Rear extension depth and the 45-degree test explained

The 45-degree test is the first thing a case officer draws on your plan, and it is not a rule in any statute. It comes from councils' own householder design guidance, where a line is struck at 45 degrees from the nearest habitable room window on the neighbouring property. If your extension crosses that line, the officer treats it as a potential harm to light and outlook and looks harder. Cross it in plan and in section, on a boundary with a north-facing neighbour, and you are into refusal territory.

None of which stops you building a 3 m single-storey rear extension under permitted development, if the house still has those rights. The test only bites when you need permission.

Where the 45-degree line comes from and what it tests

Local design guidance, adopted as supplementary planning documents, is where the line lives. It is a screening device rather than a calculation, designed to let an officer form a quick view without commissioning a daylight assessment for every householder application.

What it tests is the relationship between the mass of your extension and the neighbour's principal window. Not sunlight specifically, and not daylight in the technical sense. Outlook, enclosure and the sense of being built up against, alongside a rough proxy for light.

Because it is guidance rather than policy, an extension that crosses the line can still be approved. The officer has to weigh it, and the weighing is easier where you have shown you understand the point and mitigated it.

Plan and section application of the line

The two applications are different and both are used.

In plan, the line is drawn from the nearest edge or the centre of the neighbouring habitable room window, at 45 degrees to the plane of the rear elevation, extending towards your site. The test is whether the footprint of the extension crosses it.

In section, the line runs at 45 degrees upwards from the same window, and the test is whether the extension breaks it in height. A single-storey extension close to the boundary can pass in plan and fail in section, particularly where the neighbouring window is at ground floor and your eaves sit at 3 m.

Councils differ on which window is used, whether it is the centre or the edge, and whether the test applies to windows serving kitchens. Read your council's guide rather than working from a general description. The differences change the answer on a tight plot.

Depth limits councils publish in design guidance

Alongside the geometric test, most authorities publish indicative depth limits for rear extensions, usually varying by house type, with terraced properties treated most tightly because the gaps between rear elevations are smallest.

Those figures are indicative and they are local. There is no national depth limit for extensions that need permission. What there is instead is a set of permitted development limits, which is a different thing and frequently confused with council guidance.

Larger home extensions are not available on article 2(3) land, which includes conservation areas, National Parks, National Landscapes and World Heritage Sites. Neither are they available where permitted development rights have been removed by condition or by an Article 4 direction.

When the 25-degree BRE test is used instead

BRE guidance on site layout planning for daylight and sunlight uses a different angle for a different purpose. Its initial check asks whether the proposed development subtends an angle of less than 25 degrees to the horizontal, measured from the centre of the lowest window of a main living room. Below that, daylight is unlikely to be adversely affected and no further calculation is needed.

The two tests are not alternatives, and they are not measuring the same thing. The 45-degree line is a local design screening tool covering outlook and enclosure. The 25-degree check is the entry point to a technical daylight assessment using vertical sky component and daylight distribution.

Where a householder scheme fails the council's 45-degree test but passes the BRE check, saying so is worth a paragraph in the Design and Access Statement or the covering letter. It moves the discussion from a rule of thumb to evidence. On a larger scheme a full Daylight and Sunlight Assessment does the same job properly.

An illustrative check on a 1930s semi with a 4 m extension

Take a pair of 1930s semis with rear elevations in line, the party boundary running back from between them, and the neighbour's kitchen window 1.2 m from the boundary.

A 4 m deep single-storey extension built to the boundary line crosses the 45-degree plan line drawn from that window, because the line reaches the boundary at about 1.2 m from the rear elevation and the extension continues for another 2.8 m beyond it. In section, an extension with a 3 m eaves height at the boundary also breaks the 45-degree line from a window with a sill at about 900 mm.

Three changes fix it, in ascending order of cost to the client. Reduce the depth to 3 m. Set the extension 1 m off the boundary and pull the massing away from the neighbour. Or keep the depth and drop the eaves to 2.4 m with a mono-pitch roof rising away from the boundary.

This example is illustrative. The measurements on your own plot decide the answer, and the council's own version of the test may differ from the description above.

Design changes that resolve a failed line

The moves that work, roughly in the order officers suggest them:

1. Reduce the depth. Blunt, effective, and usually resisted by clients.
2. Step the rear wall so the part nearest the boundary is shallower and the deeper part sits away from the neighbour.
3. Lower the eaves height on the boundary side and use a mono-pitch or hipped roof.
4. Set the flank wall back from the boundary by 900 mm to 1 m.
5. Replace a solid flank with obscure glazing where the concern is enclosure rather than light.
6. Use a glazed roof section over the deepest part, which helps your own daylight rather than the neighbour's, and reads better in the officer's assessment of the design.

Talk to the neighbour before you submit. A supportive neighbour does not bind the council, and an objection from the affected window is the single most likely trigger for a refusal on this ground. Our guide on neighbour notification and site notices explains how the council publicises an application.

Draw the 45-degree line yourself, on the survey, before the design is fixed. It takes five minutes and it is the least expensive piece of design work in the project.

PlanScope drafts the householder planning documents an application needs, structured to UK planning policy, with site measurements flagged rather than assumed. Enter your postcode on PlanScope to see which reports your project is likely to need.

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.