How to measure the 45 degree line from a neighbour window
Draw the 45 degree line from the centre of the neighbour's nearest habitable room window, in plan and in section. This explains the method, the windows that count and the cases where a breach still gets permission.
The 45 degree line is drawn from the centre of the nearest habitable room window on the neighbouring property, at 45 degrees to the plane of that window's wall, and your extension should stay outside it. Break the line and the council will usually say the extension causes an unacceptable loss of light or outlook. It is a rule of thumb from council design guidance rather than anything in national policy, which is why it can be argued with and occasionally beaten.
Two versions of the test exist and most guidance uses both. The plan version controls how far the extension projects past the neighbour's window. The section or elevation version controls how high it goes. An extension can pass one and fail the other, which is the usual reason a case officer's conclusion surprises the applicant.
Where the rule comes from
Nothing in the National Planning Policy Framework, current edition published 17 August 2026, mentions 45 degrees. The test lives in local householder design guides and supplementary planning documents, and it exists as a quick proxy for the amenity policy in the adopted local plan. So the legal test the officer applies is the local plan policy on residential amenity. The 45 degree line is the shorthand they reach for first.
That matters in two directions. A scheme that passes the line can still be refused if something else about it harms amenity, such as an overbearing two-storey flank wall or a terrace overlooking a bedroom. And a scheme that breaks the line can still be approved if the applicant shows that the actual effect on the neighbour is acceptable.
The technical method, as opposed to the rule of thumb, is BRE 209 Site Layout Planning for Daylight and Sunlight, the 2022 third edition. It uses a 25 degree vertical angle screening test, vertical sky component and annual probable sunlight hours. Councils rarely ask householders for that, but it is the evidence that answers a breach of the 45 degree line properly.
Taking the measurement in plan
Work on the existing block plan at 1:200 or 1:100, drawn accurately from a measured survey rather than from title plans.
- Identify the nearest habitable room window in the neighbouring property that faces the extension.
- Mark the centre point of that window on the outer face of the wall.
- From that point, draw a line at 45 degrees to the plane of the wall, running towards your property.
- Plot the proposed extension. If any part of it sits inside the line, on the window side, the test is failed.
Two details decide most arguments. The line is drawn from the window centre, not from the nearest edge of the opening, so a wide patio door gives a more generous result than a narrow one. And it is drawn from the wall plane, so a window set back in a reveal or recessed in a bay is handled by the geometry of the wall, not the glass.
Taking it in section
The section version is drawn on the rear elevation or on a cross section through the extension and the neighbour's window.
From the same centre point on the neighbour's window, draw a line rising at 45 degrees towards the extension. The eaves and the ridge of the proposal are then tested against it. A single storey extension with a 2.4 m eaves height clears the line at a short distance. A two storey extension almost never does, which is why most councils treat two storey rear extensions on a shared boundary as needing a different justification altogether.
Where the neighbour's ground floor window sits below external ground level, or where the two gardens are at different levels, take the levels from the survey. Sloping sites are where drawings go wrong.
Which window counts
A habitable room is a living room, dining room, bedroom or study. Bathrooms, WCs, halls, landings, utility rooms, garages and conservatories are not habitable rooms, so a window serving any of those is ignored for this test.
Kitchens are the grey area. Some councils treat a kitchen as habitable, some treat it as habitable only where it is a kitchen-diner, and some exclude it. Read the local design guide before you assume the answer, because a rear extension on a terrace often turns on whether the neighbour's galley kitchen window counts.
Secondary windows are also treated differently. Where a room has two windows and the one facing your extension is the smaller or later addition, the case officer may accept that the principal source of light is unaffected. Say so on the drawing and in the supporting statement rather than leaving the officer to work it out.
A window in the neighbour's own extension still counts, but its history is relevant. A neighbour who built a deep rear extension with a flank window is in a weaker position to object to something similar.
An illustrative test on a 1930s semi
This example is illustrative, with round numbers chosen to show the method.
