The 25 degree test in section for a rear extension

The 25 degree test decides whether your rear extension needs a daylight assessment at all. Draw a line in section from the centre of the neighbour's lowest affected window, rising at 25 degrees above the horizontal towards your proposal. If the extension sits below that line, diffuse daylight to the window is unlikely to be materially affected and BRE 209 says no further analysis is needed. If it cuts the line, the question moves on to the numerical tests.

It is a screening tool, not a policy. Councils use it because it is quick, it can be drawn by anyone with a scale rule, and it matches what most objections are actually about.

How the section line is constructed

Cut a section through the proposal and the neighbouring window, taken perpendicular to the affected elevation.

Mark the reference point at the centre of the window, on the outside face of the wall, at the lowest storey with a habitable room behind it. From that point draw a horizontal datum, then a second line at 25 degrees above it, running towards your site. The test is whether the highest part of your proposal on that section rises above the 25 degree line.

Two details decide most disputes. The reference point is the centre of the window, not its head and not its cill, so a tall window and a short one at the same cill height give different results. And the line is taken from the window, which means the section has to be drawn at the window's position rather than at the most convenient point on the boundary.

Where several windows are affected, draw the test for the worst case, which is usually the lowest window closest to the proposal.

A related test appears in the same guidance for extensions to the side of a window rather than directly in front of it, using a 45 degree line taken in plan. Many councils apply their own version of that in householder design guidance, with the line drawn from the nearest edge of the neighbouring window across the proposed extension. Check which version your authority publishes, because the two produce different results on the same drawing and officers apply the local one.

Ground levels between plots

Sloping sites break more of these tests than tall extensions do.

If the neighbouring garden sits 600mm lower than yours, the window reference point drops with it and your extension eats into the angle before it has gained any height. If the neighbour sits higher, you have room you did not expect. Either way the levels have to be surveyed rather than assumed, because a section drawn on assumed level ground is worthless as evidence.

Get finished floor levels and external ground levels at the boundary from a measured survey. Show the datum on the drawing. An officer who cannot see where the levels came from will treat the section as an illustration rather than as a test.

Single storey and two storey compared

A single storey rear extension of typical proportions usually passes. Take a flat roofed addition 3m high at the boundary: the 25 degree line reaches 3m at about 6.4m from the window, so a window more than roughly that distance away is clear. That figure is arithmetic rather than policy, and it shifts with every level change and every roof form.

Two storey extensions are a different proposition. At around 6m to the top of the wall the line has to run some 12.9m from the window before it clears, which on a standard terraced or semi-detached plot it rarely does. That is why most two storey rear additions on tight plots need the fuller analysis.

Roof form matters as much as height. A pitched roof falling away from the boundary can pass where a flat roof at the ridge height would fail, because the test looks at what is actually there in section rather than at the maximum height anywhere on the plan.

What happens when the test is failed

Failing the 25 degree test does not mean refusal. It means the screening stage is over.

The next step is the vertical sky component at the affected window, compared against the BRE target and against its former value, and then daylight distribution, which looks at how much of the room still receives direct skylight. Sunlight is assessed separately for windows facing within 90 degrees of due south, using annual probable sunlight hours.

Those calculations need a three-dimensional model and someone who does them regularly. A Daylight and Sunlight Assessment prepared to BRE 209 is the document councils expect, and on householder schemes it is often the thing that turns a likely refusal into a permission with a slightly amended roof.

Where the concern is garden and amenity space rather than internal rooms, an Overshadowing Study covers different ground, and transient overshadowing is assessed on its own terms.

Where the test misleads

BRE 209 is guidance developed largely for suburban conditions, and its authors say so. Applied without judgement it produces two recognisable errors.

The first is treating a pass as the end of the matter. A proposal can clear the 25 degree line and still cause real harm through loss of outlook, a sense of enclosure, or overshadowing of a small garden. Those are separate planning considerations and neighbours raise them regardless of what the section shows.

The second is treating a fail as fatal. In dense urban areas the existing daylight to many windows is already well below the BRE targets, and a small absolute reduction produces a large proportional one. Inspectors have repeatedly accepted that the guidance is to be applied flexibly in such contexts.

There is also the question of which windows count. The test is aimed at habitable rooms. A window serving a bathroom, a garage, a utility or a stairwell attracts much less weight, and a conservatory generally attracts little.

Drawing it so the officer can use it

Put the test on a dedicated drawing rather than burying it in a general section.

• Show the neighbouring window in elevation and in section, with its cill and head levels dimensioned.
• Mark the reference point at the window centre and label the 25 degree line.
• Dimension the horizontal distance from the window to the nearest part of the proposal.
• State the source of the level data and the date of the survey.
• Where more than one window is affected, show each on the same sheet.

Then say in the covering text whether the test passes or fails, in one sentence, without decoration. Officers reading fifty applications a week reward that.

Where the extension is close to a boundary and a neighbour has already objected, consider running the test before you submit rather than after. It costs a surveyor's morning and it tells you whether the scheme is arguable. Questions about assessing daylight to a property you cannot access come up constantly here, and there are established ways to work from external measurements and typical room depths.

Privacy is the other half of the neighbour argument and it runs on entirely different conventions, including the 21 metre separation convention that many councils still apply to facing habitable windows.

Enter your postcode on PlanScope to see which supporting documents your extension is likely to need before you apply.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.