Shadow plots and dates in an Overshadowing Study
Shadow plots look simple and are easy to mislead with. The dates chosen, the times shown and what was modelled decide whether the drawings answer the question.
An Overshadowing Study answers a narrow question: how much sunlight reaches a garden, terrace or other outdoor amenity area before and after a proposed building goes up. Overshadowing study shadow plots are the drawings that show it, and the recognised guidance sets a clear benchmark. At least half of an amenity area should receive at least two hours of sunlight on 21 March, and where a development reduces the area meeting that test to less than 0.8 times its former value, the loss is likely to be noticeable.
That is a different exercise from a daylight and sunlight assessment, which looks at windows. A garden can be badly overshadowed while every window in the house passes, and the reverse happens too.
Why 21 March
The spring equinox is the reference date because it sits midway between the extremes, with the sun at a moderate altitude and day length close to twelve hours. It is a reasonable proxy for conditions across the useful part of the year.
Plots at other dates serve different purposes. 21 June shows the best case and is sometimes used to demonstrate that an area is usable in high summer. 21 December shows the worst case, with the sun low and shadows long, and it tends to be produced by objectors. Neither replaces the equinox test.
Where a study presents only midsummer plots, treat the conclusion with care. Showing that a garden gets sun on the longest day of the year answers almost nothing.
Times, and the convention that trips people up
Shadows are normally plotted hourly from about two hours after sunrise to about two hours before sunset, or across the full day where the area is enclosed. The series matters more than any single frame, because the two hour test is cumulative: an area receiving sun from 10:00 to 11:00 and again from 14:00 to 15:00 passes, even though it is in shade at midday.
State the time convention on the drawing. Plots are conventionally produced in local solar time or in Greenwich Mean Time, not in British Summer Time, and a set of drawings that shifts by an hour without saying so is the sort of error that undermines everything else in the report.
Give the sun altitude and azimuth for each frame, or reference the calculation method and the site latitude and longitude. A reviewer with those figures can check any frame in a few minutes.
What has to be in the model
The proposed building is the easy part. The rest is where studies go wrong.
Existing buildings around the site have to be modelled at their real heights, taken from a measured survey or from reliable terrain and building height data, not scaled from a photograph. Boundary walls and fences matter for small gardens, because a two metre fence casts a long shadow across a narrow plot in March.
Trees are a judgement. Evergreen trees cast shade all year and should be modelled. Deciduous trees are in leaf for part of the year only, and the convention is to show the existing condition with and without them, or to state the assumption clearly. A study that models a mature deciduous tree in full leaf on 21 March overstates the existing shade and flatters the proposal.
Ground levels matter on sloping sites. A shadow cast onto rising ground reaches less far horizontally than the flat-site calculation suggests, and a shadow cast onto falling ground reaches further.
Transient overshadowing against permanent loss
Not every shadow is a harm. A shadow that crosses a garden for forty minutes in the early morning is different from one that covers a patio from noon onwards through the growing season.
Describe the pattern as well as the totals. Which part of the amenity area is affected, at which times, and whether the part affected is the part actually used. A garden with a seating terrace close to the house and a lawn beyond is affected differently by a shadow falling on each.
Say where the existing condition is already poor. Where a garden meets the two hour test over only 30 per cent of its area before the development, the proportional test is the relevant one, and reporting only the after figure hides the baseline.
Reading a study prepared by someone else
Neighbours and objectors often receive a study with an application and have no way to test it. Three checks need no software.
Compare the existing and proposed plots frame by frame at the same time of day. If the sheets show different times, or different scales, ask why before reading any conclusion.
Look at what is missing from the existing plan. A boundary wall, an outbuilding or a mature evergreen that exists on site but not in the model changes the baseline, and omissions are easier to spot than errors in geometry.
Check the amenity area outlined. Including a driveway or a side passage in the area assessed raises the proportion meeting the test without a single sunbeam changing position.
Gardens, terraces and play space
The two hour test applies to the amenity area, which for a house usually means the rear garden. Front gardens, drives, bin stores and narrow side passages are not amenity areas and including them inflates the area assessed.
For flats, the relevant areas are communal amenity space, podium gardens and roof terraces, and each should be assessed on its own rather than aggregated. A scheme that passes across its combined amenity area while its only ground-level garden sits in permanent shade has not answered the question.
Children's play space is treated more carefully in many local policies, and some authorities set their own sunlight expectations for it. Check the development plan and any supplementary guidance before assuming the national benchmark applies, and where the scheme includes formal provision, the Play Space Assessment should be consistent with the shadow results.
Drawings an officer can read
- A site plan showing the amenity areas assessed, outlined and numbered, with their areas stated.
- Existing and proposed shadow plots for the same dates and times, on the same sheet at the same scale.
- A difference plan showing the area meeting the two hour test before and after, hatched.
- A table giving, for each amenity area, the area meeting the test before, the area after, the proportion retained and the comparison against the benchmark.
- A statement of what was modelled, including surrounding building heights and their source, trees, walls and ground levels.
- Time convention and sun position data.
The table is the item most often missing, and it is the one the case officer needs. Plots alone leave the officer to measure hatched areas with a scale rule, which nobody does.
Illustrative example, for method only. A two-storey rear extension on a neighbouring semi casts a shadow across the back third of a 90 square metre garden through the middle of the day on 21 March. Before the extension, 72 square metres of the garden met the two hour test. After, 61 square metres meet it, which is 0.85 of the former value. The area retained is above both the half-the-area benchmark and the proportional threshold, so the loss is unlikely to be treated as unacceptable on sunlight grounds, though overbearing impact and outlook remain separate matters for the case officer.
Where windows as well as gardens are affected, a separate Daylight and Sunlight Assessment is needed, and reading its output is covered in reading the results tables of a daylight assessment. The amenity case then sits alongside the design case in the Design and Access Statement.
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 is the two hour sunlight test for gardens?
Recognised guidance suggests that at least half of an amenity area should receive at least two hours of sunlight on 21 March. Where a development reduces the area meeting that test to less than 0.8 times its former value, the loss of sunlight is likely to be noticeable to the occupiers.
Why is 21 March used rather than midsummer?
The spring equinox sits midway between the extremes, with a moderate sun altitude and roughly twelve hours of daylight, which makes it a reasonable proxy for the useful part of the year. Midsummer plots show the best case and prove very little on their own.
Do the two hours have to be continuous?
No. The test is cumulative across the day, so an area receiving an hour of sun in the morning and another in the afternoon passes even though it is shaded at midday. This is why the full hourly series matters more than any single frame in the drawing set.
Should trees be included in the model?
Evergreen trees should be modelled because they cast shade all year. Deciduous trees are in leaf for part of the year only, so the usual approach is to show the existing condition both with and without them, or to state the assumption clearly. Modelling a deciduous tree in full leaf in March overstates existing shade.
Is an overshadowing study the same as a daylight assessment?
No. An overshadowing study assesses sunlight reaching outdoor amenity areas such as gardens and terraces. A daylight and sunlight assessment looks at light reaching windows in buildings. A scheme can pass one and fail the other, and contested cases usually need both.
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.