The 45 degree rule for rear extensions: a worked example
The 45 degree rule is a screening tool in council householder design guides, not national policy. Here is how the lines are actually drawn, and what the test cannot tell you.
If your rear extension crosses a line drawn at 45 degrees from the middle of your neighbour's nearest habitable room window, expect the case officer to ask questions about light. That is the whole of the 45 degree rule for a rear extension. It is a screening device used in council householder design guides, it appears in neither the National Planning Policy Framework nor any statute, and failing it is not automatic refusal. Passing it is not automatic approval either.
The test is worth understanding properly because it costs nothing to run at sketch stage, before anyone has paid for drawings.
Where the test comes from and what status it has
Most local planning authorities adopt a householder design guide as a supplementary planning document. Those guides set out how the authority reads its own local plan amenity policy, and a very large number of them include some version of the 45 degree line, sometimes called the 45 degree code or the daylight splay test.
Because the guide is supplementary, it carries weight as a material consideration but sits below the adopted plan. The wording varies by council, and so does the strictness. Some apply the line in plan only for single storey work. Some apply it in plan and in vertical section for anything two storeys. A few use 30 degrees for particularly sensitive relationships.
The more technical cousin of the test is the 25 degree rule in the Building Research Establishment's guide, Site Layout Planning for Daylight and Sunlight, BRE 209 (2022). That takes a vertical section from the centre of the lowest window and asks whether the new building subtends more than 25 degrees to the horizontal. If it does, a fuller assessment of vertical sky component and annual probable sunlight hours is needed. The 45 degree rule is the planner's short cut. BRE 209 is the daylight specialist's method, and a Daylight and Sunlight Assessment is where that work lives when a case gets contentious.
Setting up the drawing
You need three things: an accurate block plan at 1:100 or 1:200 showing both properties, the position of the neighbour's ground floor and first floor windows on the elevation facing your garden, and the proposed extension shown on the same plan.
Habitable rooms are what counts. Living rooms, dining rooms, kitchens where the household eats, bedrooms. Bathrooms, hallways, landings, utility rooms and garages are generally discounted, and a window that already serves a room lit from another direction usually attracts less weight.
Mark the centre point of the nearest habitable room window on the neighbour's rear elevation. That is the origin for every line you draw.
Measuring the plan line
- Draw the plane of the neighbour's rear wall as a straight line across the plan.
- Find the mid-point of the nearest habitable room window in that wall.
- From that mid-point, draw a line at 45 degrees away from the wall, running towards your property.
- Plot the outline of your proposed extension on the same plan, to the same scale.
- Read off whether any part of the extension sits on the far side of that line, and by how much.
The equivalent elevation line runs from the same window mid-point at 45 degrees up from the horizontal, and it tests the height and bulk of the extension rather than its footprint.
An illustrative single storey case
Take a pair of 1930s semi-detached houses sharing a party wall, both with rear elevations flush with each other. The neighbour has a dining room window set 1.5 m from the party wall, measured to the centre of the window. This example is illustrative and the dimensions are chosen to show the method, not taken from a real case.
A single storey rear extension of 3 m depth built along the party wall produces a 45 degree line that clears the extension, because the line reaches the party wall 1.5 m out from the rear elevation and the extension is 3 m deep. It breaches by roughly 1.5 m in plan. Most guides treat a modest single storey breach as acceptable where the extension is no more than 3 m deep and around 3 m high to the eaves, particularly on a north or east facing garden where the light lost is limited anyway.
Push the same extension to 6 m under the larger home extension prior approval route in Class A of Schedule 2, Part 1 to the Town and Country Planning (General Permitted Development) (England) Order 2015 and the arithmetic changes. The breach is now about 4.5 m, the neighbour has 21 days to make representations under the neighbour consultation element of that procedure, and the council has to decide the impact on amenity of adjoining premises. The 45 degree line is often exactly what the officer's report cites.
The same case at two storeys
Two storeys is where the test starts to bite properly. Applying the 45 degree line in vertical section from the ground floor window, a two storey extension 3 m deep with an eaves height of around 5 m will usually cross the line, and the shadow it casts in the morning or evening is real rather than theoretical.
This is why so many councils restrict two storey rear extensions to around 3 m in depth, ask for the flank to be set in from the boundary by at least 1 m, and look for a hipped or pitched roof falling away from the shared boundary. The same shaping is required by the permitted development rules for a two storey rear extension, which caps the depth at 3 m and requires that no part sits within 7 m of the rear boundary opposite. Those limits exist for the same amenity reason the design guide does.
Where the two storey element steps in from the boundary and the ground floor element projects further, you can often pass the test at first floor level and accept a modest breach at ground level. Officers respond well to that, because it shows the relationship was designed rather than ignored.
What the test does not tell you
It says nothing about outlook, overbearing impact or overlooking, which are separate amenity harms with their own reasoning. A single storey extension that passes the 45 degree line can still be refused for creating a tunnelling effect along a narrow boundary.
It says nothing about sunlight, as opposed to skylight. Orientation is missing from the test entirely, so a breach on a south facing rear elevation matters far more than the identical breach facing north.
And it is not a legal threshold. It is a way for a busy case officer to sort applications into the ones that need thought and the ones that do not.
What to submit alongside the drawings
Existing and proposed plans and elevations at a stated metric scale, a block plan showing both properties and the neighbour's window positions, and a site location plan. If you breach the line, add a short annotated drawing showing the 45 degree lines in plan and section with the breach dimensioned, and explain in the Design and Access Statement why the effect is acceptable here.
Say what the room is used for if you know. Say which way the garden faces. Give the distance to the boundary and the eaves height. Officers refuse what they cannot understand far more often than they refuse what they have been shown clearly, and a Planning Statement that engages with the design guide by name rather than ignoring it takes the argument seriously.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. 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 documents your extension is likely to need.
Frequently asked questions
Is the 45 degree rule a legal requirement?
No. It appears in council householder design guides adopted as supplementary planning documents, so it is a material consideration rather than a rule of law. The adopted local plan amenity policy is what the decision is actually made under, and the 45 degree line is one way of applying it. Different councils word and apply it differently.
Which windows does the 45 degree test apply to?
Windows serving habitable rooms on the neighbouring property that face the proposal, normally living rooms, dining rooms, kitchens used for eating and bedrooms. Bathrooms, halls, landings, utility rooms and garages are usually discounted. A room with a second window on a different elevation is generally given less weight because it retains another source of light.
Does the test apply in plan or in elevation?
It depends on the council's guide. Many apply the line in plan for single storey extensions and in both plan and vertical section for two storey work. Where the guide is silent, running both and presenting them is the safer approach, because it shows the officer you have considered height as well as footprint.
What if my extension breaches the 45 degree line?
A breach is not fatal. Explain the orientation, the room affected, the distance involved and any design changes you have made, such as setting the upper floor in from the boundary or lowering the eaves. Where the relationship is sensitive, a daylight and sunlight assessment using BRE 209 methods gives the officer measured figures rather than a line on a plan.
Can I use permitted development instead of applying?
Sometimes. A single storey rear extension within the Class A limits needs no application, and larger ones between those limits and 6 m or 8 m need prior approval with a 21 day neighbour consultation. Permitted development rights can be removed by an Article 4 direction or a condition, and a lawful development certificate is the only formal way to confirm that works are lawful.
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.