Overshadowing of a public park by a tall building
The sun on ground test gives one number for an amenity space. A park is many spaces. Here is how to zone it, baseline it and report it so the result survives committee.
When a tall building is proposed next to a public park, the sunlight question is governed by the same BRE criterion that applies to a back garden: at least half the area of an amenity space should receive at least two hours of sunlight on 21 March. Applying that sentence to twelve hectares of park, with a play area at one end, a bowling green in the middle and a wooded edge along the road, is where the work actually lies. The criterion is a single number. A park is not a single space.
Officers and inspectors now expect the assessment to divide the park by function and report each part separately, with a baseline that tells them what the existing condition already is.
Applying the sun on ground test to public space
The sun on ground test in the BRE guidance on site layout planning for daylight and sunlight (2022 edition) measures the proportion of the ground area of an amenity space receiving two hours or more of direct sunlight on the equinox. The output is a percentage of area, reported for the existing condition and for the proposed condition, with the difference stated.
Two points about the method are worth being clear on. The guidance is advisory rather than mandatory, and the 2022 edition is explicit that the numerical criteria are a starting point to be applied with judgement in context. And the test was written with private gardens and small amenity areas in mind, so transferring it to a large public park needs the assessor to say how the space has been treated and why.
An Overshadowing Study that reports a single whole-park percentage is doing the arithmetic and avoiding the question. A park where the formal gardens lose a quarter of their sunlit area while the playing fields lose almost nothing may still show a comfortable headline figure.
Defining the assessed area in a large park
Divide the park into zones that reflect how people use it, and agree the zones with the case officer before modelling anything. The usual divisions are:
- Children's play areas and any ball court
- Formal seating, lawns and picnic areas
- Sports pitches, greens and courts
- Paths and circulation routes
- Planted and wooded areas, and any horticultural space
Exclude from the amenity assessment the parts that are not used for sitting out: dense woodland, service yards, car parking. Say what you excluded and why, because an objector reading the report will check whether the exclusions flatter the result.
The baseline matters as much as the proposal. A park already overshadowed on its eastern edge by a Victorian terrace, with mature plane trees along the southern boundary, has an existing sunlit percentage that is nothing like the open-field theoretical maximum. Model the existing buildings and the existing tree canopy, state the canopy assumptions, and report the existing condition first.
Where a park includes allotments or a community growing space, the sunlight requirement is functional rather than amenity-based, and the approach set out for overshadowing of allotments and growing space applies to those plots instead.
Seasonal use and the assessment date
The equinox is the assessment date in the guidance, and it is the date your result will be judged against. It is not the date the park is busiest.
Report three sets of results and the argument becomes much easier to make.
| Date | What it tells the reader |
|---|---|
| 21 March | The BRE criterion and the comparison against the two-hour, half-area test |
| 21 June | Summer condition, when the park carries most of its use |
| 21 December | Winter condition, when the shadow is longest and the park is least used |
A transient shadow study, showing the moving shadow at hourly intervals across each date, does more to persuade a committee than a percentage table. Members can see that the shadow crosses the children's play area between 9am and 11am in March and has left it by midday, which is a different proposition from a shadow that sits on the play area all afternoon in June.
Be straight about the December result. A tall building will cast a long winter shadow, and the honest answer is usually that the affected part of the park has limited winter amenity use in any event. Pretending otherwise invites a challenge that discredits the rest of the report.
Wind and sunlight together
Sunlight and wind pull in opposite directions on a tall building scheme, and assessing them separately produces designs that fail on both.
A set-back and a slender tower reduce the shadow footprint. A slender tower with a hard podium edge can accelerate wind at ground level, and downdraught from the facade can make a sunlit seating area unusable. The Lawson comfort criteria categorise ground-level conditions by activity, and a space intended for sitting has a more demanding threshold than a thoroughfare.
Where the scheme is tall enough to need a Wind Microclimate Assessment, run it on the same massing options as the sunlight study and report the two together. In London, Policy D9 of the London Plan requires tall building proposals to address microclimate, including wind, daylight, sunlight, overshadowing and glare, and a committee report will usually reproduce that list.
