Reflectance assumptions in a courtyard scheme
Inter-reflected light carries the lowest floors of a courtyard. Which reflectance values to model, how to evidence them, and the design moves that shift the result more than finishes do.
Reflectance assumptions decide whether a courtyard scheme passes its own daylight test, and they are the first thing a reviewing consultant checks. Raising an internal wall reflectance from 0.5 to 0.7 in the model, or giving the courtyard facades a 0.6 external reflectance because the brick is buff rather than red, can move a ground-floor flat from failing to complying without a single change to the geometry. That is legitimate when the values match the specification, and indefensible when they do not.
Why courtyards fail daylight tests
A courtyard flat at low level sees very little sky. The obstruction angle from the window is steep, the vertical sky component is small, and under BS EN 17037 and the 2022 edition of BRE 209 the daylight provision target is assessed by climate-based modelling against a target illuminance rather than a single geometric figure.
The UK National Annex to BS EN 17037 sets minimum target illuminances by room type: 100 lux for a bedroom, 150 lux for a living room and 200 lux for a kitchen, to be met over at least half the reference plane for at least half the daylight hours. A ground-floor living room in a tight courtyard often misses that on direct skylight alone.
What carries it over the line is inter-reflected light: sunlight and skylight bouncing off the opposite facade, the courtyard floor and the window reveals. In a deep courtyard the inter-reflected component can be the larger part of what reaches the lowest floors. So the reflectance of every surface in that courtyard becomes a design variable rather than a finish decision.
Surface reflectance and its real effect
Two sets of reflectance values matter, and they behave differently.
| Surface | Conventional modelling value |
|---|---|
| Internal ceiling | 0.70 |
| Internal walls | 0.50 |
| Internal floor | 0.20 |
| External obstruction, typical | 0.20 |
| External obstruction, light rendered or pale brick | 0.40 to 0.60 |
Internal reflectance affects how light is distributed once it is in the room. It appeared explicitly in the old average daylight factor formula through the interior reflectance term, and it still matters in a climate-based model because the illuminance at the back of a room depends on bounce.
External reflectance in a courtyard affects how much light arrives at all. Here the gains are real and they are worth designing for. Moving the opposite facade from a dark engineering brick at 0.2 to a pale render at 0.6 can lift ground-floor illuminance noticeably, because that facade is most of what the window sees.
There is a limit worth stating plainly. Reflectance cannot rescue a courtyard that is simply too narrow. Where the lowest floors see almost no sky, doubling the bounce doubles a small number.
Width to height ratios that work
BRE 209 does not set a courtyard ratio, and anyone quoting one as a standard is inventing it. What the guidance does give is the obstruction angle test: where the angle to the top of an obstruction, measured from the centre of the lowest window, is no more than 25 degrees above the horizontal, skylight is likely to be adequate without further calculation.
Applied to a courtyard, that is a demanding geometry. A 25 degree angle from a ground-floor window to an eaves line five storeys up needs a courtyard perhaps three or four times as wide as it is tall, which no urban scheme delivers. In practice courtyards are modelled, and the useful working relationships are these.
Where the courtyard width roughly equals the enclosing height, upper floors comply comfortably and the lowest one or two floors need attention. Where width falls below about half the height, the lowest floors are hard to make work for living rooms and kitchens at all, and the honest answer is to put circulation, plant, cycle stores or single-aspect bedrooms there rather than to argue the model.
Corners are worse than the middle. A flat in the internal corner of a courtyard is obstructed on two sides, and the corner units are where a scheme's failures cluster. Check them first.
Balcony and reveal obstruction inside a courtyard
A balcony above a window is an obstruction. It removes the part of the sky directly overhead, which is the brightest part under an overcast sky, and it does so for the window immediately below, every time.
BRE 209 addresses this directly and expects the assessment to consider the effect of balconies. Report the results with the balconies modelled as designed, and where the loss is what pushes a unit into non-compliance, say so and test the alternatives: inset balconies over non-habitable rooms, open-tread or perforated soffits, a narrower projection, or a Juliet in place of a full balcony on the lowest levels.
Deep reveals do the same thing at a smaller scale. A window set back 300 mm into a thick wall has a reduced view of the courtyard sky, and in a courtyard where every degree counts that is measurable.
The same care over which windows serve which room applies here as in a neighbour assessment, and our note on when a room is served by two windows covers how that is handled.
