A worked 45 degree test for a rear extension
The 45 degree line is the first test a case officer applies to a rear extension. Here is how to measure it in plan and section, with illustrative figures for a 1930s semi and the design fixes that pass it.
The 45 degree rule is the quickest way to predict whether a rear extension will upset the neighbours' light, and it is the test most householder design guides apply first. Draw a line at 45 degrees from the middle of the nearest habitable room window next door. If your extension crosses it, expect a question from the case officer. Cross it in both plan and section and expect a refusal unless you can show the effect is small.
It is a rule of thumb from local design guidance, not a rule of national planning policy, and it is not the same thing as a daylight assessment. Understanding that difference is what lets you argue your way past it when the geometry is against you.
Where the rule comes from and what status it has
Most English councils publish a householder design guide or a supplementary planning document that sets out how they judge the effect of an extension on a neighbour. The 45 degree line appears in almost all of them, usually alongside a 25 degree vertical test borrowed from the Building Research Establishment's guidance on site layout planning for daylight and sunlight.
The rule is a screening tool. It was never intended to prove loss of light, only to identify where loss of light is plausible enough to warrant a proper look. Where an extension passes, officers rarely investigate further. Where it fails, the question becomes whether the actual effect on the neighbouring room is unacceptable, and that is answered with a Daylight and Sunlight Assessment using vertical sky component and no-sky line calculations rather than a line on a drawing.
Because the rule is local guidance, the detail varies. Some authorities measure from the mid-point of the window, some from the nearest edge. Some apply it only to ground floor windows of habitable rooms, some to first floor windows as well. Kitchens count as habitable rooms in some guides and not others. Read the actual document for the authority you are applying to before you draw anything.
Measuring in plan and in section
In plan, the method is: identify the nearest window of a habitable room in the neighbouring property, mark its mid-point, and draw a line at 45 degrees from that point across the rear elevation towards your extension. The extension breaches the line if its projection beyond the original rear wall is greater than the lateral distance from the window mid-point to the extension.
That is the whole of the plan test, and it reduces to a comparison of two numbers. Projection versus offset. If you project 3 m and the window mid-point sits 3.6 m away along the rear wall, you pass. Project 4 m and you fail.
In section, the line is drawn from the same window, usually at sill level or at the mid-point of the window height, at 45 degrees upwards towards the top of the extension. Here the comparison is between the height of the extension above that datum and the horizontal distance to it. A single storey extension with a 3 m eaves almost always passes the section test when it sits several metres away, and almost always fails when it runs along a shared boundary immediately beside the window.
Which neighbour window to test
Test every habitable room window that faces the extension, not just the obvious one. On a typical pair of semis that means the neighbour's ground floor rear reception window and their kitchen window, and on a terrace it may mean windows in two directions.
Ignore bathrooms, hallways, landings and utility rooms in most guides. Do not ignore a window that serves a room converted to a bedroom, even if the original drawings called it a dining room, because the use on the ground is what counts.
Where the neighbouring property has a conservatory or an existing extension of its own, the test is usually applied to the original window position behind it. A conservatory does not shield you, and it does not give you extra licence either.
Illustrative worked measurements on a 1930s semi
The figures below are illustrative. They are drawn from a typical pair of 1930s semi-detached houses with a shared rear boundary and a straight rear building line, and they are meant to show the method rather than to describe any real property.
| Input | Scheme A, single storey | Scheme B, two storey |
|---|---|---|
| Projection beyond original rear wall | 3.5 m | 3.5 m |
| Lateral distance, window mid-point to extension | 1.8 m | 4.0 m |
| Height of extension above ground | 3.0 m | 5.2 m |
| Neighbour window sill height | 1.0 m | 1.0 m |
Scheme A fails in plan. The projection of 3.5 m exceeds the 1.8 m offset, so the 45 degree line is crossed by 1.7 m. In section it fails too: the extension rises 2.0 m above the sill datum at a horizontal distance of 1.8 m, which is an angle of about 48 degrees.
