Transient overshadowing frames in an overshadowing study

Transient overshadowing frames answer a question the sun on ground test cannot: when during the day is a garden, a play area or a courtyard in shadow, and for how long at a stretch. The sun on ground analysis gives you a percentage of area meeting a benchmark on one date. The transient sequence gives you the shape of the day, which is what members and objectors actually argue about.

Both belong in an overshadowing study and they do different work. Confusing them, or submitting one and calling it the other, is the most common weakness in this part of a daylight and sunlight submission.

What transient analysis adds to the sun on ground test

The sun on ground test in the BRE guidance asks what proportion of an amenity area receives at least two hours of sunlight on 21 March, and compares the proposed position with the existing one. It is a single number against a benchmark, and it is the test an officer will look for first.

What it hides is distribution. An area where 55 per cent of the surface receives two hours can be an area that is fully sunlit from nine until eleven and then in shadow for the rest of the day, or one that is dappled all afternoon. Those are different places to sit in.

Transient frames show the shadow outline at set times across a day, usually as a series of plans. They are descriptive rather than pass or fail, and their value is in three situations: where the sun on ground result is marginal, where the amenity area has a specific intended use at a specific time, and where the shadow moves across a boundary so that the loss falls on a neighbour rather than within the site.

Choosing dates and times for the frames

Start with 21 March, because that is the date the quantified benchmark uses and because the equinox gives a sun path that is neither the best nor the worst case. Then add context dates.

Hourly frames are the normal interval. On 21 March, 09:00 through to 16:00 gives eight frames, which is enough to read the movement without producing a document nobody looks at. Half-hourly frames are worth it only where the shadow edge crosses a specific receptor during a single hour.

State the time basis. Shadow geometry depends on solar time rather than clock time, and a study that labels frames in British Summer Time without saying so will give a reader an hour of confusion. Give the latitude and longitude used, the time convention and the software, so the work can be reproduced.

Three habits improve the output. Use the same scale and orientation for every frame. Keep the existing buildings in a consistent tone so the eye tracks the new shadow rather than the drawing. And put the sun azimuth and altitude on each frame rather than only the time.

The two hour benchmark and where it comes from

The two hour figure on 21 March, applied to half the area of an amenity space, comes from the BRE guidance on site layout planning for daylight and sunlight rather than from national planning policy or from any statute. That matters in two ways.

It is advisory. The guidance itself says its figures are not mandatory and should be applied flexibly, with regard to context. A dense urban scheme in a town centre cannot be judged against the same expectation as a suburban garden, and the 2022 edition is more explicit about that than earlier versions were.

And it comes with a relative test alongside the absolute one. Where an existing area already fails the benchmark, the guidance offers a comparison against the existing position, with a reduction to less than 0.8 times the former sunlit area treated as the point at which the loss becomes noticeable. On infill sites in established areas, that relative test usually does more work than the absolute one.

Say which test you are applying and why. An assessment that quietly switches to the relative test without explaining that the baseline already fails invites the suspicion that it is doing so to produce a better answer.

Receptors worth modelling, and ones worth leaving out

The receptors that justify the effort are the ones where sitting out, planting or play actually happens: private rear gardens of neighbouring houses, communal amenity space serving flats, designated children's play space, allotments, and the usable part of a public square or park.

Receptors frequently included without value are front gardens used only for parking, service yards, roof areas with no access, and strips of landscaping under 2 m wide. Including them inflates the area denominator and makes the percentages less meaningful, which an officer will notice.

Two receptor types deserve separate treatment. Children's play space has its own quality expectations, and where a scheme relies on a courtyard to satisfy a play space requirement, the overshadowing result and the Play Space Assessment have to be consistent with each other. And where a scheme claims health or wellbeing benefits from outdoor space, those claims should be tested against the shadow result rather than asserted, which is the same discipline applied to the health determinants inside a health impact assessment.

Solar panels on neighbouring roofs are a receptor people forget. Transient frames are the right tool there, because the loss is tied to specific hours of generation.

Existing, proposed and cumulative frames

Three sets of frames answer three different objections.

Existing frames establish the baseline, and they are the set most often omitted or drawn carelessly. An officer cannot judge a loss without them, and a baseline that excludes an existing boundary wall or a mature tree will be challenged by the neighbour who lives behind it.

Proposed frames show the scheme as applied for. Model what will be built, including parapets, plant enclosures, lift overruns and balcony soffits, because those are the elements that get added after the massing study and they cast real shadows.

Cumulative frames add consented but unbuilt schemes nearby. Whether to include them is a judgement call, and the answer depends on whether those consents are likely to be implemented. Where a neighbouring consent would dominate the result, say so and present both, rather than choosing the version that suits the application.

Reading the output without burying the conclusion

The failure mode in these reports is 30 pages of shadow plans followed by a paragraph saying the effect is acceptable.

A better structure leads with the quantified result, then uses the frames to explain it. Three elements do the work: a table giving the sun on ground result for each receptor in the existing and proposed cases with the percentage change, a short written account of when each affected receptor is in shadow and when it is not, and the frames themselves as an appendix.

Write the conclusion so that someone can disagree with it specifically. "The rear garden of 12 Mill Lane retains sunlight across its outer two thirds until 14:00 on 21 March, with the loss concentrated on the paved area immediately behind the house" is a statement a planning committee can weigh. "The overshadowing effect would not be unacceptable" is not.

Where the result is poor, say so and explain what was tried. An assessment that records the massing options tested and why the chosen one performs best is more persuasive than one that presents a single scheme as self-evidently acceptable. An Overshadowing Study that reads as a design record rather than as advocacy survives scrutiny better.

Where the analysis changes a scheme

In practice, transient analysis changes designs in a small number of recognisable ways. Setting back an upper storey on the southern edge of a courtyard. Moving a lift core from the south side of a block to the north. Reducing the height of a boundary treatment. Relocating a play area from the shaded corner to the one that keeps afternoon sun. Each costs almost nothing at sketch stage and becomes impossible once the planning drawings are issued.

So run the frames during massing rather than after it. The output then becomes part of the design rationale in the Daylight and Sunlight Assessment rather than a defence of decisions already taken.

Councils expect a suitably qualified author for this work, and professional review is strongly advised before submission.

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