Daylight for a single storey rear extension on a bungalow

A single storey rear extension on a bungalow can fail a daylight test that the same extension would pass alongside a two storey house. The reason is geometry. Your neighbour's habitable room window sits at the same level as your new roof, so there is no fall of ground and no height difference to soften the effect, and the distance from window to wall does all the work.

That is why bungalow rear extension daylight questions come up so often on applications that look modest on paper. The extension is low. It is also close, and close is what matters here.

Why a low building still blocks light

Daylight to a window depends on how much sky that window can see. The standard planning measure is the vertical sky component, which expresses the amount of visible sky at the centre of a window as a percentage.

On a two storey house, a neighbouring ground floor window usually sits well below the eaves of the extension and some distance from it, so a slice of sky remains above the new roofline. On a bungalow the neighbouring window is at the same level, the new eaves may be only a metre or so above the sill, and the wall is often within two or three metres of the boundary. The result is a much lower angle of visible sky than the modest height suggests.

Orientation compounds it. A neighbouring window facing north gets no direct sun at all and relies entirely on diffuse skylight, so any reduction is felt. A south facing window losing an equivalent amount of sky keeps more usable light.

The first question to ask is not how tall the extension is. It is how far the neighbouring window is from the proposed flank wall, and what the room behind it is used for.

The tests councils actually apply

Two families of test appear in householder decisions, and they are not the same thing.

The first is the informal 45 degree rule that appears in many councils' householder design guidance. Draw a line at 45 degrees in plan from the centre of the nearest neighbouring habitable room window towards the boundary. If the proposed extension crosses that line, the council expects a closer look. A second 45 degree line is sometimes applied in section.

The second is the BRE guidance on site layout planning for daylight and sunlight, third edition 2022, which sets out the technical tests. Its screening test for an existing window is whether the new development subtends an angle of more than 25 degrees to the horizontal, measured in section from the centre of the lowest affected window. Below that angle, a substantial effect on diffuse daylight is unlikely. Above it, the vertical sky component should be calculated, with a reduction to less than 0.8 times the former value treated as noticeable, and 27 per cent used as the reference figure for a window with unobstructed sky.

For an extension on a bungalow, the 25 degree test is often the one that fails first, because the angle is driven by proximity rather than by height. Where it does, the answer is a calculation rather than an argument.

Working out the position before you design

A short section drawing answers most of the question, and it costs an afternoon rather than a survey fee.

1. Measure the distance from the neighbouring window to your boundary, and from the boundary to your proposed flank wall.
2. Measure the sill height of the neighbouring window above external ground level, and note the ground level difference between the two properties.
3. Draw a section through the neighbouring window and your proposed extension at the same scale.
4. Draw the line from the centre of that window to the top of your proposed wall or roof, and measure the angle to the horizontal.
5. If it exceeds 25 degrees, either reduce the height or depth at that end, or commission the calculation.

The technique is the same one used at a larger scale on bigger schemes, described in obstruction angles and the early massing test. It is worth doing before the drawings are finalised, because moving a wall on paper costs nothing.

Note: the room use behind the window changes how much weight the result carries. A kitchen is treated less protectively than a living room or bedroom in the BRE guidance, and that distinction affects outcomes more than most applicants expect, as set out in room use assumptions and how they change results.

Roof form, and what actually recovers light

Once the geometry is understood, the design moves that help are limited but effective.

A flat roof with a low parapet is usually the best option next to a boundary, because it keeps the mass low across the whole depth. A pitched roof sloping away from the boundary works where the ridge can be set away from the shared edge. A pitched roof with its ridge parallel to the boundary is the worst case, because it adds height exactly where the neighbour's sky view is being taken.

Setting the extension in from the boundary by even half a metre changes the angle more than reducing the height by the same amount, because the angle responds to horizontal distance as well as vertical.

Rooflights and glazing on your side of the wall do nothing for the neighbour. They improve your own daylight, which is a separate design question, and they are sometimes offered in objection responses as though they were mitigation. They are not.

Reducing the depth at the boundary end only, so the extension steps back where it meets the shared boundary and returns to full depth further away, keeps most of the floor area and removes most of the effect. That is the move that resolves the majority of bungalow cases.

Boundary treatment and the combined effect

An existing 2 metre fence or a tall hedge already restricts the neighbouring window, and that matters in two ways.

Where the existing obstruction is already substantial, the additional effect of the extension may be smaller than it first appears, because the baseline is lower. The calculation has to be run against the existing condition including that obstruction, not against an open boundary, which is the point made in mirror image baselines and existing obstructions.

But a fence can be removed and a hedge can be cut. A building cannot. Councils are alert to arguments that rely on a temporary obstruction to justify a permanent one, and an assessment that leans on an existing hedge should say what happens if it goes.

What to put in the application

For a householder application affected by this issue, the useful submission is short and specific.

Include an existing and proposed section through the neighbouring window and the extension, at a stated scale, showing ground levels on both sides. Include a plan showing the 45 degree lines from the relevant neighbouring windows. State the use of the room behind each affected window if you know it, and say how you know.

Where the geometry is marginal, a Daylight and Sunlight Assessment calculating vertical sky component for the affected windows turns an argument into a number. Councils usually expect a suitably qualified author for that work and professional review is strongly advised, but on a single window it is a contained piece of work rather than a large commission.

Address the issue in the covering statement rather than waiting for the objection. A Planning Statement that sets out the angle, the test applied and the design response reads very differently from one that is silent until a neighbour raises it.

And talk to the neighbour before submitting. A stepped-back corner agreed over a fence is faster than a refusal and an appeal, and it usually costs less floor area than the redesign forced by a refusal would.

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

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