The 45-degree rule and how officers apply it to extensions

The 45 degree rule planning test is a line drawn on a plan from the middle of your neighbour's nearest habitable room window to the nearest corner of your proposed extension. If the extension crosses that line, the officer treats it as likely to harm the neighbour's daylight and outlook, and you need either a redesign or evidence that the harm is not real.

It is a screening device, not law. No statute or national policy sets it. It survives because it is quick, it is drawn the same way by most authorities, and it catches the schemes that actually cause complaints.

Where the line comes from

Councils publish it in householder design guides and supplementary planning documents, usually under a heading about protecting residential amenity. The wording varies, and so do the details: some authorities measure from the window's mid-point, some from the nearest edge, and a few apply the line only at first-floor level.

The national evidence base sits elsewhere. BRE 209, Site layout planning for daylight and sunlight, provides the daylight methods that inspectors rely on, and it uses a 25-degree section test as its own screening step rather than a 45-degree plan line. So a scheme can cross a council's 45-degree line and still pass a proper daylight assessment. That gap is where most householder appeals on this issue are won.

Find your authority's version before you draw anything. The rule as applied in one district is not the rule as applied next door.

Drawing the line in plan and in section

The plan version is the one most officers use for a rear extension.

1. Identify the nearest habitable room window on the neighbouring property that faces the rear.
2. Mark the centre of that window on the plan.
3. Draw a line at 45 degrees from the plane of the rear elevation, running towards your plot.
4. Test whether any part of the proposed extension crosses that line.

The section version applies where the concern is height rather than depth. Draw the 45-degree line from the centre of the neighbour's window in the vertical plane, and see whether the extension rises above it. A single-storey extension at the boundary almost always passes in section and can still fail in plan if it is deep. A two-storey extension usually fails both.

Some guides use a stricter 30-degree line for the ground floor of terraced properties, or a 60-degree line where the neighbouring window serves a room with a second window. Read the diagram in the guide rather than working from the number alone.

Illustrative example: a two-storey rear extension

Illustrative example. A 1930s semi wants a two-storey rear extension 3.5 metres deep along the party boundary. The neighbour's ground-floor rear window serves a dining room and sits 1.2 metres from the shared boundary.

Drawing the plan line from the centre of that window gives a 45-degree line that the proposed extension crosses by roughly 1.9 metres at first floor. Officers would treat that as a clear breach. Two changes fix it without losing much floor area: bring the first floor back to 2 metres deep while keeping the ground floor at 3.5, and hip the first-floor roof away from the boundary. The remaining breach is at ground floor only, where the neighbour's own boundary fence already blocks the low-angle view, and a short daylight note can address it.

These dimensions are illustrative rather than drawn from a real application. The pattern, though, is the usual one: the ground floor rarely causes the objection, and the first floor almost always does.

Which windows count

Habitable rooms are living rooms, dining rooms, bedrooms and studies. Kitchens are treated inconsistently. Many guides include a kitchen only where it is the principal living space, which in practice means an open-plan kitchen and dining room counts and a galley kitchen off a hall does not.

These do not count: bathrooms, cloakrooms, utility rooms, halls, landings, garages and conservatories. Neither does a window in a wall that already faces a blank flank at close range, because there is no daylight to lose.

Two complications worth checking on site. A window serving a room that also has a second window facing another direction is given less weight, because the room retains an alternative light source. And a rooflight in a single-storey rear projection is usually treated as a secondary source rather than a protected window.

Rear depth allowances interact with all of this, so if the scheme is close to permitted development limits, read up on rear extension depth limits before committing to a full application.

The 25-degree test and when it is used instead

BRE 209's screening test works in section. Measure the angle to the horizontal, from the centre of the lowest window of the affected room, up to the top of the proposed obstruction. If the obstruction subtends less than 25 degrees, daylight is unlikely to be adversely affected and no further analysis is needed. If it exceeds 25 degrees, the assessment moves on to vertical sky component and daylight distribution.

That is a lower bar than a 45-degree plan line in most geometries, which is why a proper Daylight and Sunlight Assessment is often the answer to a design guide objection. Officers are not obliged to accept it, and a committee may prefer the simple test that residents understand. But it puts numbers into the officer report, and it gives an inspector something to weigh on appeal.

Use it where the stakes justify the fee. For a modest single-storey extension it rarely does. For a two-storey rear extension or a side-and-rear wrap-around on a tight plot, it frequently does.

What to do when your scheme crosses the line

Options, roughly in order of how often they work.

• Set the first floor back from the rear building line while keeping the ground floor deeper, so the mass that breaches the line is the part with least effect.
• Hip the roof away from the shared boundary, which pulls the bulk back at the point where the angle is measured.
• Reduce the eaves height on the boundary side, sometimes by a catslide or an asymmetric roof.
• Move the extension across the plot where there is space on the far side.
• Provide a daylight assessment showing the neighbour's window still meets the BRE guidelines.
• Obtain the neighbour's written support, which does not remove the policy objection but changes how a committee reads it.

What does not work is arguing that the neighbour's window is not really used, or that the extension matches one further down the street. Precedent arguments need the reference numbers of comparable approvals, and even then officers will distinguish them on geometry.

Set out whichever answer you choose in the Planning Statement with the line drawn on the submitted plan, rather than leaving the officer to draw it themselves. If the officer draws it, they draw it from the least favourable point.

Where the scheme is finely balanced, a short pre-application enquiry costs less than a refusal and a resubmission, and the officer's view on the 45-degree line is usually the first thing the response deals with. Enter your postcode on PlanScope to see which 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. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.