Winter sunlight and the 21 December benchmark
Annual sunlight hours hide the winter problem. Here is how the solstice benchmark works, what it does to massing, and the modelling errors that recur.
A winter sunlight assessment matters because British winter sun is low, long-shadowed and unforgiving of massing decisions that look fine in June. At the latitude of London the sun reaches only about 15 degrees above the horizon at solar noon on 21 December. Further north it is lower still, closer to 12 degrees at the latitude of Newcastle. A six storey block that casts a 20 metre shadow at the equinox casts a much longer one at the solstice, and that shadow falls across the amenity space and the ground floor windows you were relying on.
The BRE guidance splits the question in two, and the split is worth holding onto. Sunlight to windows is tested over the whole year with a separate winter component. Sunlight to gardens and amenity areas is tested at the equinox. Shadow studies are drawn at three benchmark dates, of which 21 December is one.
Why winter is assessed separately
The annual probable sunlight hours method used in the BRE guidance measures the proportion of available sunlight hours a window receives across the year, and it carries a separate minimum for the winter months from 21 September to 21 March. A window can meet the annual figure comfortably and still fail the winter figure, because summer sun arrives from a high angle and from the north-east and north-west, while winter sun arrives low and from a narrow arc to the south.
That is the practical point for massing. An obstruction to the south does disproportionate damage in winter. An obstruction to the east or west does relatively little in winter and more in summer.
Reporting both figures is not optional. A Daylight and Sunlight Assessment that quotes annual sunlight hours alone has left out the number that decides whether a south-facing living room stays usable in January.
Sun angles at the solstice and what they mean for massing
Solar geometry at the solstice is fixed and calculable, so you can reason about it before any modelling.
At noon on 21 December the solar altitude is roughly the co-latitude minus the axial tilt, which gives about 15 degrees in London and about 12 degrees in the north of England. A low angle means long shadows: at 15 degrees, an obstruction casts a shadow roughly 3.7 times its height. A 12 metre building therefore throws a shadow of around 45 metres at midday in midwinter, and considerably longer in the hours either side.
Three design consequences follow.
- Courtyards enclosed on the southern side receive little or no direct winter sun, however generous they look on plan.
- Setting back the upper storeys on a southern elevation buys more winter sun than reducing the overall height by the same volume elsewhere.
- Amenity space placed to the north of a building is a winter loss even where the equinox test is met.
Work these out on a section before the massing is committed. A single hand-drawn section at the solstice angle is worth more than a week of modelling after the layout is fixed.
Producing shadow studies at the three benchmark dates
Shadow studies are conventionally drawn at 21 March, 21 June and 21 December, usually at hourly intervals across the day. The equinox is the reference condition for amenity space testing. The solstices bracket the range.
Four practical points make the difference between a study that answers the question and one that generates more questions.
- State whether the times are solar time or clock time, and be consistent. Mixing them shifts every shadow.
- Model the surrounding context accurately, including buildings outside the site that cast onto the receiving spaces.
- Include existing site buildings in the baseline, so the comparison is against what is there rather than against an empty site.
- Show the existing and proposed conditions side by side at matching times, at the same scale and orientation.
Where the assessment covers a garden or a courtyard, add a plan showing the proportion of the area receiving at least two hours of sun on 21 March, which is the test councils apply for amenity space, with the comparable winter picture shown for information rather than as a pass or fail.
A standalone Overshadowing Study is often the right document for a scheme where the issue is amenity space rather than windows, and choosing which date to lead with is part of the judgement covered in this note on assessment dates for sunlight studies.
