An overshadowing study asks one practical question: will a new building cast so much shadow that a neighbouring garden, courtyard or amenity space loses its sunlight? Under the current BRE guidance, the answer turns on a single, deliberately chosen day — the spring equinox on 21 March — when the sun behaves as a fair, average representative of the whole year.
If you are designing an extension, an infill block or a larger residential scheme, the 21 March equinox test is often the part of a daylight and sunlight assessment that decides whether amenity space remains usable. This guide explains what the test measures, why that date was chosen, what the pass threshold is, and how to read the results before they reach a planning officer.
Why overshadowing matters in planning
Sunlight to open space is treated separately from daylight and sunlight to windows. A scheme can satisfy the window-based metrics — the ones covered in our guide to VSC, NSL and APSH — and still fail on overshadowing if it plunges a neighbour’s garden into shade for most of the day. Local planning authorities across England, Scotland, Wales and Northern Ireland rely on the BRE methodology to judge whether the loss of sunlight to gardens and amenity areas is acceptable, and the test is frequently cited in officer reports and at appeal.
The reason is straightforward. A shaded garden is a garden people use less. Where a development materially reduces the sunlight reaching a shared courtyard, a children’s play area or a private rear garden, that loss of amenity is a legitimate planning consideration — and an overshadowing study is the accepted way to quantify it.
The 21 March equinox test: what it measures
The headline test is the sun-on-ground assessment. The BRE guide recommends that, on 21 March, at least half (50%) of an amenity area should receive at least two hours of direct sunlight. If a proposed development reduces the sunlit area so that less than 50% gets two hours, and the area that still receives two hours falls to less than 0.8 times its former value, the loss is likely to be noticeable and may be judged unacceptable.
In practice an assessor builds a three-dimensional model of the proposal and the surrounding context, then runs a shadow analysis for 21 March. The output is usually a pair of plans — existing and proposed — shading the parts of the amenity space that fail to achieve two hours of sun. The difference between the two tells you how much sunlit ground the development removes.
Why 21 March and not midsummer or midwinter
The choice of the spring equinox is not arbitrary. On 21 March the sun rises due east and sets due west, and its path sits roughly midway between the high arc of the June solstice and the low, weak arc of December. That makes it a sensible “average” day: testing in midsummer would flatter almost any scheme because the sun is so high, while testing in midwinter would condemn schemes that perform perfectly well for most of the year.
The equinox also aligns the UK approach with BRE BR 209 (2022) and the European daylight standard BS EN 17037, both of which use the equinox as a reference condition. It is the same date that underpins the sunlight-to-windows assessment for new dwellings, so a single reference day governs both the indoor and outdoor sunlight checks — a deliberate simplification introduced in the 2022 edition.
What changed under BRE 2022
The 2022 edition of the BRE guide brought the sunlight methodology into line with BS EN 17037. For sunlight to new dwellings, the older “probable sunlight hours” approach was replaced by an assessment of the time a window can receive direct sunlight on 21 March under cloudless conditions. The sun-on-ground test for amenity space, however, has remained broadly stable: the two-hour, 50% benchmark on the equinox continues to be the working standard, and it sits alongside the indoor daylight provision tests introduced in the same revision.
The practical effect is consistency. Whether an officer is looking at sunlight reaching a living-room window or sunlight reaching a back garden, the same date and the same cloudless-sky assumption apply, which makes results easier to compare and harder to game.
How the test plays out for common projects
For a modest single-storey rear extension, overshadowing of a neighbour’s garden is often the deciding issue — particularly where gardens are short, the boundary runs north of the extension, or the plot orientation throws afternoon shadow across next door. A taller two-storey or wrap-around addition raises the stakes further.
For larger schemes, the amenity space in question is frequently the development’s own communal garden or podium. Here the study does double duty: it protects neighbours and demonstrates that the new residents will have usable, sunlit outdoor space of their own. Catching a shortfall at design stage — by lowering a ridge, pulling back a massing line, or reorienting a block — is far cheaper than discovering it after a refusal.
Reading an overshadowing result before it reaches the council
A few questions help you interpret the output quickly:
- What proportion of the amenity area receives two hours on 21 March in the existing case? If it is already below 50%, the 0.8× retained-value test becomes the one to watch.
- How much sunlit ground does the proposal remove? A drop from, say, 70% to 65% is usually comfortable; a drop below the 50% line, or to under 0.8 of the former value, needs justification.
- Is the affected space genuinely amenity space? Driveways, narrow side passages and hardstanding are not weighted the same way as a usable garden or shared green.
- Does the local plan add anything? Some authorities set their own amenity-space or sunlight expectations on top of the BRE benchmark, so the national figure is a floor, not always the ceiling.
Common misunderstandings
Two points trip people up. First, the test is about direct sunlight, not general brightness — a space can feel bright and open yet still fail because tall obstructions block the sun’s low equinox path. Second, the cloudless-sky assumption is a modelling convention, not a weather forecast; it gives every scheme the same yardstick rather than predicting real conditions on any particular March day.
It is also worth remembering that the BRE figures are guidance, not statute. They are advisory targets that planners apply with judgement, and a well-argued report can explain why a numerical shortfall is acceptable in context — for instance in a dense urban setting where some mutual overshadowing is unavoidable.
How Fortress Associates can help
If your project risks casting shadow over a neighbour’s garden or your own amenity space, our daylight and sunlight report service includes the full 21 March sun-on-ground assessment to BRE 2022 and BS EN 17037, with clear existing-and-proposed shadow plans you can submit with your application. We turn most reports around in 4–5 working days, we work nationwide across the UK, and we take no advance payment. Where a scheme falls short, we set out practical mitigation so you can adjust the design before it reaches committee. See our full list of services or get in touch for a fixed quote.
Sources & further reading
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