The 45 degree rule and a rear extension

The 45 degree rule extension test is a council device, not national policy and not part of BRE 209. It draws a line at 45 degrees from a neighbour's habitable room window and asks whether the proposed extension crosses it. Break the line and the officer treats loss of daylight and an overbearing effect as likely. Stay behind it and the amenity objection usually falls away without any calculation at all.

Because it is local guidance, the version that applies is the one in the design guide published by the council determining your application. The geometry differs between authorities more than most people assume.

What the 45 degree line measures

The rule is a screening device. It substitutes a simple piece of geometry for a daylight calculation that would be disproportionate on a householder application.

In plan, a line is drawn at 45 degrees from the nearest edge of the neighbour's habitable room window, usually taken from the mid-point of that window, running across the boundary towards the proposed extension. If the extension projects past the line, the test is failed.

In section, a line at 45 degrees is taken from the same window, normally from the centre of the window or from the window head, rising towards the extension. A single storey extension close to the boundary often passes in section and fails in plan. A two storey extension usually fails in section first.

Some councils require both tests to be satisfied. Others treat the plan test as the primary check and use the section test only for two storey work. A few apply the line from the window at 45 degrees in plan and 25 degrees in section, which is a stricter combination. Read the guide.

Taking the line from the right window

Most arguments about the 45 degree rule are arguments about which window it is drawn from.

The window has to serve a habitable room. Bathrooms, cloakrooms, halls, landings, utility rooms and garages do not count. Kitchens are treated as habitable by most authorities, though a minority treat a small galley kitchen as secondary. Where a room has more than one window, the test is applied to the one nearest the proposal.

Two further points come up repeatedly. A window in a single storey rear addition of the neighbour's house counts, and it is usually the closest window to any new work, which is why side return schemes fail this test more often than rear extensions of the same depth. And a window that the neighbour has themselves added under permitted development still counts, whatever the applicant thinks about the fairness of that.

Where the neighbouring house has been extended and the room behind the window is now an open plan kitchen and dining space, argue the point in the planning statement rather than assuming an officer will read the plan the way you do.

Where the rule sits against BRE 209

These are two different tests measuring two different things, and conflating them weakens an application.

BRE 209 provides an initial check based on a 25 degree angle: where a new obstruction subtends an angle to the horizontal of less than 25 degrees measured at the centre of the lowest window of a neighbouring building, diffuse daylight is unlikely to be materially affected. If that check is passed, no further calculation is needed. If it is failed, the assessment moves to the vertical sky component and daylight distribution tests, with 27 per cent vertical sky component and the 0.8 former value comparison as the reference points.

The council's 45 degree rule is about amenity in a broader sense, taking in overbearing effect and enclosure as well as light. It is a planning judgement expressed as geometry.

So a scheme can pass the BRE check and still fail the council's rule, and the officer is entitled to rely on their own adopted guidance. The reverse also happens, and it is the more useful position for an applicant: where a scheme breaks the 45 degree line by a small margin, a short Daylight and Sunlight Assessment showing that the BRE tests are comfortably met is the evidence most likely to carry the day.

Single storey against two storey cases

For single storey rear extensions the plan test does most of the work, because the height is limited and the projection is what affects the neighbour.

For two storey extensions the section test dominates. A two storey rear extension on a boundary will break a 45 degree line from a ground floor window almost regardless of depth, which is why most approvals on tight plots take the form of a two storey element set back from the boundary with a single storey element continuing to it.

Side extensions behave differently again. A two storey side extension built to the boundary usually engages the neighbour's flank windows and their sense of enclosure rather than their rear windows, and many councils apply a separate 1 m set-back or first floor set-in rule for those. Related thinking on first floor elements and their effect on neighbours is set out in balconies and the overlooking test.

When a council applies 25 degrees instead

Some authorities have adopted 25 degrees in section rather than 45, which is a materially tighter test and derives from the BRE initial check rather than from amenity guidance.

Where that is the case, the practical effect is that extension height at the boundary matters far more than depth. A 3 m deep extension with a 2.4 m boundary wall may pass where a 2.5 m deep extension with a 3.2 m parapet fails.

Check the guide rather than guessing, and check its date. Design guides adopted before the 2022 revision of BRE 209 sometimes cite figures that have moved, and while the council will apply its own adopted document, knowing where its numbers came from helps when arguing a marginal case.

Design changes that pass the line

In rough order of how much they buy you:

1. Step the extension in plan so the section nearest the boundary is shallower, taking full depth on the far side.
2. Lower the eaves or parapet at the boundary, which moves the section test more than anything else.
3. Set the flank wall 0.5 m to 1 m off the boundary, which opens the plan line and gives maintenance access.
4. Hip or slope the roof away from the boundary instead of running a flat roof at full height.
5. Drop the two storey element back and keep only single storey at the boundary.

A scheme redesigned this way usually loses less internal floor area than the owner fears, because the lost corner is generally circulation rather than usable room.

Presenting the geometry on a drawing

Officers will not construct the line for you, and an application that leaves them to do it invites a cautious answer.

Include a plan at a stated scale showing each affected neighbouring window, its use labelled, the mid-point marked, the 45 degree line drawn and the clearance or breach dimensioned. Include a section on the same line showing the window head, the proposed flank wall and eaves, and the angle. Put a north point on every sheet.

Then explain it in words. A paragraph in the planning statement setting out which window the line has been taken from, why that window, and what the result is, does more than any amount of additional drawing. Where design is contested as well as amenity, the same reasoning belongs in the Design and Access Statement. Schemes that fill a side return face the test at its tightest, as side return infill on a Victorian terrace shows.

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 documents a householder application is likely to need, enter your postcode on PlanScope.