Transient overshadowing and how it is assessed
Transient shadow moves across a garden through the day. How hourly diagrams are produced, which assessment dates matter and how officers read them.
Transient overshadowing is shadow that moves. It falls on a garden, a terrace or a window for part of the day and then leaves, and it is assessed by looking at duration and timing rather than at the area covered in any single image. That distinction decides most arguments about shadow studies, because an objector holding one screenshot of a garden in shade at 4pm in December is describing a moment, not an effect.
The assessment that answers them shows the same garden across a whole day, hour by hour, on dates that mean something.
What transient overshadowing means
The BRE guidance on site layout planning for daylight and sunlight sets out two different ways of looking at sunlight on open space. One is a fixed test: whether a defined proportion of an amenity area receives a stated period of sunlight on the equinox. The other is the shadow study, which shows where the shadow of the proposed building falls at intervals through a day.
The fixed test gives a pass or fail. The shadow study gives context, and it is the document that tells a case officer whether a garden loses its morning sun, its afternoon sun, or an hour in the middle of the day when nobody is home.
Neither test is a rule of law. Both are guidance, applied with judgement, and an authority can accept a technical failure where the design justification is strong or refuse a technical pass where the harm is obvious on the ground.
Producing hourly shadow diagrams
A usable study is a set of plan views of the site and its surroundings, each showing the shadow cast by the existing situation and by the proposal, at hourly intervals across the daylight hours of the chosen date. Include the existing baseline on every image. A shadow diagram of the proposal alone tells the reader nothing about change.
Build the model from survey data rather than from assumptions. Neighbouring building heights should come from a measured survey or from a reliable height model, boundary treatment should be shown, and existing trees should be included with a note of whether they are in leaf on the modelled date. Ground levels matter on sloping sites, where a flat model can understate or overstate shadow substantially.
Label every image with the date, the time, the projection and the orientation. Studies get separated from their covering report inside a case file, and an unlabelled image is worthless.
Assessment dates and why they are chosen
The equinox on 21 March is the standard assessment date because it sits at the midpoint of the year, with day length and sun angles close to the average. The BRE guidance uses it for the sun on ground test, so it is the date a technical assessment has to include.
21 June and 21 December are often added for context. The June date shows the best case, when the sun is high and shadows are short, and it matters for outdoor space in the months people use it. The December date shows the worst case, with a low sun casting long shadows, and it is the one objectors quote.
| Date | What it shows | How officers usually treat it |
|---|---|---|
| 21 March | Average conditions, the basis of the sun on ground test | The date the assessment turns on |
| 21 June | Long days and high sun angles, best case for gardens | Useful context for amenity space in summer use |
| 21 December | Low sun and long shadows, worst case | Informative, but rarely decisive on its own |
Say on the drawing which dates were modelled and why. If a study shows only June, the reader will assume the other dates were unflattering.
Reading duration rather than extent
The question is how long sunlight is lost, at what time of day, to a space that people use. Two hours of shade across the middle of a small garden matters more than four hours of shade on the strip beside a boundary fence that was already shaded.
Read the sequence, not the frames. Track one point in the garden across the hourly images and count the hours it is in shadow before and after the proposal. Do that for two or three representative points: the seating area near the house, the middle of the lawn, and the end of the garden. Those three numbers are what a committee report can use.
Timing carries weight too. Loss of morning sun to a breakfast terrace, or of evening sun to a west facing garden, is a different harm from a shadow that passes at 11am. Where the difference matters, our note on transient overshadowing against permanent loss of sunlight sets out how the two effects are weighed.
Where transient shadow matters most
Gardens and communal amenity areas are the usual subject, but three other cases deserve attention. Children's outdoor space attached to a school or nursery is assessed against the same sun on ground approach, and our note on sunlight to playgrounds and nursery outdoor space covers what those assessments need. Solar panels on a neighbouring roof raise a different question, since shading part of an array can reduce output disproportionately. And allotments and food growing areas depend on sunlight in a way that lawns do not.
Roof terraces on the proposal itself are worth modelling too, since a terrace in permanent shade from the building's own massing is a design problem rather than a neighbour problem.
Common presentation errors
Four errors turn a sound study into a disputed one.
- Omitting the existing baseline, so change cannot be seen
- Modelling only one date, usually the flattering one
- Using a north point inconsistent with the site plan, which reverses the whole analysis
- Presenting a single rendered view instead of plan projections, because a perspective exaggerates whatever is in the foreground
Add a short written reading of the images. Consultants often submit 18 diagrams and no interpretation, leaving the officer to draw conclusions from pictures. Two paragraphs identifying the receptors, the hours lost and the residual condition will be copied into the report almost verbatim.
Answering an objection based on one image
Objections built on a single moment are answered by putting that moment back into its sequence. Confirm the date and time in the objector's image, show the same view at hourly intervals across the day, state the hours of direct sunlight the space retains before and after, and compare the result with the guidance benchmark.
Do it without dismissing the concern. An objector who reports that their garden goes dark in the afternoon is reporting something real, and the answer is evidence about how long and when, not an assertion that the guidance is satisfied. Where the loss is genuine, look at design: a set back at upper level, a step in the massing, a change to the roof form, or relocating the taller element to the northern part of the site.
An overshadowing study covers the shadow question, while window based daylight and sunlight tests are a separate exercise set out on the daylight and sunlight assessment page. Submitting one does not answer the other, and officers ask for whichever is missing.
PlanScope drafts the narrative documents that accompany technical studies and flags where modelling inputs are missing. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. Shadow modelling should be produced by a suitably qualified consultant with access to survey data.
This is general guidance, not legal advice. Local design guidance can set its own amenity standards, so check with your local planning authority before relying on any benchmark.
Frequently asked questions
Which dates should a shadow study model?
The equinox on 21 March is the standard date, because it sits close to average sun angles and underpins the sun on ground test in the BRE guidance. Many studies add 21 June and 21 December for context. Always state which dates were modelled, since a study showing only the flattering date invites suspicion.
What is the difference between transient overshadowing and loss of sunlight?
Transient overshadowing describes shadow that moves across a space for part of the day, assessed by duration and timing. Loss of sunlight in the BRE tests is measured against fixed benchmarks for windows and for amenity areas. A space can fail one and pass the other, so both are usually reported.
Does a shadow diagram need to show the existing situation?
Yes. Without the existing baseline at the same date and time, the reader cannot see what the proposal changes. Submitting proposal-only images is the most common reason a study is treated as advocacy rather than evidence, and it usually prompts a request for further information.
How should the study be interpreted for a committee report?
Track two or three representative points in each affected space across the hourly images and state how many hours of direct sunlight each retains before and after. Those figures, with a note of the time of day affected, are what an officer can use. Diagrams without a written reading rarely help.
How do I answer an objection based on one photograph?
Establish the date and time it shows, then present the same view at hourly intervals across that day so the moment sits in its sequence. State the retained hours of sunlight and compare against the guidance benchmark. Where the loss is real, look at massing changes rather than arguing the point.
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.