Where winter results drive the design
Winter results change schemes in a small number of recognisable situations.
| Situation | What winter analysis usually shows |
|---|---|
| Perimeter block with an enclosed courtyard | Courtyard in permanent shadow from November to February unless the southern side is lowered or opened |
| Tall element on the southern boundary of a site | Long shadows across neighbouring gardens for most of the winter day |
| North-facing ground floor flats | Little or no direct sun at any time of year, which makes the daylight case more important |
| Dense terraced streets | Rear extensions that pass the equinox test can still remove all winter sun from a neighbour's rear window |
| Rooftop amenity space | Frequently the only space with winter sun, which makes its shading design worth attention |
The honest position is that a scheme in an urban context will often fail some winter targets. That is not automatically a reason for refusal. What matters is that the shortfall is identified, quantified, explained and mitigated where it can be, rather than omitted.
Effects on solar gain and the overheating strategy
Winter sun is the useful kind. It reduces heating demand, and losing it to an obstruction pushes energy consumption up in the season when it matters most.
That interacts with the overheating assessment, because the glazing and shading strategy designed to control summer gains also reduces winter gains. Fixed horizontal shading works well for high summer sun and barely affects low winter sun, which is why it is usually preferred to deep reveals on southern elevations. Vertical fins do the opposite.
Where the scheme is being tested under CIBSE TM59, the shading assumptions used in the Overheating Risk Assessment should be the same ones described in the sunlight report. Two documents making different assumptions about the same brise soleil is the sort of inconsistency a careful officer finds.
Presenting seasonal results without burying the answer
Lead with the conclusion. State which receptors meet the annual and winter targets, which do not, by how much, and what the design does about it. Then present the tables.
Common presentational failures include burying the failures in an appendix, using a colour scheme where pass and fail are hard to distinguish in print, presenting only the proposed condition without the existing baseline, and quoting percentages without the absolute hours behind them.
One image usually does more than any table: the existing and proposed shadow plots at the same time on the same date, side by side. Put it early.
Common errors in solstice modelling
A short list, all of which appear regularly.
Using the wrong latitude, or a default location from the software. Modelling shadows with no context buildings. Forgetting that trees in leaf are a summer condition and that deciduous trees admit winter sun, which cuts both ways depending on whether you are relying on them for shading or for screening. Failing to model balconies and overhangs, which remove a surprising amount of low-angle sun from the windows beneath them. And taking measurements to the centre of a window rather than following the guidance's reference point convention.
Check the model against a hand calculation at one point before trusting the output. A single section drawn at 15 degrees will catch a model set to the wrong hemisphere faster than any review.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and work from the current edition of the BRE guidance.
If you are assembling the amenity evidence for a scheme, enter your postcode on PlanScope to see which reports your site needs. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Why is winter sunlight assessed separately from annual sunlight?
Because the annual probable sunlight hours method carries a separate minimum for the winter months from 21 September to 21 March. A window can meet the annual figure while failing the winter one, since summer sun arrives from high angles and a wide arc, whereas winter sun comes low and from a narrow southern arc.
How high is the sun at the winter solstice in the UK?
At solar noon on 21 December the altitude is roughly the co-latitude minus the axial tilt, giving about 15 degrees at the latitude of London and around 12 degrees in the north of England. At 15 degrees an obstruction casts a shadow approximately 3.7 times its own height at midday.
Which dates should shadow studies be drawn at?
Conventionally 21 March, 21 June and 21 December, usually at hourly intervals. The equinox is the reference condition for testing sunlight to gardens and amenity areas, while the solstices bracket the range. Always show existing and proposed conditions side by side at matching times and the same scale.
Does failing a winter sunlight target mean refusal?
Not automatically. Urban schemes frequently fall short of some targets, and the BRE guidance is guidance rather than a standard. What matters to an officer is that any shortfall is identified, quantified, explained in context and mitigated where it can be, rather than omitted from the report.
How does winter sunlight relate to overheating assessment?
The same shading strategy affects both. Fixed horizontal shading controls high summer sun while barely reducing low winter sun, whereas vertical fins do the reverse. Make sure the shading assumptions in the sunlight report match those used in the CIBSE TM59 overheating assessment, because inconsistency between the two is easily spotted.
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.