Transient overshadowing plots and how officers use them
Transient shadow diagrams show extent at an instant, not duration. Here is how to produce them properly and keep them separate from the BRE sun on ground test.
Transient overshadowing shadow plots show where the shadow of a proposed building falls at a single instant, usually at hourly intervals across a chosen day. They are supplementary analysis. They are not a test, they have no pass or fail criterion, and an officer who treats a 09:00 December plot as a finding has misread the drawing. What the BRE sun on ground assessment measures is different: the proportion of an amenity area receiving at least two hours of sunlight on 21 March, before and after the development.
Both have a place. Problems start when one is presented as the other.
What a transient plot shows that the sun on ground test does not
The sun on ground test in the BRE guidance reduces a whole day to one number for the whole area. That is its strength and its limitation. It tells you half the garden still gets two hours on the equinox. It does not tell you that the half losing sunlight is the half with the terrace and the back door.
A transient plot answers the spatial question. At 15:00 on 21 June, which part of this courtyard is in shade? Where should the seating go? Will the children's play area be usable after school in the shoulder seasons? For an Overshadowing Study covering public space or shared amenity areas, that is often the more useful output, because the design decision is about where things go rather than whether a threshold is met.
The pairing works like this. Use the duration assessment to establish whether the effect is acceptable in principle. Use the transient plots to show where the shade sits and to justify the layout that follows.
Choosing dates and time intervals
Three dates do almost all the work, and each answers a different question.
21 March, the equinox, is the date the BRE sun on ground criterion uses, so a transient set on that date sits alongside the numerical assessment and can be read against it.
21 June shows the best case and the usable extent of outdoor space in the summer. It is the date residents care about, and the date an applicant naturally wants to show.
21 December shows the worst case, with the sun low and shadows long. Objectors reach for it. Present it anyway, because withholding it reads as concealment and an objector will produce it themselves.
Hourly intervals are the normal convention, often from an hour after sunrise to an hour before sunset, which keeps out the grazing-angle shadows that extend for hundreds of metres and mean nothing. Half-hourly intervals are worth producing where a specific receptor matters, such as a single garden terrace or a nursery play space.
State whether the times are clock times or solar times, and whether summer time has been applied. A set of plots labelled 09:00 to 17:00 on 21 June with no statement about British Summer Time is ambiguous by an hour, and that hour changes which part of the courtyard is in shade.
Modelling conventions that have to be stated
An unlabelled shadow plot is not evidence. Six conventions should appear on the drawing or in the method section:
- The north point used, and whether it is true north or grid north. The difference is small in England but it is not zero, and the plots are sensitive to orientation.
- The site latitude used to derive solar altitude and azimuth.
- Whether times are Greenwich Mean Time, British Summer Time or local solar time.
- What is in the model: the proposal, the existing buildings being removed, surrounding buildings and their heights, and the source of those heights.
- Whether trees, boundary walls and fences are included. Deciduous trees in December are not the same obstruction as in July, so either exclude vegetation and say so, or model it twice.
- The ground plane and any level changes, since a sloping site shifts the shadow geometry.
The source of surrounding building heights is the one most often skipped. Heights taken from aerial survey data should be cited as such, and heights taken from a measured survey are better. Where a neighbouring roof height has been estimated, say so, because that estimate is the first thing a competent objector will test.
Where the plots help and where they mislead
They help when the question is spatial. Where to put a play space. How to orientate a terrace. Which flats get a balcony. Whether a courtyard works in the afternoon as well as at noon.
They mislead in three ways. A single frame taken out of a set exaggerates, because it shows extent without duration: a shadow crossing a garden at 10:00 and gone by 11:00 reads identically to one that sits there all day. A low winter sun produces enormous shadows that look alarming and matter little, because nobody is sitting in the garden on 21 December. And a plot with no criterion attached invites the reader to supply their own, which is how a reasonable scheme ends up described as plunging a garden into darkness.
The honest answer to all three is to publish the full set, add a duration output, and say in the text what the plots do and do not demonstrate. The way local plan amenity policies use the BRE guidance usually tells you which of the two the officer will rely on.