The practical consequence is that mitigation has to be assessed against both tests. A line of canopy trees along the podium edge may fix a wind problem and worsen a sunlight one on the same seating area.
Mitigation through massing and set-backs
Four moves do most of the work, in rough order of effectiveness.
- Orientation and position. Moving the mass to the northern side of the site, or shifting the tower along the frontage, can remove the shadow from the most-used part of the park entirely.
- Slenderness. Reducing the plan depth of the upper floors shortens the time any point stays in shadow, even where the shadow still passes over it.
- Set-backs at the park-facing elevation. These help the immediate edge of the park and the footway, and they do less for the middle distance than applicants hope.
- Rearranging the park. Where the applicant is also delivering the public open space, relocating the play area or the seating to a part of the park that stays sunlit is sometimes the cleanest answer, and it needs the landowner's agreement early.
Test the options as a sequence, record the numbers for each, and put the comparison in the report. A design evolution table showing four massing options and the sunlit percentage for each is the most persuasive page in an overshadowing report, because it shows the shadow was designed out rather than merely measured.
Illustrative example: a 22-storey tower at the south-west corner of a park, with the park's children's play area 60 metres to the north-east. The first scheme reduced the sunlit area of the play zone on 21 March from 71 per cent to 48 per cent, below the half-area criterion. Rotating the tower and reducing the upper floor plate restored it to 56 per cent, with the remaining loss falling on a planted bank. The figures are illustrative; the method of reporting each zone separately is the transferable part.
Consultation responses from park users
Representations about parks are different in character from neighbour objections. They come from users rather than owners, they often come through a friends group or a parish council, and they are usually about the experience of the space rather than about a measurable loss.
Three responses tend to be effective. Publish the transient shadow diagrams in the consultation material rather than only the percentages. Identify which zone each result relates to in plain words, naming the play area or the bowling green. And answer the summer question directly, because that is what people are actually asking when they say the park will be in shadow.
Where the local plan carries its own amenity standard, read it before relying on the BRE figures, because the way local plan amenity policies use BRE 209 varies and some authorities set a stricter test for designated open space.
Presenting the results at committee
A committee has twenty minutes and no appetite for a 90-page appendix. The Daylight and Sunlight Assessment and the overshadowing work should produce a two-page summary with one plan, one table and three shadow diagrams.
Put the following on that summary: the zones assessed, the existing and proposed sunlit percentage for each on 21 March, the June position for the most-used zone, what mitigation was tested and rejected, and one sentence on residual effect. State the residual effect honestly. A report that concedes a measurable loss on one zone and explains why it is acceptable in the planning balance survives scrutiny better than one that claims no effect at all.
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Frequently asked questions
What is the sun on ground test?
It is the BRE criterion that at least half the area of an amenity space should receive at least two hours of direct sunlight on 21 March. The assessment reports the sunlit proportion of ground area for the existing and the proposed condition. The guidance is advisory, and the 2022 edition asks for the numerical criteria to be applied with judgement.
Does the BRE criterion apply to public parks?
The test was written with gardens and small amenity areas in mind, and it is routinely applied to public open space. Because a park contains different functional spaces, the assessment should divide it into zones and report each separately, stating which areas were excluded from the amenity assessment and why.
Which dates should a shadow study cover?
The equinox on 21 March for the BRE criterion, 21 June for the summer condition when the park carries most of its use, and 21 December to show the longest shadow. Transient diagrams at hourly intervals on each date explain the effect far better than a percentage table on its own.
Do I need a wind assessment as well?
For a tall building near open space, usually yes, and the two studies should run on the same massing options. Slender forms that reduce shadow can accelerate ground-level wind, and the Lawson comfort criteria set a more demanding threshold for sitting areas than for thoroughfares. In London, Policy D9 of the London Plan requires both to be addressed.
How should the existing condition be modelled?
With the existing buildings and the existing tree canopy in place, and with the canopy assumptions stated. A park already shaded by a terrace and mature trees has an existing sunlit percentage well below the theoretical open-field maximum, and reporting the proposal against that baseline is what makes the comparison meaningful.
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.