Scheme B passes in plan, because 3.5 m of projection sits inside a 4.0 m offset. But it fails in section. The extension rises 4.2 m above the sill at a horizontal distance of 4.0 m, roughly 46 degrees. This is the combination people miss: a two storey extension set well away from the boundary can clear the plan test and still break the section line, and it is the section failure that produces the "overbearing" wording in a refusal notice.
Scheme A would pass both tests if the projection came down to 1.8 m, or if the extension were pulled 1.7 m off the boundary and kept to a lower eaves. Scheme B would pass if the first floor element were set back from the rear wall of the ground floor by about 1.5 m, which is the classic design fix and usually costs little.
Where the rule is only a starting point
Three situations where a breach does not mean refusal.
The first is where the neighbouring window serves a large room with another window on a different elevation. Loss of light to one of two windows is a different proposition from loss of light to the only one.
The second is orientation. A breach on the northern side of a neighbour's house has a smaller effect on sunlight than the same geometry to the south, and the assessment methods recognise that with separate sunlight tests based on annual and winter probable sunlight hours.
The third is the existing baseline. Where the neighbour's window already faces a boundary wall, an outbuilding or a mature hedge, the additional effect of an extension may be modest. That argument needs evidence, not assertion: photographs, a measured survey of what is there, and preferably calculated results.
On the other side of the ledger, passing the 45 degree test does not immunise a scheme. Outlook, sense of enclosure, overlooking and the effect on a rear garden are separate considerations, and an extension that clears the line can still be refused on privacy grounds.
Presenting the drawings a case officer expects
Put the test on the drawing rather than leaving the officer to construct it.
- A block plan showing the neighbouring windows, annotated with room uses where you know them.
- A rear elevation at the same scale showing both properties, with the 45 degree line drawn from the relevant window and dimensioned.
- A plan at 1:100 with the 45 degree line drawn from the window mid-point, with the projection and offset dimensioned in metres.
- A short paragraph in the covering statement stating the result of each test and, where a line is crossed, what the actual effect on the room is.
- Existing and proposed sections through the boundary where the section test is close.
If the scheme has already been refused on neighbour amenity grounds, the same drawings become the backbone of an appeal case, and the structure of an appeal statement for a householder refusal shows where they sit in the argument.
A Design and Access Statement is not required for most householder applications, but a short design note carrying these drawings does the same job and answers the objection before it is made.
This is general guidance rather than legal advice, and each authority applies its own version of the 45 degree test, so check your council's householder design guide or speak to a qualified professional before relying on it.
PlanScope drafts the supporting statement that carries this analysis, structured to UK planning policy, with site facts flagged where they are missing. 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 comes from local householder design guides and supplementary planning documents, so the detail varies between councils. Some measure from the window mid-point, others from the nearest edge, and the rooms treated as habitable differ. Always read the guidance published by the authority deciding your application.
What happens if my extension breaches the line?
A breach is not an automatic refusal. It triggers a closer look at the actual effect on the neighbouring room, which is assessed using daylight and sunlight methods such as vertical sky component and the no-sky line. Evidence about room size, other windows and the existing baseline can show the effect is acceptable.
Which windows does the test apply to?
Windows serving habitable rooms facing the extension, normally living rooms, dining rooms and bedrooms, and kitchens in some guides. Bathrooms, hallways, landings and utility rooms are usually excluded. Test the actual use of the room rather than the label on the original drawings, because a converted dining room used as a bedroom counts.
Can an extension pass in plan and fail in section?
Yes, and two storey extensions often do. A first floor element set several metres from the boundary can clear the plan test while still rising above the 45 degree line in section. Setting the first floor back from the rear wall of the ground floor is the usual fix and rarely costs much floor area.
Does passing the test guarantee permission?
No. Outlook, sense of enclosure, overlooking, privacy and the effect on the neighbouring garden are separate considerations, as are design, materials and any constraint such as a conservation area. The 45 degree test answers one question about light, not the whole application.
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.