Public open space and seasonal use
For a park, square or play space the seasonal question is the one that matters, because use is concentrated in the warmer months and in the after-school and after-work periods.
So pick the time bands to match the use rather than the solar geometry. A pocket park beside offices is used at lunchtime, so the 12:00 to 14:00 band on 21 March and 21 June is the useful evidence. A play space is used from 15:30 onwards in term time. A seating area outside a cafe is used in the late afternoon.
Growing space is different again, because plants respond to accumulated light across a season rather than to comfort at a moment. Transient plots are weak evidence there, and a sunlight hours analysis across the growing season is what the question deserves. The same logic applies to overshadowing of allotments and orchards.
Labelling supplementary analysis clearly
Put a short paragraph at the front of the plots that says four things. That the analysis is transient and supplementary. That the BRE guidance contains no pass or fail criterion for instantaneous shadow extent. That the numerical assessment of sunlight to amenity areas is reported separately against the two hour criterion on 21 March. And that the plots were produced on the stated conventions.
Number the frames and keep the viewpoint, scale and north point identical across the set, so frames can be compared. Produce an existing and a proposed version of each frame rather than only the proposed, because the comparison is the evidence. Where the existing site has a building on it, the removal of that building sometimes improves the position in part of the day, and a proposed-only set hides that.
An illustrative example. A four storey infill block sits to the south-west of a row of terraced gardens. The 21 March proposed plots show shade reaching the rear third of the gardens from about 16:00. The existing plots show the same gardens already shaded from about 16:30 by a two storey workshop being demolished. The sun on ground assessment shows each garden retaining more than half its area with two hours of sunlight. Taken together the picture is a modest afternoon change rather than a loss. Those figures are illustrative and a real site will differ.
Answering an objection built on a shadow image
Objectors increasingly produce their own shadow images from freely available modelling tools, and some of them are competent. Three responses work.
Check the conventions in their model first. Wrong north point, wrong date, summer time applied inconsistently, or surrounding buildings omitted are all common, and identifying the error is more persuasive than arguing about judgement.
Then supply duration where they have supplied extent. An image showing a shadow at one instant is answered by a figure for how many hours the receptor retains.
Then give the policy test. Amenity policies and the Daylight and Sunlight Assessment are assessed against stated criteria, and an argument with no criterion behind it carries less weight than one with a measured result. A Daylight and Sunlight Assessment is a report councils usually expect from a suitably qualified author, and professional review is strongly advised before it is relied on in a committee report.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. Enter the site postcode on PlanScope to see which amenity and daylight documents the constraint check flags for your scheme.
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Frequently asked questions
Is there a pass or fail standard for transient shadow plots?
No. The BRE guidance contains a criterion for sunlight to amenity areas, assessed on 21 March against a two hour threshold and a comparison with the existing position, but nothing governs instantaneous shadow extent. Transient plots are supplementary spatial analysis. Say so on the drawings, and report the numerical sunlight assessment separately so the two are not confused.
Which dates should shadow plots cover?
21 March, because the sun on ground criterion uses the equinox and the plots can then be read against the numerical result. 21 June for the best case and summer usability, and 21 December for the worst case. Produce the December set even though it looks poor, because withholding it reads as concealment and objectors will generate it themselves.
Do shadow plots need to state summer time?
Yes. A set labelled 09:00 to 17:00 without saying whether times are Greenwich Mean Time, British Summer Time or local solar time is ambiguous by up to an hour, and that hour changes which part of a courtyard is shaded. State the convention, the latitude used and the north point, and keep them consistent across the whole set.
Should trees be included in the model?
Either exclude vegetation and say so, or model it twice, because a deciduous canopy in July and the same tree in December are very different obstructions. Boundary walls and fences should be treated the same way. The point is that a reader can tell what is in the model, so an unstated assumption is worse than a conservative one.
How do I answer an objector's own shadow image?
Check their conventions first, since wrong north point, wrong date, inconsistent summer time and omitted surrounding buildings are common errors and easier to demonstrate than a difference of judgement. Then supply duration where they supplied extent, giving hours retained at the receptor. Then point to the policy criterion the assessment is measured against.
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.