The 45 degree rule for rear extensions and where it comes from
The 45 degree line is a screening test in council design guides, not a national standard. How it is drawn, when it misleads, and the evidence that answers a failure.
The 45 degree rule rear extension test is a line drawn on a plan from your neighbour's window. If your proposed extension crosses it, the council's design guidance treats that as an indication that the extension will block too much daylight or feel overbearing. It is a local rule of thumb, written into householder design guides by individual councils. It appears nowhere in national planning policy and it is not part of the British Research Establishment guidance that daylight specialists use.
That matters, because passing the 45 degree test does not guarantee an approval and failing it does not guarantee a refusal.
What the rule tests
The rule is a proxy for two separate concerns rolled into one line. The first is daylight: whether enough sky will still be visible from the neighbour's window. The second is enclosure: whether a large structure close to the boundary will make the neighbouring room or garden feel hemmed in.
Neither is measured by the test. The line simply says that beyond a certain angle, experience suggests a problem is likely. Councils adopted it because it can be checked in 30 seconds on a submitted drawing by an officer who has 40 other applications that week.
Understanding it as a screening device rather than a standard changes how you use it. A scheme that passes comfortably rarely attracts a daylight objection. A scheme that fails narrowly needs a better answer than an argument about where the line was drawn from.
Why it appears in local design guidance
Local planning authorities publish householder design guides, usually as supplementary planning documents, to give applicants and officers a consistent starting point on extensions. Almost all of them include some version of the 45 degree test, and a good number also include a 25 degree version for taller structures further away.
The wording varies more than people assume. Some guides measure from the centre of the neighbouring window. Some measure from the nearest edge. Some apply the test only to habitable room windows, which usually means living rooms, dining rooms, kitchens used as a dining space and bedrooms, and disregard bathrooms, landings and utility rooms. Some apply it in plan only, others in plan and section.
So read your own council's guide before drawing anything. A scheme designed against a neighbouring borough's version of the rule will be tested against the local one.
Applying it in plan and in section
In plan, the line is drawn on the ground floor layout. Find the nearest habitable room window on the neighbouring property that faces the rear, mark the reference point your council's guide specifies, and draw a line at 45 degrees to the wall containing that window, running towards your site. The rear wall of the proposed extension should sit behind that line.
In section, the same angle is drawn in the vertical plane, from the window towards the extension, to test the height as well as the depth. A shallow extension with a tall parapet can fail in section while passing in plan.
Work through it in this order:
- Obtain accurate neighbour window positions, measured rather than scaled off an aerial image.
- Identify which of those windows serve habitable rooms.
- Apply the reference point and the angle your council's guide specifies.
- Draw both the plan and the section, on the same drawing sheet if the guide allows.
- Show the result clearly, with the line annotated, rather than leaving the officer to construct it.
Where the boundary is not at right angles to the rear elevation, which is common in Victorian terraces, the geometry gets awkward and the honest answer is to draw it properly rather than approximate. The same problem is discussed in obstruction angles and the early massing test, where the angle is used at a larger scale.
Ground floor windows against first floor windows
This is where most applications come unstuck.
A single-storey rear extension is normally tested against the neighbour's ground floor windows. A two-storey extension is tested against the first floor windows as well, and the first floor test is the harder one to pass, because the extension is closer in height to the window it affects.
| Proposal | Windows usually tested | Common outcome |
|---|---|---|
| Single storey rear extension | Ground floor habitable windows | Passes where depth is modest and the boundary wall is low |
| Two storey rear extension | Ground and first floor habitable windows | Often fails at first floor unless set in from the boundary |
| Rear dormer | First floor and roof-level windows | Assessed on roof form more than on the angle |
| Side extension near the boundary | Side and rear windows of the adjoining house | Depends heavily on window positions |
The usual fix for a failing two-storey scheme is to set the first floor in from the boundary while keeping the ground floor at full width. That produces a stepped form which most design guides encourage anyway.
Passing the rule and still causing harm
The test says nothing about outlook, overshadowing of a garden, or loss of privacy. Each of those is a separate ground on which permission is refused, and each can arise on a scheme that clears the 45 degree line comfortably.
