonly pitched at around 45 degrees, which is why the figure appears again in loft conversion conversations.

None of the three has anything to do with the others. It is worth checking which one a person means before agreeing with them.

Right to light is a different animal entirely

A private property right, enforceable in court, and the council has no role in it whatsoever.

A right to light can be acquired under the Prescription Act 1832 where a defined window has enjoyed light uninterrupted for twenty years. It attaches to the aperture rather than to the room or the person, and it is a matter of property law between two owners.

The distinctions that matter are these. The test is not the planning test, and a scheme that satisfies a council’s daylight guidance can still infringe a right to light. The council cannot take it into account, will not check it, and a planning permission is no defence to it. The remedies are damages or, in the more serious cases, an injunction requiring work to be altered or taken down, which is a materially worse outcome than a refusal. It is rarely engaged by a modest suburban rear extension. It is worth taking advice on where you are building close to an old building with long established windows facing your plot.

Where this leaves your drawing

Ask for a section through the boundary early, showing your extension and the neighbour’s nearest window with the angles drawn on it. It is a small piece of work for a technician who has already measured the house, and it converts the whole subject from an argument into a fact. On the prior approval route for a deeper rear extension it is more valuable still, because amenity is the only thing the council may weigh, so this drawing effectively is the application.

None of this describes the light in your own new room, incidentally. The Building Regulations set no minimum daylight standard for a habitable room, so an extension can be entirely lawful and still be gloomy at the far end. That is a design question rather than a planning one, and it is answered with rooflights and a set back rather than with an angle.

We look at the boundary relationship at the survey, before anything is drawn, because on a tight plot it usually shapes the roof and sometimes the depth. If a neighbour has already raised the 45 degree rule with you, bring that up at the first visit and we will tell you honestly whether it has legs.

Local guidance is revised periodically and differs between authorities, and how any of it applies depends on your property and the relationship to the windows next door. Treat this as an explanation of how the tests work rather than as confirmation of what you can build.

Last reviewed August 2026. Planning and Building Regulations change, and your property may differ.