Worked example: the 45 degree test on a semi-detached house
The 45 degree line from a neighbour's habitable window sets the depth of a rear extension. How to measure it, draw it, and three design changes that recover the lost area.
The 45 degree test asks a single question: does your extension break a line drawn at 45 degrees from the nearest habitable room window of the house next door. On a semi-detached house with the usual back to back layout, a full width rear extension built right up to the party boundary passes only if it is shallower than the distance from that window to the boundary. That is the whole of the 45 degree test worked example, and it explains why so many rear extensions are approved at 2.5 metres deep and refused at 3.5.
It is a local design guide tool, not national policy. Every council words it slightly differently and some apply it in plan only while others apply it in plan and section. Read your own authority's householder design guide before you measure anything.
Choosing the correct window
The test is applied to habitable room windows, and the choice of window decides the answer.
Most design guides treat living rooms, dining rooms, bedrooms and studies as habitable. Most treat bathrooms, WCs, hallways, landings, utility rooms and garages as not habitable. Kitchens are the disputed category: some authorities include them, some exclude them, and some include a kitchen only where it is the room's sole source of daylight.
You want the nearest habitable window on the ground floor facing the rear. Where the neighbour has two, test the one closest to the boundary, because that is the one that fails first. Where the neighbour's rear elevation is stepped, measure from the actual window position rather than from the building line.
Two further points. A window serving a room that was originally a garage conversion counts if the room is now habitable. And patio doors count as a window.
Measuring the window position
Three measurements, taken on site rather than scaled off a drawing.
The horizontal distance from the party boundary to the midpoint of the neighbour's window, measured along the rear elevation.
The window's width, which you need so you can find the midpoint.
The height of the window's centre above external ground level, for the section test.
Most design guides take the line from the midpoint of the window. Some take it from the edge nearest the boundary, which is stricter and reduces the permitted depth. Check which, because the difference on a 2 metre wide window is a full metre of extension depth.
Record the measurements with a photograph and a dated note. A case officer who cannot verify your dimension will take the neighbour's version.
Drawing the line in plan
Set the party boundary as your reference. Mark the neighbour's window midpoint on the rear elevation, and note how far it sits from the boundary. Draw a line at 45 degrees from that point, running away from the house and across the boundary into your garden.
Your extension passes if no part of it crosses that line.
The arithmetic falls out neatly because the angle is 45 degrees. At any lateral distance from the window midpoint, the permitted depth equals that lateral distance. So if the window midpoint is 2.2 metres from the boundary, a full width extension can be 2.2 metres deep. If you set the extension back 1 metre from the boundary, the nearest corner is 3.2 metres from the window midpoint laterally, so the permitted depth becomes 3.2 metres.
That relationship is the single most useful thing to understand about the test. Every metre you pull the extension away from the shared boundary buys you a metre of depth.
| Distance from window midpoint to nearest corner of extension | Permitted depth under the plan test |
|---|---|
| 1.5 m | 1.5 m |
| 2.2 m | 2.2 m |
| 3.0 m | 3.0 m |
| 3.2 m | 3.2 m |
| 4.0 m | 4.0 m |
Drawing the line in section
Where the design guide applies a section test as well, the line is taken from the centre of the neighbour's window, in a vertical plane, rising at 45 degrees away from the house. The extension's roof has to stay below it.
A single storey extension with eaves at around 2.4 metres and a roof sloping away from the boundary usually passes the section test wherever it passes the plan test. A two storey extension frequently fails the section test even where the plan test is satisfied, which is why two storey rear extensions on semis are so often reduced to single storey at the boundary with the first floor set in.
Keep this separate in your mind from the BRE guidance on daylight and sunlight, which uses a 25 degree vertical line from the centre of the lowest window as one of its tests, along with vertical sky component and no-sky line calculations. The two are different tools with different thresholds, and a council applying its own 45 degree guide is not applying BRE 209. Where the scheme is larger or the relationship is unusual, a daylight and sunlight assessment to the current BRE guidance is the document that carries weight, and councils expect a suitably qualified author for it.
Reading the result against the design guide
Failing the test is not automatic refusal. Design guides are guidance, and most say in terms that the 45 degree line is an indicator rather than a rule, to be applied with judgement.