Splayed reveals and light shelves
Splaying the reveal opens the view of the sky without enlarging the glass. On the lowest courtyard floors a splay of 100 to 150 mm on each side, with a pale finish, gives a real gain and costs almost nothing in a masonry opening. It is the most underused move available to an architect on this problem.
Light shelves work better on a bright facade than in a deep courtyard, because they rely on light arriving from above the shelf. In a courtyard the arriving light is mostly diffuse and comes from a narrow patch of sky, so the shelf has less to work with. A pale window head and soffit, by contrast, works with diffuse light and is easier to detail.
Courtyard floor treatment is worth a paragraph in the assessment. A pale paving or light-coloured gravel at the base of a courtyard raises the reflected component to the lowest floors, whereas dark paving or dense planting reduces it. If the daylight case relies on the paving, the paving belongs in the condition schedule.
Justifying the reflectance values used
This is where assessments lose arguments. A reviewing consultant will ask for three things, and having them ready shortens the process.
- A source for every value. Internal reflectances from the finishes schedule or a conventional set, external reflectances from the material schedule or a manufacturer's figure for the specified brick, render or cladding.
- A maintenance allowance. Reflectance falls with soiling, and a courtyard facade in an urban setting will not stay at its as-built figure. State the allowance made and why.
- A sensitivity test. Re-run the model with external reflectance at 0.2 and report the difference. If compliance survives, you have answered the objection before it is raised. If it does not, you have found a design problem while you can still fix it.
Record all of this in the Daylight and Sunlight Assessment rather than in a modelling file nobody reads. Where the courtyard doubles as amenity space, sun on ground results belong alongside, and an Overshadowing Study is the usual vehicle.
Design changes that move the numbers
The interventions that actually shift courtyard daylight, in rough order of effect, are geometric before they are material. Widen the courtyard or step the upper floors back. Chamfer or open one corner to admit light and a view of sky. Raise floor-to-ceiling heights and window head heights on the lowest levels, since head height governs how far light reaches into a room. Reduce room depth for courtyard-facing units.
Then the finishes. Pale courtyard facades, pale reveals and soffits, pale paving, and glass with a higher light transmittance where solar gain allows it.
And one honest uncertainty. Climate-based modelling results are sensitive to the weather file, the reflectance set and the room geometry together, so two competent consultants can reach modestly different numbers for the same building. That is an argument for transparency about inputs rather than for precision claims the method cannot support. The approach to bedroom daylight in non-standard room types, set out in our piece on daylight to hotel and care home bedrooms, raises the same question.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. Daylight modelling is specialist work, so councils expect a suitably qualified author and professional review is strongly advised.
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Frequently asked questions
What reflectance values should a courtyard daylight model use?
Internal surfaces conventionally take 0.70 for ceilings, 0.50 for walls and 0.20 for floors. External obstructions are commonly modelled at 0.20, rising to 0.40 or 0.60 for pale render or light brick where the material schedule supports it. Any value above the conventional figure needs a source, a maintenance allowance and a sensitivity test at 0.20.
Is there a minimum courtyard width to height ratio?
No published standard sets one. BRE 209 gives an obstruction angle test instead: where the angle to the top of the obstruction from the centre of the lowest window is 25 degrees or less, skylight is likely to be adequate. Urban courtyards rarely achieve that, so they are modelled, and internal corner units are where failures cluster.
How do balconies affect courtyard daylight results?
A balcony above a window blocks the overhead sky, which is the brightest part of an overcast sky, and it does so for the window immediately below. BRE 209 expects the assessment to consider this. Options include insetting balconies over non-habitable rooms, perforated or open-tread soffits, shallower projections, or Juliet balconies on the lowest floors.
What daylight targets apply to new flats?
The UK National Annex to BS EN 17037 sets minimum target illuminances of 100 lux for bedrooms, 150 lux for living rooms and 200 lux for kitchens, achieved over at least half the reference plane for at least half the daylight hours. BRE 209, third edition 2022, aligns with that approach and uses climate-based modelling rather than a single geometric figure.
Do pale finishes need securing by condition?
If the daylight case depends on them, yes. Where compliance rests on a pale courtyard facade, light-coloured reveals or reflective paving, those materials should appear in the approved materials condition rather than only in a modelling input file. Otherwise a value-engineered darker brick can quietly remove the basis on which permission was granted.
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.