An illustrative worked example of the 45 degree test
The 45 degree test is local guidance rather than national policy. Here is how it is drawn, where councils differ, and a worked illustrative example.
The 45 degree rule extension test is a rule of thumb, not a policy, and knowing that changes how you use it. You draw a line at 45 degrees from the nearest habitable room window of the neighbouring house. If your proposed extension crosses that line, the council's design guidance treats it as likely to harm the neighbour's daylight and outlook, and the officer will ask you to reduce it. If it stays clear of the line, you have a starting point for the argument rather than a permission.
The test appears in the householder design guidance of most English local planning authorities. The wording differs between them, sometimes materially, and there is no national version to fall back on.
Where the test comes from
It began as a drafting shortcut for officers dealing with high volumes of householder applications. A full daylight and sunlight analysis takes days. A 45 degree line takes two minutes with a scale rule, and for the great majority of rear extensions on ordinary suburban plots it gets to broadly the same answer.
Because it lives in supplementary planning documents and design guides rather than in the development plan itself, its weight is that of guidance supporting a policy on amenity. The policy is what refuses an application. The test is the evidence the officer uses to apply it.
That has a practical consequence. An appeal inspector is not bound to apply a council's 45 degree formulation, and inspectors regularly say that a numerical breach does not by itself demonstrate unacceptable harm, or that compliance does not cure a scheme that plainly overbears. Treat the test as the first question rather than the last.
Drawing the line in plan and in section
Most councils apply the test twice.
In plan, the line is drawn from the neighbouring window at 45 degrees to the plane of the rear wall, projecting away from the house. The extension should sit outside it. This limb controls depth, and it is the one that catches deep single-storey rear extensions on narrow plots.
In section or elevation, a line is drawn from the window at 45 degrees upwards, and the extension should sit below it. This limb controls height, and it is the one that catches two-storey rear extensions and raised eaves.
A single-storey extension that passes in plan can still fail in section if it has a high parapet or a pitched roof close to the boundary. Officers who only check the plan miss this, and so do applicants, until the neighbour's agent points it out.
Some authorities use 45 degrees in plan and a shallower angle in section, and a few apply the section test only above a stated height. Read the actual document.
Measuring from the correct window
This is where most disagreements start.
Three variables decide the answer. Which window: the test applies to habitable room windows, meaning a living room, dining room, bedroom or a kitchen used as a habitable space, and not to a bathroom, hall, landing or utility. Where on the window: some guides measure from the mid-point of the window, others from the nearest edge to the proposal, and the difference on a 2 m wide window is a metre of permitted depth. And what height: the section test may start at the sill, the window centre, or a fixed height above the internal floor.
Get these three from the council's own document before you draw anything. An extension designed against the mid-point convention and assessed against the nearest-edge convention will be a metre too deep.
Where a neighbouring window serves a room with two windows, the test normally applies to the one most affected. Where the neighbour has a rear extension of their own, the relevant window is usually the one in that extension, which can make a proposal easier rather than harder.
An illustrative worked example
The following is illustrative, to show how the numbers move rather than as a result to rely on.
Take a pair of 1930s semis with a shared rear boundary running square off the back wall. The neighbour has a dining room window in the original rear elevation, 1.4 m wide, its nearest edge 1.2 m from the party boundary. The proposal is a rear extension across the full width of the applicant's plot.
Applying the plan test from the nearest edge of that window, the 45 degree line reaches the party boundary 1.2 m back from the rear wall. An extension of 3 m depth built up to the boundary therefore crosses the line by 1.8 m at the boundary and fails.
Three ways to resolve it, each with a different cost:
- Reduce the depth at the boundary to 1.2 m and step the extension out to 3 m away from the boundary, giving an L-shaped plan. Floor area lost, roughly 2 m².
- Hold 3 m depth but set the whole extension 1.8 m off the boundary. More area lost, and a narrow unusable strip created.