Three circumstances where a breach is commonly accepted.
The affected window serves a room with another window on a different elevation, so the room is not reliant on the one being affected.
The orientation means the extension causes no meaningful loss of light, for example where it sits to the north of the window.
There is a substantial level difference, or an existing outrigger or boundary structure already produces much the same effect.
Three where it is not. The affected window is the only one serving a main living room. The extension is two storey. Or the authority's guide states the test as a standard rather than as guidance, which some do.
Adjusting the design to pass
In rough order of how much depth they buy you:
- Set the extension in from the shared boundary, which buys depth metre for metre.
- Reduce the depth at the boundary only, keeping full depth at the far side, producing an angled or stepped rear wall.
- Drop the eaves height and slope the roof away from the boundary.
- Replace a pitched roof with a flat roof and a rooflight or lantern, which lowers the mass at the boundary.
- Make the first floor element shallower than the ground floor on a two storey scheme.
Option two is the one most often overlooked and the one that preserves the most internal area. A rear wall angled back towards the boundary looks deliberate rather than apologetic if it is drawn properly, and it reads well in the design and access statement as a response to the neighbour rather than as a compromise forced on you.
An illustrative test on a 1930s semi
The following is an illustrative example. The figures are chosen to show the method and are not taken from a real property.
A pair of 1930s semis with rear elevations in line. The neighbour's ground floor rear window serves a dining room, is 2.0 metres wide, and its nearer edge sits 1.2 metres from the party boundary, so its midpoint is 2.2 metres from the boundary. The window centre is 1.3 metres above external ground level. The authority's guide applies the test from the window midpoint in plan, and in section for two storey elements only.
The owner wants a 3.5 metre deep full width single storey rear extension.
Plan test. At the boundary the 45 degree line sits 2.2 metres back from the rear wall. A 3.5 metre extension breaks it by 1.3 metres, so the scheme fails as drawn.
First option. Reduce the depth to 2.2 metres across the full width. The internal gain drops from roughly 3.5 metres to 2.2 metres of added depth, which may not deliver the kitchen diner the owner wanted.
Second option. Keep 3.5 metres of depth but set the extension 1.3 metres in from the party boundary, with the strip left as a side passage. The nearest corner is then 3.5 metres from the window midpoint laterally, which matches the 3.5 metre depth and sits on the line.
Third option. Angle the rear wall, taking 2.2 metres at the boundary and running back to 3.5 metres at the far side. The plan test is satisfied along its whole length and most of the internal area is kept.
The third option is usually the one worth drawing. Then show the neighbour before you submit, which the neighbour engagement template sets out, because an unopposed application that just satisfies the test is a far better position than a contested one that comfortably satisfies it.
This is general guidance, not legal advice; check your authority's own householder design guide and 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
Which window is the 45 degree test measured from?
The nearest habitable room window on the neighbour's rear elevation, usually a living room, dining room or bedroom. Bathrooms, hallways, landings and utility rooms are generally excluded, and kitchens are treated differently by different authorities. Where there are two candidates, test the one closest to the party boundary because it fails first.
Is the line drawn from the middle or the edge of the window?
Most design guides use the midpoint, but some take the edge nearest the boundary, which is stricter. On a two metre wide window that choice changes the permitted extension depth by a full metre, so read the authority's householder design guide rather than assuming the version you have seen elsewhere.
Does failing the 45 degree test mean refusal?
Not automatically. Most guides describe the line as an indicator applied with judgement. Breaches are commonly accepted where the affected room has another window, where orientation means no meaningful light loss, or where an existing structure already has a similar effect. Some authorities do state it as a standard rather than guidance.
How much extra depth do I gain by moving away from the boundary?
One metre of depth for every metre you set the extension in, because the angle is 45 degrees. If the window midpoint is 2.2 metres from the boundary, a full width extension can be 2.2 metres deep, and setting it in one metre takes the permitted depth to 3.2 metres.
Is the 45 degree test the same as the BRE daylight guidance?
No. The 45 degree test is a local design guide tool. The BRE guidance on site layout planning for daylight and sunlight uses a 25 degree vertical line from the centre of the lowest window, along with vertical sky component and no-sky line calculations. They are separate tools with different thresholds.
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.