Transient overshadowing and permanent loss of sunlight

Transient overshadowing planning arguments usually start from a misunderstanding. A neighbour looks at a shadow diagram, sees their garden covered in grey at 9am on 21 March, and objects on the basis that the extension will put their garden in shade. But the shadow at 9am is not the shadow at noon, and the test the council applies is not about any single moment. It is about how much of the garden gets two hours of sun across the whole day.

That is the difference between a shadow passing over a space and sunlight being taken away from it. One is normal and unavoidable in any built up area. The other is a planning harm that can refuse an application.

What an hourly shadow study shows

A shadow study is a set of plan drawings showing where the shadow of the existing buildings falls, and where the shadow of the proposal falls, at stated times on stated dates. Hourly intervals from early morning to late afternoon is the usual format, with the existing and proposed shadows shown in different tones so the added shadow is visible.

What it is: a visualisation. It shows the geometry, and it is honest about it if the dates, times and north point are right.

What it is not: a test. There is no threshold in the shadow study itself. Nobody has ever written a policy saying a garden may not be shaded at 10am. The numerical tests sit elsewhere, and a shadow study supports them rather than replacing them.

Which is why a study submitted on its own, with no accompanying assessment, tends to persuade nobody. The case officer cannot write a report that says the shadow looks acceptable.

Why a moving shadow is treated differently

Sunlight to gardens and open spaces is assessed in the guidance published by BRE in Site Layout Planning for Daylight and Sunlight by looking at how much of the area receives at least two hours of sunlight on 21 March. The recommendation is that at least half the amenity area should achieve that, and that a reduction to less than 0.8 times the former sunlit area is likely to be noticeable.

Notice what that method does. It integrates across the day. A shadow that covers the patio at 9am and has moved off by 10.30am does not reduce the two hour figure at all, provided the patio gets its two hours somewhere between sunrise and sunset. The moving shadow is already accounted for.

Sustained loss is different. A three storey flank wall three metres from the boundary on the southern side of a garden removes sun from that strip for the whole day, every day. The two hour area shrinks, the 0.8 factor is breached, and the harm is real and measurable.

So the question to ask about any shadow is not how large it is. It is whether the area it covers gets its two hours anyway.

Selecting representative dates and times

The dates matter more than most submitted studies acknowledge.

• 21 March, the spring equinox, is the reference date for the sun on ground assessment. If only one date is modelled, this is the one.
• 21 June shows the shallowest shadows and the best case. Useful for showing what a garden gets in summer, and easy to misuse.
• 21 December shows the deepest shadows and the worst case. Objectors like it. Councils know that almost every garden in the country is heavily shaded at midwinter, so it proves less than it appears to.
• 21 September is sometimes requested because it matches the equinox from the other direction and picks up late season use of gardens.

Times should be stated in true solar time or clearly labelled as clock time with the offset. A diagram labelled 3pm on 21 June that has silently applied British Summer Time is showing the sun an hour further round than the reader thinks. That single error has undermined more studies than any modelling failure.

The model itself needs the existing buildings, boundary walls and significant evergreen trees included. Leaving out a two metre boundary fence overstates the added shadow near the ground, and leaving out a neighbour's existing garage understates the baseline.

Reading a shadow diagram correctly

Four things to check on any diagram handed to you:

1. Is the north point correct, and does it match the site plan?
2. Are existing and proposed shadows distinguished, so the added shadow can be seen separately?
3. Are the dates and times labelled, with the time convention stated?
4. Is the amenity area outlined, so the proportion affected can be judged rather than guessed?

Then look at what the shadow does over the sequence rather than at any one frame. A shadow that sweeps from west to east across a lawn between 8am and 11am and then retreats is the normal behaviour of a building on the eastern boundary. A shadow that sits on the same half of the garden in every frame is telling you something else.

An Overshadowing Study that shows both the sequence and the two hour sunlit area calculation gives a case officer everything needed. One that shows only the sequence leaves the judgement to whoever objects most loudly.

Where transient shading still matters

The distinction is not a licence to dismiss every shadow complaint. Moving shadow does cause real harm in specific situations, and an honest assessment says so.

Solar panels are the clearest case. A photovoltaic array shaded for two hours in the middle of the day loses generation, and the loss is not offset by the sun returning later. Where a neighbour has roof mounted panels on the affected elevation, expect the point to be raised and deal with it directly.

Then there are spaces used at a particular time of day. A west facing patio that is the only usable sitting area, shaded from mid afternoon by a new extension, has lost the part of the day it was used for, even if the two hour figure is still met somewhere on the plot. Greenhouses, vegetable beds and small courtyard gardens with a single sunlit corner behave the same way.

Habitable room windows follow separate tests altogether. Sunlight to windows is assessed through annual probable sunlight hours, with a recommendation of at least 25 per cent of annual sunlight hours including at least 5 per cent in the winter months for windows facing within 90 degrees of due south. Daylight is assessed separately again through the vertical sky component. Those tests, unlike the garden test, are sensitive to the direction the window faces, and for flats the analysis gets more involved, as our note on single aspect north facing flats and amenity policy sets out.

Common misunderstandings in objections

The five that come up most often, and the short answer to each.

A shadow in the diagram means a loss of light. No. It means a shadow at that moment.

Midwinter shadows show what the garden will be like. Only in midwinter, when most gardens are shaded anyway.

The extension blocks the morning sun, so the garden will be dark. The garden may still meet the two hour test from afternoon sun.

I have a right to light. Rights of light are a civil property matter arising under the Prescription Act 1832 and the general law, wholly separate from planning. A council cannot refuse permission for breach of a right to light, and a grant of permission does not authorise an interference with one.

The council must apply the BRE figures strictly. It must not. The guidance itself says the numerical values are advisory and should be applied flexibly, and local plan amenity policies are what actually bind the decision.

Combining shadow studies with the numerical tests

For a householder application where overshadowing is the live issue, a proportionate package is:

1. Shadow diagrams at hourly intervals on 21 March, with 21 June and 21 December for context, existing and proposed distinguished.
2. The two hour sunlit area calculation for each affected garden, before and after, with the 0.8 comparison stated.
3. Where habitable room windows are affected, annual probable sunlight hours and vertical sky component results for those windows.
4. A short written assessment tying the numbers to the council's amenity policy wording.

On a larger scheme a full Daylight and Sunlight Assessment will cover all of that, and where facing windows are the issue the separation distances between facing windows usually come into play as well. If you are testing amenity inside your own proposed units rather than the neighbours', that is a different exercise, covered in assessing daylight inside your own proposed flats.

The honest position is that these tests were written for layout design, not for adjudicating disputes between two suburban neighbours, and applying them to a small extension can look disproportionate. It is still better than trading assertions about shadows.

This is general guidance, not legal advice. Amenity policies and the weight given to guidance vary by authority, so check the position with your local planning authority or a qualified professional before relying on it.

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