The 45 degree test for rear and side extensions

The 45 degree rule is a council design guide test, not a national standard and not part of the Building Research Establishment guidance. A line is drawn at 45 degrees from the nearest habitable room window on your neighbour's house, and if the proposed extension crosses it the council treats that as a warning sign that daylight or outlook may be unacceptably harmed. Crossing the line does not mean automatic refusal. It means the burden shifts to you to show the effect is acceptable.

Most householder refusals on amenity grounds start here, so it is worth knowing exactly how your council draws the line.

Where the line is drawn from

There are two versions of the test, and councils apply one or both.

In plan, the line is taken from the mid-point of the nearest habitable room window on the neighbouring elevation, drawn at 45 degrees across the boundary towards the proposed extension. If the extension's rear corner falls beyond that line, the test is failed in plan.

In elevation or section, the line runs from the same window at a set height, usually the centre of the ground floor window, at 45 degrees upward towards the extension. If the extension's roof crosses it, the test is failed in section.

The distinctions that decide the answer are these. A habitable room means a living room, dining room, kitchen used for dining or bedroom. Bathrooms, halls, landings and utility rooms are not habitable rooms, and windows to them do not attract the test. Where a room has two windows the test is applied to the one nearest the extension. Conservatories, which many guides treat as non-habitable, often fall outside it altogether.

Some authorities apply a 60 degree line instead, some apply 45 degrees in plan only for single storey extensions and both for two storey, and some measure from the boundary rather than the window. Read the adopted householder design guide before assuming.

What crossing the line actually means

The rule is a screening device. Its purpose is to identify the cases that need a closer look, not to determine them.

An officer faced with a scheme that crosses the line will consider the orientation of the neighbouring window, the separation distance, the existing situation including any boundary wall or outbuilding already there, the room the window serves, whether the room has another window, and what the actual loss of light would be.

South-facing neighbouring windows lose more from a given projection than north-facing ones, which is why two identical extensions on opposite sides of a street can get different answers. That looks inconsistent and it is not.

Where the scheme crosses the line substantially, or where the neighbouring room depends on that single window, a refusal on amenity grounds is likely. Where it crosses marginally and the neighbouring room has a second window on a return elevation, officers usually accept it.

The Building Research Establishment guidance contains its own 25 degree screening line for vertical sky component and a separate test for sunlight, and a formal daylight and sunlight assessment works to those numerical criteria rather than to the 45 degree code. On a contested householder scheme, that assessment is what displaces the rule of thumb with measured evidence.

Design changes that pull a scheme back inside

The good news is that this is a geometry problem, and geometry has solutions.

• Shorten the projection: the depth of a rear extension is the variable the line responds to most directly
• Pull the extension off the shared boundary by half a metre, which moves the corner away from the line and creates space for a maintenance strip
• Drop the roof: a mono-pitch falling towards the boundary, or a flat roof with a parapet, clears an elevational line that a hipped or gable roof crosses
• Step the rear wall so the part nearest the boundary is shallower and the deeper section sits away from it
• Reduce the eaves height on the boundary side, which is often enough on its own for a single storey extension
• Reposition the neighbour-facing corner by angling the rear wall, which is rarely elegant but does work

Combining two small changes usually beats one large one. Taking 600 mm off the depth and 300 mm off the eaves changes the geometry more than either alone, and it costs less floor area than a straight reduction.

Check the effect on your own permitted development position at the same time. A single storey rear extension that would otherwise be permitted development under Class A of the General Permitted Development Order has its own limits on depth, eaves height at the boundary and overall height, and a redesign to satisfy the council's 45 degree guidance sometimes brings the scheme back inside those limits, removing the need for an application at all. A lawful development certificate is the only formal way to confirm that.

Evidence to submit with a householder application

Councils see a great many applications where the drawings simply do not answer the question, so the officer has to guess and guesses conservatively.

Submit an existing and proposed block plan at a stated scale showing the neighbouring windows and their use. Add a plan-based 45 degree line drawing showing the test applied. Where the elevational version applies, add a section through the boundary showing the same. Include photographs of the boundary condition from both sides where you can get them.

State the room use of each affected neighbouring window, because that is the fact the officer cannot establish from the site.

A short Design and Access Statement that deals with the amenity question directly, rather than describing the materials for two pages, is worth more than the drawings on a marginal case. Where the geometry is tight and the neighbour has objected, an overshadowing study or a full daylight assessment gives the officer something to rely on other than the rule of thumb.

Talking to the neighbour first

None of this is a legal requirement and all of it helps.

A neighbour who has seen the drawings before they appear on the council's website objects less often than one who receives a notification letter out of the blue. If they raise a specific concern about a particular window, dealing with it in the design costs less than dealing with it after a refusal.

Where the neighbour objects anyway, their objection will be considered on planning grounds only, and a case officer weighing an objection against a scheme that passes the 45 degree test in both plan and section has an easy decision to make.

One honest note. The 45 degree rule is a blunt instrument, it was never intended as a scientific measure of daylight, and it produces some results that make little sense on the ground. Officers know this. That is why a well evidenced case explaining why the geometry overstates the harm on your particular site is worth making, and why it sometimes succeeds.

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. Enter your postcode on PlanScope to see which reports your project is likely to need.