The 25 degree line test before you commission modelling

The 25 degree line daylight test exists so that you do not pay for modelling you do not need. Draw a section through the proposed building and the neighbouring window, take a line at 25 degrees above horizontal from the centre of the lowest affected window, and if no part of the proposal rises above that line, the guidance treats daylight to that window as unlikely to be adversely affected. No vertical sky component calculation, no no-sky line, no consultant fee.

It is a screening tool and it behaves like one. A pass ends the enquiry for that window. A fail tells you nothing except that the enquiry continues.

What the preliminary check screens out

The British Research Establishment guidance on site layout planning for daylight and sunlight sets the test up as a first filter. Most low rise development against a reasonably separated neighbour passes it comfortably, which is why a single storey rear extension at the far end of a long garden does not usually need a daylight consultant.

The test applies to existing dwellings and to other buildings with a reasonable expectation of daylight, which includes schools, surgeries and some offices. It is applied window by window. One window failing the check does not fail the scheme and does not require every window on the elevation to be modelled, only the ones that fail.

Two things it is not. It is not the 45 degree rule that many councils publish in householder design guidance, which is a plan-based test for extensions and sits in local supplementary planning documents rather than in the national daylight guidance. And it is not a sunlight test, because sunlight availability has its own separate screening involving orientation within 90 degrees of due south.

Drawing the section correctly

The section has to be taken perpendicular to the affected window, through the centre of that window, and it has to show true levels rather than assumed ones.

Include the ground level at both buildings taken from a topographical survey, the finished floor levels of the neighbouring property where they can be established, the full height of the proposal including parapets, plant enclosures, lift overruns and any roof-mounted equipment, and the horizontal separation measured on the same section rather than scaled off a plan.

Three errors account for most disputed sections. Using a plan distance where the section is not perpendicular, which shortens the apparent separation. Omitting rooftop plant, which is often the only part of the proposal that breaks the line. And drawing to the eaves rather than the ridge on a pitched roof.

The obstruction angle approach is closely related and useful earlier in the design, and the way it is used at massing stage is set out in obstruction angles and the early massing test.

Where the reference point sits on the window

The reference point is the centre of the window, at the level of the lowest window serving a habitable room. Not the cill, not the head, and not the middle of the elevation.

For a bay window, take the main plane of the window rather than the projecting flank. For a patio door, the same rule applies and the effective centre sits lower than most assessors assume, which makes the test harder to pass rather than easier. Where a room has two windows at different levels, the lowest one governs.

Rooms that are not habitable sit outside the test. Bathrooms, hallways, garages and utility rooms have no protected expectation of daylight in the guidance, although a council may still take a view on them in amenity terms. Identify the room use before you take the measurement, because assuming a ground floor rear window serves a kitchen when it serves a bedroom changes nothing about the geometry but changes everything about whether the result matters. Room use assumptions have a wider effect on results than most people expect, as room use assumptions and how they change results sets out.

Sloping ground and mixed storey heights

Level changes are where the test quietly goes wrong.

Where the proposal sits uphill of the neighbour, the effective obstruction height is the building height plus the level difference, and a scheme that passes on a flat section fails once the true levels go in. Where it sits downhill, the reverse applies and a taller building can still pass.

Mixed storey heights within one proposal need the section drawn at the point of greatest obstruction rather than at an average. On a stepped terrace, that means several sections rather than one. On a scheme with a set-back upper floor, the set-back often takes the upper storey below the line even where the parapet of the lower block does not.

Take the levels from a survey. An assumed 2.7 metre floor to floor height applied across five storeys can be out by more than a metre against the real building, and the whole test turns on a line drawn to a single point.

What a pass at this stage proves

A pass means that detailed daylight calculations for that window are not required by the guidance. That is all it means.

It does not address sunlight to the window, which is tested separately through annual probable sunlight hours where the window faces within 90 degrees of due south. It does not address sunlight to gardens and amenity space, which is a separate overshadowing exercise and is dealt with through an overshadowing study. It does not address outlook, privacy or a sense of enclosure, all of which are amenity considerations a council can weigh regardless of the daylight result. And it says nothing about daylight inside the proposed building.

Councils sometimes ask for the calculation even after a pass, particularly where objections have been received. Providing the section and the stated result usually answers that, and it costs a drawing rather than a report.

When to move to full assessment

A fail at the preliminary stage moves the window into detailed assessment: vertical sky component at the window face, daylight distribution through the no-sky line where internal room layouts are known, and the corresponding sunlight tests.

Do not treat the move as a bad outcome. Plenty of windows that fail the 25 degree check pass the detailed tests comfortably, because the preliminary line is deliberately conservative and takes no account of the width of the obstruction. A narrow building directly in front of a window blocks far less sky than a long terrace at the same height and distance, and only the detailed calculation sees that difference.

Where the existing condition is already obstructed, the baseline matters as much as the proposal, and the treatment of that baseline is covered in the mirror image baseline and existing obstructions.

Using the test to steer massing

The most useful application of the check is not in the submission at all. Run it at the earliest massing stage, across every affected neighbouring window, and you get a three dimensional envelope that the design can sit inside.

That envelope tends to produce the same moves each time. Set the upper floors back from the boundary facing the neighbour. Step the height down towards the shorter separation. Put the circulation core, rather than habitable rooms, on the elevation that causes the problem. Rotate the block so that the long elevation faces the longer separation.

Doing this early is cheaper than arguing about vertical sky component ratios at committee, and it produces a daylight and sunlight assessment that reports compliance rather than one that explains departures.

One honest limitation. The 25 degree line takes no account of reflected light from surrounding surfaces or of the actual sky conditions at the site, and in a dense urban context it will flag windows that no reasonable assessor would regard as harmed. Use it as a filter and not as a policy test.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it.