A 1930s semi-detached house has a rear projection and a proposed single storey rear extension across the full width, 3.5 m deep, with a flat roof at 2.8 m and a parapet. The attached neighbour's ground floor rear window serves a dining room. Measured on the survey, the centre of that window sits 1.8 m from the party boundary.
In plan, a 45 degree line from the window centre reaches the party boundary 1.8 m back from the rear wall. The proposal projects 3.5 m along that boundary. The breach is therefore 1.7 m of depth measured at the boundary.
In section, the line from the window centre at, say, 1.4 m above the neighbour's ground level clears 2.8 m of height at 1.4 m distance. Along the boundary the extension is 2.8 m high from the rear wall onwards, so the section test fails as well.
Two changes fix it. Reducing the depth to 1.8 m at the boundary and stepping out to 3.5 m away from it passes the plan test while keeping most of the floor area. Or keeping 3.5 m of depth and dropping the boundary section to a mono-pitch with a 2.2 m eaves, then setting the extension 1 m off the boundary, passes both. Which one you choose depends on the internal plan, and a wrap-around extension is often the version that survives both tests because the depth sits away from the shared boundary.
Where the line is broken and permission still follows
Orientation does most of the work. A neighbour's window facing north loses little direct sunlight in any case, and the annual probable sunlight hours test in BRE 209 may show no material change. Officers accept that argument when it is supported by a proper daylight and sunlight note rather than asserted.
Other factors that have carried weight:
- An existing outbuilding, garage or boundary wall already obstructing the same view.
- A long garden where the window is set well back from the boundary.
- A room with a second, larger window on another elevation.
- A 45 degree breach measured in plan only, with the section test comfortably passed by a low mono-pitch roof.
- Local precedent along the same terrace, which the officer can see for themselves on the mapping.
Where you are relying on any of these, put the reasoning in a Planning Statement with the measured geometry attached. The case officer has to write a report that survives a complaint from the neighbour, so give them the sentences.
Recording the test on a drawing
Put the 45 degree line on the proposed block plan as a dashed line, annotated with the window it is taken from and the room that window serves. Repeat it on the rear elevation for the section test. Add a short note giving the measured distance from the window centre to the boundary and the projection depth.
Where a Design and Access Statement accompanies the application, the amenity paragraph should refer to the drawing number rather than repeating the numbers, so the two documents cannot drift apart.
Check the local guide before you draw anything. Some authorities take the line from the window frame rather than its centre, some use 30 degrees for two storey elements, some replace the whole method with a fixed depth allowance such as 3 m for a terrace and 4 m for a detached house, and a few apply a 60 degree line in denser areas. The geometry is only as good as the guide it comes from.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.
Enter your postcode on PlanScope to see which supporting documents your extension is likely to need. 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 council householder design guides and supplementary planning documents as a quick proxy for the residential amenity policy in the adopted local plan. The National Planning Policy Framework does not mention it. That means the line is a starting point for the officer's judgement rather than a rule you either pass or fail outright.
Do I measure the 45 degree line from the centre or the edge of the window?
Most guidance takes it from the centre point of the window opening, measured on the outer face of the wall. A minority of councils take it from the nearest frame edge, which gives a tighter result. Check the local design guide, because the difference on a wide patio door can be half a metre of extension depth.
Does a kitchen window count as a habitable room window?
It depends on the council. Some treat any kitchen as habitable, some only a kitchen-diner, and some exclude kitchens entirely. Bathrooms, WCs, halls, landings, utility rooms and garages are never habitable rooms for this test. On a terraced rear extension the answer often decides whether the scheme passes, so confirm it before drawing.
Can an extension that breaks the 45 degree line still be approved?
Yes. The line is a proxy, and a daylight and sunlight note prepared to BRE 209 can show that the actual effect on the neighbouring window is acceptable. Orientation, an existing garage or boundary wall already blocking the view, a second window serving the same room and local precedent along the terrace have all carried weight.
How do I show the 45 degree test on my drawings?
Add the line as a dashed annotation on the proposed block plan and repeat it on the rear elevation for the section test. Label the window it is taken from and the room it serves, and note the measured distance from the window centre to the boundary. Reference the drawing number in the supporting statement rather than repeating figures.
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.