Transient overshadowing: what a shadow study must show
Transient shadow plots show when a receptor loses sun, rather than only how many hours it keeps. Here is how to choose dates, model the baseline and present plots an officer can read.
A transient overshadowing study plots where the shadow of a proposed building falls, hour by hour, on stated dates. It is a presentational exercise rather than a numerical test, and that is exactly why it gets done badly. There is no threshold to hide behind, so the quality of the study rests on the assumptions, the dates and whether an officer can read the drawings without a covering note.
Done properly it is one of the more persuasive documents in an amenity case. Done as an afterthought it invites the objection it was meant to answer.
What transient overshadowing adds to the sun on ground test
BRE 209, Site layout planning for daylight and sunlight, sets a numerical test for sunlight to open space. On 21 March, at least half of an amenity area should receive at least two hours of sunlight, and where the proposal reduces the sunlit area it should not fall below 0.8 times its former value. That is the sun on ground test, and it produces a pass or a fail.
The trouble is what it hides. An amenity area can pass while being in shadow through the entire morning, because the test counts total hours rather than when they fall. A garden that is sunlit from 15:00 onwards passes on 21 March and is useless for breakfast in June.
Transient plots show the distribution. They also cover receptors the numerical test does not reach, such as a stretch of pavement, a children's play area used at a particular time of day, or a neighbour's patio that sits outside the assessed amenity area boundary.
They are qualitative. Say so in the report. An officer who thinks they are being handed a pass or fail result will look for a threshold that does not exist.
Choosing assessment dates and time intervals
Three dates cover almost every case.
| Date | What it shows | Why include it |
|---|---|---|
| 21 March | The equinox condition used by the BRE numerical tests | Comparable with the sun on ground assessment, and the standard reference |
| 21 June | Longest day, highest sun, shortest shadows | Shows the summer amenity condition when gardens are used most |
| 21 December | Shortest day, lowest sun, longest shadows | The worst case, useful where an objector claims permanent loss |
Some authorities ask for 21 September as well. It is optically identical to 21 March, so include it only where a local validation list or a specific request calls for it, and say in the report why it duplicates the equinox.
Hourly intervals are the norm. Half-hourly plots on the equinox are worth producing where a single hour decides whether a receptor passes the two-hour test, because they show whether the sunlit period is continuous.
State the time convention explicitly. Sun position is calculated in local apparent solar time or in Greenwich Mean Time, and a plot labelled 13:00 means two different sun positions depending on whether British Summer Time has been applied. Label every sheet with the convention used. This is the single most common thing a reviewing consultant picks apart.
Modelling the existing and proposed condition
A shadow study with no baseline proves nothing. Model two scenarios and present them side by side on each sheet: the existing site as built, and the proposed development. Where the site is currently vacant, the existing condition is the vacant site, not the previously demolished building, unless the demolition is part of the same application.
Get these inputs right.
- Latitude and longitude of the site, not of the city
- True north, taken from the survey rather than from a drawing convention
- The full surrounding context to a radius that captures every building tall enough to cast a shadow onto your receptors
- Existing boundary walls and fences, which matter enormously for ground-level receptors
- Terrain, where the site slopes more than a metre or so across the receptor area
Trees are a judgement call. Deciduous trees in leaf are not modelled for the March and December cases, and modelling them in June overstates the existing shading. The defensible approach is to exclude them and state that you have, noting that the existing condition is therefore presented as sunnier than it is in summer.
Which receptors to plot and why
Pick receptors before you model, and justify each one. A study that plots everything within 200 metres is unreadable, and a study that plots only the applicant's own courtyard is transparent.
Neighbouring private gardens are the priority, particularly the part of the garden nearest the house, which is where people sit. Communal amenity space within the scheme comes next, then any public open space, play space or allotments. Balconies and roof terraces on adjoining buildings are receptors too, and are frequently missed because they do not appear on the Ordnance Survey base.
