How loss of light is assessed in planning applications
“Loss of light” is one of the most common reasons councils give for refusing planning permission. It is also one of the areas of the planning system that causes the most confusion.
Most people assume there must be some clear rule — that if you block a certain amount of light, the application will be refused, and if you don’t, it will be approved. In reality, it does not work like that at all.
It often comes as a surprise to clients that there is no simple or consistent way in which loss of light is assessed. Like many aspects of planning, it is ultimately a matter of judgement.
What councils are actually trying to assess
Planning policies tend to say that development should not cause harm to neighbouring properties in terms of light, outlook or general living conditions. That sounds straightforward enough, but it does not tell you where the line is drawn.
Importantly, the test is not whether there is any impact. In built-up areas, particularly in London, almost any extension will have some effect on neighbouring properties. Houses sit close together, gardens are relatively short and extensions are often built along boundaries. In those circumstances, some degree of impact is inevitable.
The real question is whether the impact becomes unacceptable, and that is not very straightforward to asses..
The role of guidelines (and why they are only guidelines)
Most councils publish guidance for extensions, usually in the form of a supplementary planning document. These often include rules of thumb intended to help officers reach a view.
The most well-known example is the 45-degree rule, where a line is drawn from the centre of a neighbouring window and the extension is assessed against it. If the extension crosses that line, it is said to be likely to reduce light. We won a recent case in Three Rivers on the application of the 45-degree rule.
There are also common limits on the height and depth of extensions — for example, keeping a rear extension within around three metres in height and projection.
These measures are useful, but they are crude. They cannot account for every situation, and they are not applied rigidly. As you know from practice, a proposal can breach them and still be acceptable, or comply with them and still be refused.
What really matters in practice
When you read officer reports and appeal decisions over time, a pattern emerges. The same underlying considerations tend to come up again and again.
Orientation is one of the most important. A south-facing window will receive a large amount of sunlight throughout the day, whereas a north-facing window may already receive very little direct light. An extension that affects the latter may have a much more limited real-world impact.
The position of the window also matters. Windows in front and rear elevations tend to be given more weight because they serve the main outlook over a property’s own garden. Side-facing windows are often different. They typically look towards the boundary at close range, and there is less expectation that they will enjoy particularly good levels of light or outlook. That is something inspectors regularly pick up on in appeal decisions.
The existing situation is equally important. If a window already looks onto a boundary fence or a neighbouring wall at close proximity, then the additional impact of a new extension may be relatively modest. Planning is concerned with the change caused by the development, not with achieving an ideal situation.
It is also relevant to consider how a room is used and how it is lit overall. A side window serving part of a larger open-plan room may be much less important if there is a large rear window bringing in light from the garden. In those cases, even if the side window is affected, the room as a whole may still function perfectly well.
These are the kinds of judgements that sit behind decisions, even if they are not always fully explained.
Daylight and sunlight reports (the BRE guidance)
In more contentious cases, applicants sometimes commission a professional daylight and sunlight report. These are prepared in accordance with the BRE guidance and use technical measures to assess light levels.
They can be helpful, particularly where a proposal is finely balanced or where the council has raised a clear concern about light. A well-prepared report can provide a more objective assessment and, in some cases, demonstrate that the impact falls within accepted limits.
However, they are not required in most cases. The majority of householder applications are determined without them.
Cost is also a factor. A typical report will often be in the region of £1,000 to £2,000, which many homeowners reasonably see as disproportionate for a relatively modest extension.
When it is worth getting one
There are situations where a report can be worthwhile.
If the relationship with a neighbour is particularly tight, if the extension is pushing the limits of what is normally accepted, or if loss of light is clearly the main issue in a refusal, then a report can add real value. The same applies at appeal stage, where you may want to address the issue more robustly.
One point that is sometimes overlooked is that a report is not just something you submit at the end of the process. It can be used to shape the design. If the initial results are poor, a consultant can often advise on how far the extension needs to be reduced, or adjusted, in order to fall within acceptable limits.
A note on neighbour objections
Neighbours will almost always raise concerns about loss of light where an extension is proposed close to a boundary. That is entirely understandable.
However, those concerns are not determinative in themselves. The planning system does not operate on the basis of whether someone feels affected, but on whether the impact reaches the threshold of unacceptable harm when properly assessed.
In practice, many objections are based on a fear of losing light rather than a clear understanding of how the issue is judged.
A real example: when the council got it wrong
In one recent case, our client applied for a two-storey side extension to a semi-detached house. The council refused the application, arguing that the extension would have two main impacts.
First, they said it would create a “terracing effect”, making the pair of houses appear joined together rather than remaining visually separate.
Second, they argued that it would lead to a loss of light affecting a side-facing window in the neighbouring property.
On paper, both concerns sound familiar. They are the kind of standard reasons you see in refusal notices all the time.
However, when you look more closely at the actual situation, the picture was quite different.
In terms of the “terracing effect”, the extension was set back from the front elevation and maintained a clear gap at first floor level. At ground floor level, there were already boundary treatments and built form that limited any meaningful perception of separation. In other words, the visual distinction between the houses was not being materially eroded in the way the council suggested.
The loss of light point was similar. The window the council was concerned about was side-facing and already looked directly onto the boundary and the existing built form. It was also not the primary source of light to the room, which formed part of a larger open-plan space with a significant rear window.
We commissioned a daylight and sunlight assessment to test this properly. While some technical reductions in light levels were identified, the overall conclusion was that the room would continue to receive adequate light, particularly from the rear elevation. The report also applied the “mirrored massing” approach, showing that a similar form of development on the neighbour’s side would produce a comparable situation.
On that basis, the inspector allowed the appeal. The key point was that, although there was some impact, it did not amount to unacceptable harm when viewed in context.
The key point
There is no single rule that determines whether a proposal causes an unacceptable loss of light.
The 45-degree rule, separation distances and BRE guidance all have a role to play, but they do not provide definitive answers. Ultimately, decisions are made by applying judgement to the specific circumstances of each case.
That is why outcomes can sometimes appear inconsistent — and why a well-presented case can make a real difference.
Need advice on a refusal?
If your application has been refused on the basis of loss of light, it does not necessarily mean the proposal is unacceptable. It may simply mean that the council has taken a cautious view of a situation that is more balanced than it first appears.
If you want a clear view on your prospects — whether that is revising the scheme or pursuing an appeal — you can get a free assessment from our chartered town planners.
