Shadow path diagrams and which dates to model

Model 21 March first. The spring equinox is the reference date in the BRE guidance on site layout planning for daylight and sunlight, it is the date the two hour sun on ground criterion for gardens and amenity areas is tested against, and a shadow study that offers only June and December has answered a question nobody asked. Add the solstices afterwards, as context, clearly labelled as such.

Shadow path diagrams do something the numerical tests cannot. Annual probable sunlight hours and the sun on ground percentage both compress a year of sun geometry into a single figure, which is fine for a pass or fail judgement and useless for explaining what a neighbour will experience at four o'clock on an April afternoon. A set of hourly plots shows where the shadow sits, how fast it moves, and which part of the garden it never reaches. Members understand them immediately, which is both their value and their risk.

Why the equinox

At the equinox the sun's path is the same in spring and autumn, the altitude at noon is mid range for the latitude, and the result sits roughly halfway between the extremes. That makes it a fair single date for comparison, which is why the BRE guidance builds the amenity criterion around it: at least half of an amenity area should receive at least two hours of sunlight on 21 March, and a reduction below that which also leaves less than 0.8 times the former sunlit area is where a loss becomes material.

Two consequences follow for the drawing set. The 21 March plots are the evidence, so they need to be complete from sunrise to sunset at hourly intervals, or half hourly where the garden is small enough for a thirty minute step to change the answer. And the same date has to be modelled for the existing and the proposed case, on the same sheet, at the same scale, or the comparison cannot be made.

Adding 21 June and 21 December

The solstices are context rather than test dates, and they are worth including for different reasons.

21 June shows the best case. It answers the member who says the objector's garden will be in permanent darkness, and on a scheme to the north of a garden it often shows almost no effect at all. 21 December shows the worst case, with the sun low and shadows long, and on any site it looks alarming. Include it anyway. A study that omits the winter condition invites the suggestion that something is being hidden, and an objector's consultant will produce it.

Label all three date sets with the criterion that applies, or with a note that none applies. The common misunderstanding at committee is that the December plots are being assessed against the two hour criterion, which they are not.

Solar time, true north, and the two errors that follow

Shadow diagrams are plotted in apparent solar time, not clock time. On 21 March in Britain, British Summer Time has already started, so a plot labelled "10:00" in clock time is showing roughly the 09:00 solar position. Every shadow in the set is in the wrong place by about an hour.

State on the sheet which time base you have used. "Hourly intervals, apparent solar time" removes the ambiguity. If a council's own guidance asks for clock time, give both in the labels rather than converting silently.

The second error is north. Shadow geometry is driven by orientation, and a plan rotated to grid north or taken from a magnetic compass reading will be out by enough to matter on a tight garden. Take true north from the topographical survey, state the source on the drawing, and check it against a satellite image before you run the model. The wider point about model accuracy, survey input and massing is covered in the piece on daylight model accuracy, survey and massing.

What goes in the model

The model has to contain everything that casts a shadow and is likely to stay there.

• Existing buildings within the relevant radius, with heights from survey rather than from estimated storey counts. Parapets, chimneys, roof plant and lift overruns all cast shadow.
• Boundary walls and fences at their actual height. A 1.8 metre close boarded fence on the south boundary of a small garden does more to the morning and evening condition than the proposed extension does.
• The ground surface, modelled with its slope. Shadows plotted on a flat plane across a sloping garden land in the wrong place, and the error grows with the gradient.
• The proposed scheme at its full extent, including balustrades, planters and any structure permitted by the application.

Trees are the judgement call. The usual convention follows the BRE guidance in treating trees as impermanent, since they can be felled, they are seasonally variable and a deciduous canopy is largely transparent in March. So the assessed baseline normally excludes them. Where a tree is protected by a Tree Preservation Order, or is a mature evergreen that plainly will remain, show a second set of plots with it included and say why. Excluding a protected yew from a study and leaving the officer to spot it is a short route to a request for revisions.

Annotating the amenity area under test

The plots are only half the output. The assessment needs the area under test defined, and the definition is where most studies are weak.

1. Draw the amenity area boundary on the plan and dimension it. Exclude the building footprint, hardstanding used for parking, bin and cycle stores, and any strip too narrow to use.
2. State the total area in square metres, and the area receiving two hours or more of sunlight in the existing and the proposed case, with the percentage for each.
3. Hatch the two hour area on a single summary plan rather than leaving the reader to overlay the hourly plots.
4. Give the ratio of proposed to existing sunlit area, because the 0.8 test is the second limb of the criterion and it is routinely left out.
5. Where several gardens are affected, repeat the table per garden and number them to match the location plan.

That table is what an officer copies into the report. If it does not exist, the officer writes their own sentence instead, and it will not be the sentence you wanted.

Errors that make a study unusable

The ones that recur, roughly in order of how often they appear:

• Proposed case only, with no existing baseline, so no comparison is possible.
• Clock time labels with no statement of the time base.
• Grid or magnetic north instead of true north.
• Existing obstructions missing, particularly fences, outbuildings and the neighbour's own extension.
• Shadows cast onto a flat plane where the ground slopes.
• No scale bar and no north point on the plots themselves, only on the location plan.
• Rendered perspective images of shadows instead of plan diagrams. They look better and prove nothing, because a reader cannot measure an area from them.
• A study of the applicant's garden rather than the neighbour's, which happens more often than you would think.
• Intervals coarse enough to miss the effect, typically two hourly steps on a garden where the shadow transits in ninety minutes.

Two other points worth noting. The sun on ground criterion is about amenity areas, and the separate annual probable sunlight hours tests deal with sunlight to windows. A full Daylight and Sunlight Assessment covers both, and an Overshadowing Study on its own only answers the amenity area question. Councils expect a suitably qualified author for this work, and professional review is strongly advised where the outcome is contested.

The BRE guidance is itself guidance rather than policy. The numerical criteria are advisory, the guide says so, and inspectors apply them with regard to context, which matters a great deal in dense urban areas where existing conditions already fall short. How that plays out on appeal is set out in how inspectors weigh daylight breaches at appeal.

Produce the 21 March set properly, label the time base, define the amenity area, and the rest of the argument is about planning judgement rather than about your drawings.

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