Where the scheme adjoins a school playground, a care home garden or a bowling green, name it and treat it separately. Those receptors have a time-of-day sensitivity that a generic assessment will not capture.
Presenting hourly plots that officers can read
The presentation decides whether the study does its job.
- One sheet per date, with the hourly plots laid out in a grid reading left to right, existing above proposed.
- A consistent scale, north point and key on every sheet.
- Shadow rendered as a solid tone with the receptor boundaries drawn over it, not underneath it.
- Building outlines legible through the shadow, so the reader can see what is casting it.
- A short caption per date stating the finding in words, for example that the northern half of the rear garden at number 14 falls into shadow after 15:00 in March under both conditions.
- A summary table of receptors against dates, with a plain description of the change.
Print it at A3 and look at it. If you cannot tell existing from proposed at arm's length, neither can the case officer or the committee member.
Common modelling errors and omissions
Six errors account for most of the studies that get taken apart.
Omitting existing buildings that are outside the red line but inside the shadow path. Using the drawing north point rather than true north, which on some surveys differs by several degrees. Presenting British Summer Time and Greenwich Mean Time plots on the same sheet without labelling. Modelling the proposal without its parapets, lift overruns and plant enclosures, which are often the tallest elements. Assessing only the equinox, then being asked for December at committee and delaying the decision by a cycle. And using a massing model whose heights came from the planning drawings while the surrounding context heights came from a national terrain dataset with a different vertical datum.
That last one is subtle and worth checking. Mixing height sources produces shadows that are wrong by metres and nobody notices until an objector's consultant re-runs it.
Using the study to change the massing
The most useful moment for a shadow study is before the design is fixed. Run the December case on the massing option at feasibility and you will often find that pulling back the top two storeys on the southern edge, or shifting a core, recovers an hour of morning sun in a neighbour's garden for very little area.
Bring it forward again if the scheme is refused on amenity grounds. Transient plots showing the difference between the refused scheme and a revised one give a case officer something concrete to report, and they read well in a statement of case. They belong alongside the numerical work in a Daylight and Sunlight Assessment rather than instead of it, and a standalone Overshadowing Study is the right document where the concern is amenity space rather than windows.
Councils expect a suitably qualified author for daylight and sunlight work, and professional review is strongly advised. PlanScope drafts the assessment structured to UK planning policy, with a "to be confirmed" flag wherever modelling outputs are missing, so your consultant fills in the numbers rather than the narrative. This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. Enter your postcode to see which reports your site is likely to need.
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 use?
The equinox on 21 March is the standard reference because it matches the BRE numerical tests. Add 21 June for the summer amenity condition and 21 December for the worst case. Some validation lists ask for 21 September, which is optically the same as the March case, so include it only where it is specifically required and say why it duplicates.
Is transient overshadowing a pass or fail test?
No. It is a qualitative presentation of where shadows fall at stated times. The numerical test is the sun on ground assessment, which looks at whether half an amenity area receives two hours of sunlight on 21 March. State clearly in the report that the transient plots are illustrative, or an officer will look for a threshold that does not exist.
Do trees need to be included in the shadow model?
Generally no, and the reason should be stated. Deciduous trees are not in leaf at the March and December dates, and modelling them in June overstates existing shading, which flatters the proposal. Excluding them means the existing baseline is presented as sunnier than reality in summer, so say so rather than leaving the reader to work it out.
Why does the time convention on shadow plots matter?
A plot labelled 13:00 in British Summer Time shows a different sun position from one labelled 13:00 in Greenwich Mean Time, so shadows fall in different places. Reviewing consultants check this first. Label the convention on every sheet and use the same one throughout the study.
When is a shadow study most useful?
At feasibility, before the massing is fixed. Running the December case on early options often shows that setting back the upper storeys on the southern edge returns an hour of morning sun to a neighbouring garden for very little floor area. Producing it only after a refusal means arguing rather than designing.
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.