Shadow path diagrams and transient overshadowing
A shadow path diagram illustrates an effect; it does not test one. Here are the conventions that make a transient overshadowing study checkable, and the cases where it adds nothing.
A shadow path diagram shows where the shadow of a building falls at a given moment on a given date. It is a picture, not a test. The quantified sunlight test for open space in BRE 209 asks what proportion of an amenity area receives at least two hours of sun on 21 March, and a shadow path study is how that answer is illustrated and how a committee reads it. Confuse the two and the analysis gets attacked on the grounds that it proves nothing.
Used properly, a transient shadow study answers a question a quantified table cannot: what does the garden look like at four o'clock in April, and how much of that shadow was already there.
What a shadow path study shows
Each frame in a shadow path study is a plan of the site with the shadow cast by existing buildings and by the proposal at a stated date and time. Run the frames in sequence and you see the shadow sweep across the ground through the day.
Three things must appear on every frame or the study is not checkable. The date. The time, with the time base stated. And the sun altitude and azimuth used for the calculation.
The time base catches people out. Shadow geometry follows solar time, and British Summer Time runs an hour ahead of it for much of the year. A study labelled in clock time without saying so shows the afternoon shadow an hour earlier than a reader standing in the garden will experience it. State whether the times are Greenwich Mean Time or local clock time, and be consistent across every frame.
Standard dates and time intervals
Use the conventional dates and a reader can compare your study with any other.
| Date | What it shows | Why it is used |
|---|---|---|
| 21 March | Equinox | The date used for the BRE sun on ground test |
| 21 June | Summer solstice | Highest sun, shortest shadows, best case |
| 21 December | Winter solstice | Lowest sun, longest shadows, worst case |
| 21 September | Autumn equinox | Near identical to March, rarely needed |
Hourly intervals are the norm, running from around two hours after sunrise to two hours before sunset on the equinox, and across the shorter usable day in December. Half-hourly frames are worth producing where the shadow crosses a small garden quickly, because an hourly sequence can miss the passage entirely and understate the effect.
Presenting only the June frames is the oldest trick in this field, and officers know it. If you produce June, produce March as well.
Transient and cumulative analysis are different things
A transient frame is an instant. A cumulative analysis aggregates across the day and produces a contour or a shaded area showing how many hours of direct sun each part of the ground receives, or which parts receive none at all.
The cumulative version is what connects to the quantified test. Where the BRE guidance asks whether at least half of an amenity area receives two hours of sun on 21 March, the cumulative plot shows exactly which half, and the comparison between existing and proposed shows how much of that area is lost. An Overshadowing Study that reports the percentage without the plot leaves the reader unable to see whether the lost area is the part of the garden people actually use.
Produce both. Transient frames for the narrative, cumulative plots for the test, and a single sentence tying the two together.
Reading existing and proposed together
Every frame needs its pair. Show the existing condition and the proposed condition at the same date, time, scale and orientation, and place them side by side on the sheet rather than on consecutive pages.
Better still, add a third drawing showing only the additional shadow: the area shadowed in the proposed case that was sunlit in the existing case. That overlay is the most honest drawing in the set, because it separates the effect of the proposal from the effect of buildings that are already there. It is also the drawing most likely to win an argument where the neighbour's garden was already overshadowed by a mature boundary tree or by the neighbour's own extension.
Setting the baseline correctly is its own problem, and the conventions around it are covered in the mirror image baseline and existing obstructions.
Where the study misleads
Shadow studies mislead in predictable ways, and a reviewer should check for all of them.
- Omitting existing obstructions. A model containing only the application site produces shadows that appear to fall on sunlit ground which is already in shade.
- Flat ground assumptions. On a sloping site, shadows projected onto a horizontal plane are the wrong length and land in the wrong place.
- Missing trees. Deciduous trees are legitimately excluded from some analyses because they are seasonal and can be felled, but the exclusion has to be stated, not silent.
- Generic massing. A model built from the planning envelope rather than the proposed roof form overstates shadow at the edges.
- Cherry-picked frames. Six frames from a twelve frame sequence, chosen after the sequence was run.
- Clock time labelling. As above, and it shifts the whole afternoon sequence.
Each of these is fixable, and each costs far less to fix before submission than after an objector's consultant has found it.
Presentation conventions
Keep the graphic language simple and consistent. Existing shadow in one tone, additional shadow in a second, sunlit ground left white. A north arrow and a scale bar on every frame. Garden boundaries and the assessed amenity areas outlined, with the same references used in the written assessment.
Number the frames so the text can refer to them. "Frame 07, 21 March, 15:00 GMT" is a reference a case officer can quote in a report; "the afternoon shadow plot" is not.
Where the study accompanies a quantified Daylight and Sunlight Assessment, put the shadow frames in the same document and cross-refer, rather than issuing them as a separate drawing pack that arrives on the file a week later. Making the results legible to a non-specialist audience is a skill in itself, and presenting daylight results a committee can read goes further into it.
When a study adds nothing
Not every scheme needs one. A shadow path study adds little in three situations.
Where the quantified sun on ground test passes with a wide margin and no amenity area loses meaningful sun, the plots simply confirm a result already stated. Where the relevant objection concerns daylight to windows rather than sun to gardens, shadow plots are the wrong instrument, because vertical sky component and annual probable sunlight hours are calculated at the window and are not visible in a plan shadow. And on a small householder extension in a dense terrace, the geometry is usually resolved by a section and the standard angular tests rather than by a twelve frame sequence, as rear extensions and overshadowing in dense terraces sets out.
Spend the fee where the argument is. A scheme fighting on garden amenity needs the frames, the cumulative plots and the additional shadow overlay. A scheme fighting on window daylight needs the numbers, and a shadow sequence in that case is decoration.
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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Frequently asked questions
What dates should a shadow path study use?
21 March is the essential one, because it is the date used for the BRE sun on ground test. 21 June and 21 December give the best and worst case shadow lengths. The September equinox is near identical to March and rarely adds anything. A study presenting only June frames will be treated as selective.
Should shadow times be in GMT or British Summer Time?
Shadow geometry follows solar time, so calculations are normally run in Greenwich Mean Time. The important thing is to state which base you used and apply it consistently. A study labelled in clock time without saying so places the afternoon shadow an hour earlier than a person standing in the garden would experience it.
What is the difference between transient and cumulative shadow analysis?
A transient frame shows the shadow at a single instant. A cumulative analysis aggregates across the day to show how many hours of direct sun each part of the ground receives. The cumulative version connects to the quantified test, because it identifies which part of an amenity area loses its two hours of sun.
Do I need to model the neighbour's existing buildings and trees?
Existing buildings, yes. A model containing only the application site produces shadows that appear to fall on sunlit ground that is already shaded, which overstates the effect of the proposal. Deciduous trees are sometimes excluded because they are seasonal and can be removed, but that exclusion has to be stated rather than left silent.
When is a shadow study not worth producing?
Where the quantified sun on ground test passes comfortably, where the objection concerns daylight to windows rather than sun to gardens, and on small householder extensions in dense terraces. In the second case the relevant measures are calculated at the window and are not visible in a plan shadow, so the frames answer a question nobody asked.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.