The 45-degree rule: how councils apply it in plan and section

The 45 degree rule planning test is a line drawn from the middle of your neighbour's nearest habitable room window, at 45 degrees, towards your proposed extension. If the extension crosses that line, the council treats it as a warning that light and outlook will be materially harmed. Cross it in plan and in section and refusal becomes likely.

It is a rule of thumb, not law. No statute contains it and the National Planning Policy Framework does not mention it. It lives in council householder design guides and supplementary planning documents, which is why the version applied in one borough differs from the one next door.

Where the rule comes from

Councils need a quick, repeatable way to judge whether a rear extension will take too much light from the house next door. Full daylight modelling is proportionate on a block of flats and absurd on a 3 m single-storey addition, so authorities adopted a geometric shortcut and published it in householder design guidance.

That guidance is a material consideration. An officer refusing an application will cite the adopted design guide policy, not the 45-degree rule on its own, and the rule is the evidence that the policy test on residential amenity is failed.

Because it is local guidance, the detail varies. Some authorities apply 45 degrees in plan and 45 degrees in section. Some apply 45 in plan and a shallower angle in section. Some measure from the window's nearest edge rather than its centre. Read your council's householder design guide before you draw anything, and read the version currently adopted rather than the one that appears first in search results.

Drawing the line in plan

The plan test is the one most homeowners can do themselves on a site plan.

1. Identify the nearest habitable room window in the neighbouring property that faces the shared boundary or looks along it.
2. Mark the centre point of that window on the plan.
3. Draw a line at 45 degrees from that point, measured from the plane of the wall the window sits in, running across your garden.
4. Overlay the footprint of the proposed extension.
5. Note whether any part of the extension sits inside the angle, and by how much.

An extension that stops short of the line is usually accepted on amenity grounds without further evidence. One that crosses it needs an argument, and the argument has to be about the specific relationship rather than about how much you want the space.

Two practical points. Measure from the actual window position, which means visiting the boundary or working from an accurate measured survey, because online mapping is not precise enough at this scale. And where the two houses are staggered, the line from a window set forward of yours can clip an extension that looks perfectly modest on paper.

Drawing the line in section, and why it bites harder

The section test takes the same 45-degree line and applies it vertically, from the mid-point of the neighbouring window up towards the roof of your extension. It captures the thing the plan test misses: height.

This is where two-storey extensions and rear dormers fail. A single-storey extension 3 m deep might sit comfortably outside the plan line and still breach in section once you add a pitched roof rising to 4.5 m at the party boundary. Flat roofs and pitches that fall away from the boundary are the standard fix, and hipping the roof away from the shared side does more for the section test than shortening the extension by half a metre.

Ground level matters too. Where your garden sits higher than the neighbour's, the effective height at the boundary is greater than the drawing suggests unless levels are surveyed and shown.

Which windows the rule protects

Habitable rooms are the target: living rooms, dining rooms and bedrooms, with kitchens treated differently by different authorities. Some design guides protect kitchens fully, some only where the kitchen is the principal living space.

Windows that are usually outside the test include bathrooms, toilets, halls, stairs, landings, utility rooms, garages and any window that is already obscure-glazed as a condition of an earlier permission. Secondary windows serving a room that has another main window are also given less weight, though the officer will want to see that the other window exists.

Conservatories and later additions on the neighbour's side sit in an awkward middle ground. A window in a conservatory built under permitted development still lights a room somebody uses, and most councils will consider it, while giving it less protection than an original window in the main rear elevation.

When councils accept a breach

A breach is a trigger for scrutiny, not an automatic refusal. Officers accept them regularly where the wider relationship shows no material harm:

• The boundary runs north of the neighbouring window, so the extension takes little direct sun.
• Separation distance is large enough that the geometry overstates the effect.
• An existing outbuilding, wall or mature boundary planting already produces the same enclosure.
• The affected window serves a room with a second, unobstructed window.
• The breach is small, at the far end of the extension, and the room's outlook is retained.

Where you are relying on one of these, evidence it. Photographs from inside the neighbour's room, taken with permission, carry more weight than an assertion in the covering letter. A sun path study or a short Overshadowing Study can settle an orientation argument in a page. And where the relationship is marginal, a numerical Daylight and Sunlight Assessment against BRE 209 will either support you properly or tell you to redesign before you have paid a fee.

Recording the test on your drawings

Officers work through householder applications quickly. Make the test easy to verify and you remove a reason to object.

Show the 45-degree line on both the proposed block plan and a proposed section, with the neighbouring window positions marked and dimensioned. Annotate the window use where you know it. Include a levels note where the gardens differ. Then explain the result in the Design and Access Statement in three or four sentences: what the rule requires, whether the scheme complies, and if not, why the effect is acceptable.

Drawings that hide the line invite the officer to draw it themselves, and their version will be the pessimistic one.

If the scheme fails

Redesign is almost always faster than argument. The moves that recover a failing test are well worn: reduce the depth against the boundary while keeping it further out, step the rear wall so the deepest part sits away from the neighbour, drop to single storey at the boundary with the second storey set in, change a pitched roof to a lower flat or mono-pitch roof, and move glazing to the garden elevation.

If the application has already been refused, a householder appeal to the Planning Inspectorate runs on a shorter timetable than a full appeal, and inspectors do apply council design guides as material considerations while looking at the actual relationship on site. That is an argument worth making only where the site visit will support it.

To see which supporting documents a householder scheme is likely to need before you submit, enter your postcode on PlanScope and start from the constraints it picks up.

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.