Transient overshadowing diagrams: how many and at what times
Shadow plots make sun-on-ground numbers legible. Here are the dates, intervals, comparison cases and captions a planning submission is expected to carry.
Transient overshadowing diagrams show where the shadow of a proposed building falls at stated moments on stated dates. They are not a numerical test, and no threshold attaches to them. Their job is to make the sun-on-ground figures legible, which is why a submission carrying shadow plots with no accompanying quantified assessment usually gets criticised, and why plots for 21 June alone get dismissed outright.
Plot the equinox. That is the date the quantified test uses, and it is the date that decides the argument.
What a transient study adds
The quantified sunlight test for open space in the BRE guidance asks whether at least half of an amenity area receives at least two hours of sunlight on 21 March. That produces a number and a pass or fail, and it tells a reader nothing about where the shadow sits at four in the afternoon in a neighbour's garden.
Transient plots fill that gap. They let a case officer, a committee member or an objector see the shape and movement of the shadow, compare it against what exists today, and understand which parts of a space lose morning light rather than afternoon light. On schemes where the numbers pass but neighbours are unhappy, the plots are what makes the case comprehensible.
They are a supplement rather than a substitute. Submit them alongside the sun-on-ground analysis in an Overshadowing Study, not instead of it.
Dates and hours to plot
The conventional set is three dates, and adding a fourth rarely helps.
| Date | Why it is plotted | Usual hours |
|---|---|---|
| 21 March | The equinox, and the date of the BRE sun-on-ground test | Hourly from around 09:00 to 16:00 |
| 21 June | Summer solstice, showing the best case and the longest usable day | Hourly from around 08:00 to 19:00 |
| 21 December | Winter solstice, showing the worst case with the lowest sun | Hourly from around 10:00 to 14:00 |
Hourly intervals are the norm. Half-hourly plots on the equinox are worth producing where a single receptor is contentious, because the two hour test can turn on a narrow window.
State the time basis on every drawing. Greenwich Mean Time, British Summer Time and true solar time give different shadow positions for the same clock reading, and a set of plots labelled 13:00 without saying which one it is invites a challenge that is tedious to answer later. Most practitioners work in GMT for consistency across the three dates and say so.
21 September is sometimes added. Its sun path is close enough to 21 March that it rarely earns its place.
Choosing the receptors to illustrate
Plotting the whole site at hourly intervals produces a lot of drawings nobody reads. Choose receptors and label them.
The ones that matter are:
- neighbouring private gardens, particularly those to the north of the site
- communal amenity space within the scheme
- children's play space, school playgrounds and nursery outdoor areas
- allotments
- public squares and sitting-out areas with seating
- any space the local plan or a supplementary planning document identifies as protected open space
Mark each receptor on a key plan with a letter, and use the same letters in the quantified table. A reader moving between the plots and the numbers should never have to guess which garden is which.
Where the scheme is arranged around a courtyard, the shared space inside it is usually the receptor that decides the application, and the approach to those is set out in the note on courtyard blocks and sunlight to shared amenity space.
Existing, proposed and cumulative
Three cases, presented side by side at the same scale, on the same sheet where the page size allows.
The existing case shows shadows cast by what is on the ground today, including any building on the application site that will be demolished. Omitting an existing building from the baseline overstates the effect of the proposal, and including a building that has already been demolished understates it. The reasoning behind baseline choices, including where a mirror image assumption is appropriate, is covered in the note on the mirror image baseline.
The proposed case shows the scheme. The cumulative case adds consented but unbuilt schemes nearby, and on any site in an area of active development this case is the one an objector will ask for if you have not provided it.
Colour the shadows so that the three cases are distinguishable in greyscale, because committee reports are printed in black and white more often than anyone expects.
Static plots or animation
Static plots are the submission document. They print, they appear in the officer report, they can be annotated and they can be compared.
An animation is a useful supplement for a presentation or a website, and it is close to useless as evidence, because nobody can cite a frame. Where an animation is provided, extract the hourly stills and include those in the report.
Perspective views and rendered images cause more problems than they solve. A shadow plot should be a plan, drawn to a stated scale with a north point, because a plan can be measured against the site layout and a render cannot.
Criticisms that land
Six recur, and all of them are avoidable.
Summer only. Plots for 21 June alone show the scheme at its most flattering and read as an attempt to do exactly that.
No baseline. Proposed-case shadows with nothing to compare them against say nothing about change.
Midday only. A single plot at 12:00 or 13:00 misses the long shadows that cause the actual loss of amenity.
No time basis stated. Covered above, and easy to fix.
No quantification. Plots without the sun-on-ground percentages leave the reader with an impression rather than a finding.
No cumulative case on a site surrounded by consents.
Fixing all six costs a day of drawing time, and each of them is a reason an officer can give for asking a question that delays determination by a fortnight. The wider point about presenting this material legibly is made in the note on presenting daylight results a committee can read.
Captioning the plots
Every drawing needs six pieces of information in the title block or the caption, and they take a line each:
- The date plotted, written in full.
- The time, with the time basis stated.
- The case: existing, proposed or cumulative.
- The scale and a scale bar.
- A north point.
- The receptor references shown on the sheet.
Add a one-line finding under each plot where one can honestly be made, such as noting that the rear garden of a named receptor is in full sun at that time in both cases. Resist the temptation to editorialise on every sheet. A caption that draws a conclusion the drawing does not support is worse than no caption, and officers notice.
Number the sheets so the set reads in date and time order, and include a contents page. A committee member with ten minutes and thirty drawings will look at the first three and the contents page.
Where the numbers behind the plots are close to a threshold, commission the modelling before the massing is fixed rather than after, and set the Daylight and Sunlight Assessment alongside the shadow work so the two documents describe the same building.
This is general guidance, not legal advice. Daylight, sunlight and overshadowing modelling should be carried out by a suitably qualified specialist, and BRE guidance is advisory and applied with judgement. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Which dates should a shadow study plot?
21 March, 21 June and 21 December. The equinox is the date the BRE sun-on-ground test uses and carries the most weight. The solstices show the best and worst cases. Adding 21 September rarely helps, because its sun path is close to that of 21 March and it produces near-identical plots.
How often should shadows be plotted through the day?
Hourly is the convention, roughly 09:00 to 16:00 on the equinox, 08:00 to 19:00 in June and 10:00 to 14:00 in December. Half-hourly plots on the equinox are worth producing where one receptor is contentious, because the two hour sunlight test can turn on a narrow window.
Do I need to state whether times are GMT or BST?
Yes, on every drawing. Greenwich Mean Time, British Summer Time and true solar time place the shadow differently for the same clock reading, so a plot labelled 13:00 without a basis is ambiguous. Most practitioners work in GMT across all three dates for consistency and say so in the title block.
What is the cumulative case in an overshadowing study?
The proposed scheme modelled together with consented but unbuilt development nearby. On a site in an area of active development an objector or officer will ask for it if it is not provided. Present it alongside the existing and proposed cases at the same scale so the three can be compared directly.
Are shadow animations acceptable as planning evidence?
They are a useful supplement for a presentation and poor evidence on their own, because no one can cite a frame or measure it. Extract hourly stills from the animation and put those in the report as plans, drawn to a stated scale with a north point, so they can be annotated and printed.
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.