- Keep 3 m depth against the boundary and argue the case on the actual effect, with a sun path study and a section showing the single-storey height.
Option 1 is what gets built most often. Option 3 succeeds where the neighbour's window is north-facing, or already obstructed by their own outbuilding, or where the room has a second window. It needs evidence, not assertion.
Now change one input. Make the extension two storeys. The section test comes into play, the 45 degree line from the window rises steeply, and a two-storey flank wall 3 m deep on the boundary will breach it on almost any plot of this width. The practical answer on a semi is usually a two-storey element set in from the boundary with a single-storey element beyond it.
Where local guidance differs
Differences worth checking before design work starts include the angle used in section, the measuring point on the window, whether the test applies to first-floor windows as well as ground floor, whether a 45 degree breach is treated as a presumption against approval or as a trigger for further assessment, and whether single-storey extensions under a stated depth are exempt.
A handful of authorities use a different device altogether, such as a fixed maximum depth for boundary development, or a sky visibility test. If the design guide is more than ten years old, check whether it has been superseded by a newer document rather than assuming the version on the website is current.
How this relates to the BRE method
The 45 degree rule and the BRE guidance on site layout planning for daylight and sunlight are different tools and they do not always agree.
The BRE approach measures actual daylight and sunlight: the vertical sky component at the window, annual probable sunlight hours, and the sun on ground test for gardens. Its screening angle in the vertical section is 25 degrees, not 45, and passing that screen means daylight is unlikely to be affected at all.
So a proposal can breach a council's 45 degree line and still perform acceptably on the BRE tests. That is the standard shape of a householder appeal on amenity grounds, and where the stakes justify it, a Daylight and Sunlight Assessment is the evidence that makes the argument. Our note on reading the results tables of a daylight assessment explains what the numbers in one actually mean.
If the scheme fails and you proceed anyway
Some applicants do, for good reasons. If you go that way, do the work properly.
Put a section and a plan in the application showing the 45 degree line and the breach, measured and labelled. Officers trust an applicant who draws the problem more than one who leaves it to be discovered. Then set out the evidence on actual effect: orientation, the existing obstructions, the room served, the window's other openings, and the sun path. Address outlook separately from daylight, because they are different objections and a single-storey extension can pass on light and still be refused for enclosure.
Explain the design response in the Design and Access Statement, including the options tested and rejected. If it is refused, the same material forms the backbone of an appeal statement of case.
Enter your postcode on PlanScope to see which documents a householder application on your site is likely to need.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Is the 45 degree rule national planning policy?
No. It appears in the householder design guidance and supplementary planning documents of most English authorities, with wording that differs between them. The development plan policy on amenity is what an application is refused under, and the 45 degree line is the evidence an officer uses to apply that policy.
Which window is the line measured from?
A habitable room window in the neighbouring property, meaning a living room, dining room, bedroom or kitchen used as habitable space, rather than a bathroom, hall or utility. Whether the measurement starts at the mid-point of the window or the nearest edge depends on the council's own document, and the difference can be a metre of depth.
Can I get permission if my extension breaches the line?
Yes, quite often. A breach shows the rule of thumb has been crossed rather than that harm has been proven. Cases succeed where the neighbouring window faces north, is already obstructed, or serves a room with another window. You need measured drawings showing the breach and evidence on the actual effect on light and outlook.
How is the 45 degree rule different from the BRE guidance?
They measure different things. The BRE guidance assesses actual daylight and sunlight using the vertical sky component, annual probable sunlight hours and the sun on ground test, and its screening angle in the vertical section is 25 degrees. A scheme can breach a council's 45 degree line and still perform acceptably on the BRE tests.
Does the test apply in section as well as plan?
In most authorities, yes. The plan limb controls depth and catches deep single-storey rear extensions on narrow plots. The section limb controls height and catches two-storey rear extensions, raised eaves and high parapets. A scheme that passes in plan can still fail in section, which is often missed until a neighbour's agent raises it.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.