Overshadowing is the most common. A structure to the south of a small garden can leave it in shade for most of a winter afternoon without coming near a habitable window. That is tested by modelling sun on the ground across the year, not by an angle, and where it is likely to be an issue an Overshadowing Study is the document that answers it. The particular difficulties in tightly packed streets are covered in rear extensions and overshadowing in dense terraces.
Privacy is the other. A new first floor window or a raised terrace looking into a neighbouring garden will attract objections whatever the plan geometry shows, and obscure glazing with a restricted opening secured by condition is the usual answer.
Note: a neighbour's objection is not by itself a reason for refusal. What counts is the effect on living conditions, described in planning terms.
Failing the rule and what follows
A failure is a prompt to look more closely, not an automatic refusal. Design guides are material considerations, and they are applied with judgement rather than as fixed standards.
Three routes out. Amend the scheme so it passes, which usually means reducing depth, setting the upper floor in, or lowering the eaves at the boundary. Show that the window the test uses does not serve a habitable room, with evidence rather than assertion. Or demonstrate with a technical assessment that the actual daylight effect is acceptable.
The third route is where a Daylight and Sunlight Assessment prepared to the British Research Establishment guidance earns its cost. That guidance uses measures such as vertical sky component and the annual probable sunlight hours, and it allows for the daylight a window already receives rather than treating every window as if it faced open sky. A window on a narrow London terrace may already be well below the 27 per cent vertical sky component figure the guidance uses as a benchmark, in which case the guidance's proportional test, comparing the new value against the former one, is the relevant comparison. That analysis frequently shows an acceptable result on a scheme that failed the 45 degree line.
It is not a cure-all. Where the objection is really about enclosure, a set of daylight numbers will not answer it.
Evidence to submit with a householder application
Put the 45 degree analysis on the drawings. An officer who has to construct the line themselves, from a site plan with no neighbour windows marked, will draw it conservatively.
Submit existing and proposed rear elevations showing the neighbouring house alongside, a section through the boundary, and a plan with the neighbour's habitable room windows marked and dimensioned. Add photographs from the neighbour's side of the fence where you can get them.
Then address the guide's wording directly in the covering statement. Name the design guide, quote the relevant paragraph, and say how the scheme responds. Where the scheme departs from it, say so and explain why the departure causes no harm in this particular case, which is a far stronger position than hoping nobody checks.
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. To see which supporting documents your extension needs, enter your postcode on PlanScope.
Frequently asked questions
Is the 45 degree rule in national planning policy?
No. It appears in local householder design guides published by individual councils, usually as supplementary planning documents. National policy does not contain it, and neither does the British Research Establishment guidance that daylight specialists use. It is a screening device that flags schemes needing a closer look, which is why passing it does not guarantee permission.
Which windows does the test apply to?
Most guides apply it to habitable room windows, meaning living rooms, dining rooms, bedrooms and kitchens used for dining. Bathrooms, landings, utility rooms and secondary windows to a room that has another are usually excluded. The reference point on the window varies between councils, with some measuring from the centre and others from the nearest edge, so read the local guide.
My extension fails the 45 degree line. Will it be refused?
Not necessarily. Design guidance is a material consideration applied with judgement, not a fixed standard. You can amend the scheme, show that the window used for the test does not serve a habitable room, or commission a technical daylight assessment showing the actual effect is acceptable. That last route often succeeds where the neighbouring window already has restricted sky view.
Does passing the test mean there is no daylight objection?
It makes one less likely but does not rule it out. The test says nothing about overshadowing of a garden, loss of outlook or privacy, and each of those is a separate reason for refusal. A structure south of a small garden can leave it in shade for much of a winter afternoon without approaching any habitable window.
Do I need to apply the rule to a permitted development extension?
No. The 45 degree rule is design guidance applied when a planning application is determined, so it has no role where works are carried out under permitted development rights. Where the larger home extension prior approval route is used, neighbours can comment on the effect on their amenity, and the council considers that in deciding whether to grant approval.
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.