The 45 Degree Rule Assessment: How Councils Judge Daylight and Outlook Loss From Extensions

If you are planning an extension, loft conversion or upward extension in 2026, there is a good chance your council will, at some point, draw a line at 45 degrees from your neighbour's window and ask whether your new roofline or wall crosses it. This is the 45 degree rule assessment, one of the most common (and most misunderstood) tools UK planning officers use to judge whether a proposed extension will unreasonably block daylight, sunlight or outlook to a neighbouring property.

It is not written into the planning system as a national law, and it does not appear in the General Permitted Development Order. It is a practical daylight and outlook test that many local planning authorities have adopted, in various forms, as part of their design guidance. For homeowners, understanding how it works, and using it before you submit drawings rather than after a refusal, can save months of delay and thousands of pounds in redesign costs.

This guide explains what the assessment actually measures, how it is applied to extensions, loft conversions and upward extensions, and how it interacts with the permitted development rules that already limit what you can build without planning permission.

What Is the 45 Degree Rule Assessment?

At its core, the 45 degree rule assessment is a way of checking whether a proposed extension will cut off an unacceptable amount of light or outlook from a neighbouring habitable room, usually a lounge, kitchen or bedroom window.

Planning officers (or your own architect, working proactively) draw an imaginary line at a 45 degree angle from a fixed reference point, most commonly the centre of the nearest affected window in the neighbouring property. If the proposed extension, roof extension or additional storey breaches that line by a significant margin, it is treated as a strong indicator that the daylight or outlook to that window will be materially harmed.

The test is usually run in two dimensions:

• In plan view, looking down from above, to check how far a side extension projects past the line drawn at 45 degrees from the neighbour's window wall.
• In section view, looking at a vertical slice through the properties, to check whether the height of a rear extension, dormer or additional storey breaches a 45 degree line drawn from the centre of the window.

Where a proposal breaches the line by a small margin, officers will usually look at the wider context (orientation, distance between properties, existing boundary treatments) before deciding whether the loss of light is significant enough to justify refusal or a reduction in size.

Where the Test Comes From, and Why It Varies by Council

The 45 degree assessment is not a single, nationally consistent formula. It grew out of general daylight and sunlight guidance (the kind of principles found in the Building Research Establishment's guidance on site layout planning), and different councils have adapted it in their own supplementary planning documents and residential design guides.

Some authorities apply a strict 45 degree line from the centre of the nearest habitable room window. Others use a 25 degree or 30 degree line instead, particularly for terraced or closely spaced housing where a 45 degree test would be almost impossible to satisfy. A smaller number of councils use a combination: a 45 degree test in plan and a separate vertical test measured from a point on the boundary rather than the window itself.

The practical implication for homeowners and developers is this: before you commission drawings, check your own council's residential extensions guidance or householder design guide. Search the council's planning policy pages for terms like "daylight and sunlight," "45 degree," or "extensions design guidance." The angle, the reference point and the tolerance for a "minor" breach can all differ from one authority to the next, even between neighbouring boroughs.

How the Assessment Works in Practice

The Plan (Bird's-Eye) Test

Imagine a semi-detached house with a side return extension proposed along the boundary shared with next door. The officer identifies the nearest habitable room window in the neighbouring property, typically a kitchen or living room window facing the boundary. From the centre of that window, a line is drawn at 45 degrees across the boundary. If the proposed side extension's flank wall stays behind that line, it generally passes. If the wall extends past the line and close to the neighbour's window, it is likely to be flagged as a daylight and outlook concern, particularly for single-aspect rooms that rely heavily on that one window.

The Section (Vertical) Test

For a rear extension, loft dormer, or additional storey, the same principle is applied vertically. A line is drawn at 45 degrees from the centre of the neighbour's window, rising upward and outward. If the ridge, parapet or dormer roofline of the new work breaches this line, officers treat it as a signal that the extension may significantly reduce direct sunlight and sky visibility to that window, especially where the neighbouring room faces south, south-east or south-west.

This vertical version of the test becomes especially important for two-storey rear extensions and for upward extensions that add whole additional storeys, because height, rather than depth or width, tends to have the biggest effect on how much sky is blocked from a nearby window.

When Councils Apply the 45 Degree Rule

The assessment tends to come into play in a handful of recurring scenarios.

Side Extensions and Wraparounds

Because side extensions run parallel to a shared boundary, they are the classic case for the plan-view 45 degree test. This matters particularly on article 2(3) land, which covers conservation areas, National Parks, Areas of Outstanding Natural Beauty, the Broads and World Heritage Sites. On this land, side extensions are not permitted development at all under the relevant permitted development class, meaning any side extension needs a full planning application, and will very likely be assessed against the council's daylight and outlook guidance, including a 45 degree check, as a matter of course.

Two-Storey and First-Floor Rear Extensions

A two-storey rear extension has the greatest potential to block light to a neighbour's ground-floor window, which is why the vertical (section) version of the assessment is applied so often here. In conservation areas and other article 2(3) land, rear extensions of more than a single storey are not permitted development, so any two-storey rear addition must go through full planning permission, where daylight and outlook impact, often tested using the 45 degree method, will be a central consideration.

Loft Conversions and Dormer Windows

Roof extensions such as dormers can also trigger a 45 degree check where they add significant height or bulk close to a boundary, particularly where a rear dormer is large or positioned close to a party wall overlooking a neighbour's garden or window. It is worth remembering that roof extensions of this kind are not permitted development at all on article 2(3) land: any dormer in a conservation area, National Park, AONB or World Heritage Site needs a planning application, which brings the daylight and outlook assessment into play from the outset. Roof balconies are treated even more cautiously and are not covered by permitted development rights anywhere, so a balcony created during a loft conversion will always need consent and will typically be scrutinised for overlooking and outlook effects on neighbours, alongside light.

Upward Extensions (Additional Storeys)

Since the introduction of the permitted development route allowing homeowners to add up to two additional storeys to a house with two or more storeys, or one additional storey to a single-storey house, councils have had to grapple with daylight and amenity impacts in a new way. This route requires prior approval, not a full planning application, but the prior approval process specifically requires the council to consider the impact on the amenity of neighbouring premises, including the impact on the amount of light received by neighbouring properties. In practice, many councils use a 45