Choosing assessment dates for sunlight studies

Sunlight assessment dates are not a matter of preference. The default is 21 March, because BRE 209 sets its sunlight and overshadowing tests at the spring equinox, and an assessment plotted on a different date without explanation invites the objection that the modeller picked a flattering day. Where a council's own guidance names additional dates, those are added to the equinox rather than substituted for it.

The date question sounds procedural. It decides outcomes, because shadow length at this latitude roughly triples between midsummer and midwinter, and a scheme that looks harmless in June can bury a garden in December.

Why the equinox is the standard date

On 21 March the sun follows a path close to the annual average, and the same path repeats around 21 September. Modelling one date therefore describes two, and it sits between the extremes rather than at either of them.

The two BRE tests that use it are worth stating precisely. For amenity space, at least half the area should receive at least two hours of sunlight on 21 March. Where an existing space already meets that and a proposal reduces the sunlit area to less than 0.8 times its former value, the loss is treated as noticeable. The same 0.8 relative threshold runs through the window-based sunlight tests, which work on annual probable sunlight hours rather than on a single day.

That combination of an absolute test and a relative test is why the equinox matters so much. Both halves of the amenity test are anchored to it, so a study run on any other date cannot answer the question the guidance asks. The window-based side of the analysis, including the winter probable sunlight hours split, belongs with the wider Daylight and Sunlight Assessment rather than with shadow plots.

What the summer and winter dates add

Neither 21 June nor 21 December forms part of the BRE numerical tests. Both are still worth plotting, for different reasons.

June shows the best case, and it is the honest answer to an applicant who says the courtyard will be usable in summer. It also shows where a tall element casts a shadow across a neighbouring garden in the early evening, which is when people are actually outside.

December shows the worst case. Low sun angles produce very long shadows, and a December plot is what an objector will produce at committee if you do not. Including it and explaining it is a stronger position than omitting it and being asked why.

A useful discipline: if you plot June and December, say plainly that they are context rather than compliance, and keep the compliance conclusion anchored to the equinox.

Dates and intervals councils specify

Local guidance varies and it overrides your default. Some authorities require plots at additional dates, some specify intervals, and some ask for sun-on-ground diagrams in addition to shadow outlines.

Check the supplementary planning document before you start. Redoing a set of plots because the council wanted half-hourly intervals costs a fee round and a committee cycle, and the requirement is usually written down somewhere.

Time intervals and how many to plot

Hourly is the working standard. Half-hourly is worth using where a shadow crosses a small garden quickly, because hourly steps can miss the moment of worst effect entirely.

Run the plots between sunrise and sunset for the date, not between arbitrary office hours. A set that starts at 09:00 and stops at 16:00 on 21 June cuts off several hours of real sunlight and will be read as selective.

Show the existing situation and the proposed situation at identical time steps, on identical base drawings, at the same scale. Reviewers compare frames, and any difference in framing between the two sets slows that comparison and raises suspicion. Where the site sits among tall neighbours and existing overshadowing is already severe, say so in the baseline rather than letting the proposed plots carry the whole story, which is the approach set out for daylight assessment in dense historic townscapes.

Clock time, solar time and true north

Three errors recur, and all three are avoidable.

The first is British Summer Time. Sun position calculations work in local solar time, and clocks in the United Kingdom run one hour ahead from late March to late October. A 21 June plot labelled 13:00 without saying whether that is clock time or solar time is ambiguous, and the difference is a full hour of sun movement. Label every frame with the convention used.

The second is north. Building models are often drawn to grid north taken from the ordnance survey base, while the sun path needs true north. The difference varies across the country, and on a tight site it is enough to move a shadow edge off a boundary or onto it.

The third is site coordinates. Solar geometry depends on latitude, and a model left at a software default rather than set to the site's own position will be wrong by a noticeable margin in the north or south of England. State the latitude and longitude used.

Presenting the seasonal comparison

Put the compliance answer first. The percentage of each amenity area receiving two or more hours of sun on 21 March, existing and proposed, with the ratio between them, in a table. Then the plots.

Keep one page per date, with existing above proposed, and a short caption stating the date, the interval, the time convention and the north reference. Anything an objector might reasonably ask should be answerable from the caption.

Where the proposal falls short, say so and quantify it rather than burying it. A study that concedes a 0.72 ratio on one garden and explains why the harm is limited reads far better than one that reports compliance and is then contradicted at committee. That honesty matters most in an Overshadowing Study submitted alongside a contested application, where the officer is looking for a reason to trust the numbers.

Neighbouring solar installations deserve a separate paragraph if any are present, since panel output is affected by shading in a way that garden amenity is not, and that argument is handled differently, as set out in the note on overshadowing of a neighbour's solar panels.

Agreeing dates before you model

Ask the case officer, in writing, before the modelling starts. A single email confirming the dates, intervals, time convention and which amenity areas are to be assessed costs nothing and removes the most common reason for a resubmission.

Where pre-application advice is being taken anyway, fold the question into it. Where it is not, a short scoping note sent at validation stage does the same job. Record the reply and reproduce it in the methodology section of the report, so the person reviewing it in six weeks can see the scope was agreed rather than chosen.

Daylight and sunlight work is a specialist field, and councils generally expect a suitably qualified author, with professional review strongly advised before submission.

To see which assessments a site is likely to need before you scope the modelling, enter your postcode on PlanScope and work through the guided questions.

This is general guidance, not legal advice, so check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.