Courtyard schemes and daylight to the lower floors

Courtyard daylight is decided at massing stage, and by arithmetic that does not care how good the rest of the scheme is. A perimeter block obstructs its own lower floors, and the controlling variable is the angle from a ground-floor window up to the eaves of the building opposite. The rule of thumb in the BRE guidance on site layout planning for daylight and sunlight is that an obstruction angle below about 25 degrees, measured from a point roughly two metres above ground level, suggests daylight is likely to be adequate. Convert that into plan dimensions and a courtyard needs to be somewhere near twice as wide as the surrounding buildings are tall before the ground floor is comfortable.

Most urban schemes cannot afford that. So the practical question becomes which units take the hit, and what goes in them.

The geometry that governs courtyard daylight

Three dimensions do almost all the work: the height of the opposing facade above the window being assessed, the horizontal distance to it, and the head height of the window itself.

The first two set the obstruction angle. The third sets how far daylight penetrates into the room, because sky visible high in the window reaches deeper than sky visible low in it. A room with a 2.1 metre window head is a different proposition from the same room with a 2.6 metre head, even though the obstruction has not changed.

Floor-to-floor height is therefore a daylight variable as much as a cost and planning-height variable. Where a scheme is squeezing storey heights to fit a height limit, the ground-floor units pay for it twice.

Width to height ratios that work

Here is the arithmetic, and it is illustrative rather than a standard.

Two caveats. The 25-degree test is a screening rule, and failing it does not mean the scheme fails. It means the detailed assessment against the illuminance targets in the daylight standard has to do the work. And the height that matters is measured above the window in question, so a scheme stepping down towards its own courtyard performs better than the storey count suggests.

Where the ratio cannot be met, the honest response is to change the unit mix rather than to change the assumptions.

Corner units and the double obstruction problem

A window near an internal corner sees sky through a narrower wedge than a window in the middle of the same elevation, because two facades obstruct it rather than one.

Corner plans make this worse when the designer does what is natural and puts a single window on each face of the corner room. Each window is close to a return wall, both look into the obstructed wedge, and the room's total sky access is lower than either window suggests in isolation.

Better moves exist. Chamfer or set back the corner to open the wedge. Place the corner unit's habitable rooms on the outer elevations and put the bathroom and circulation in the corner itself. Or accept a non-residential use in that position at ground and first floor.

The assessment has to reflect the real geometry on both sides of the boundary, which is the same consistency point that arises in a mirror-image massing test.

Ground and first floor unit types

This is where a scheme is won or lost, and it is a layout decision rather than a modelling one.

Dual-aspect units are the single most effective response. A unit with a window to the street and a window to the courtyard is not relying on the obstructed elevation for its daylight. Single-aspect units facing only into a deep courtyard at ground floor are the ones that generate the long list of failures in an appendix.

Maisonettes help, by putting the living space at first floor where the obstruction angle is lower and the bedroom below. So does raising the ground floor slab half a storey, which lifts every window head and gives the ground floor a defensible threshold at the same time.

And some ground-floor courtyard frontages should not be homes. Cycle stores, plant, a concierge, a workspace unit or a commercial tenancy in the worst positions is a better answer than five dwellings that need explaining at committee.

Balconies compounding the loss

Inset and recessed balconies on a courtyard elevation are the combination that produces the worst numbers, because the soffit above the window removes the highest and most useful part of the sky view before the courtyard geometry has even been considered.

The mitigations are known and each has a cost. Reduce the balcony depth. Make it a projecting balcony rather than an inset one, so the soffit is outside the window plane. Use a glazed or perforated balustrade. Keep balconies off the lowest two floors of the courtyard elevation and provide amenity space elsewhere for those units.

Model the balconies as built, with the soffit as the darker surface it is. An assessment that models an inset balcony as an open reveal is not modelling the scheme.

Light-coloured finishes and their real effect

Pale courtyard facades do help, and the effect is real in a tight court where much of the light arriving at a lower window has bounced off the wall opposite. It is also limited, dependent on the specification being secured, and reduced by weathering.

Treat it as the last percentage point rather than the fix. If a scheme only works with high external reflectance, the officer needs to know that, and a condition securing the finish becomes reasonable.

Reporting the worst units rather than an average

Block averages hide exactly what a planning officer is being asked to accept.

Report by unit reference and room. Give the number and proportion of rooms meeting the target illuminance, the number falling short, and by how much. List the worst ten rooms with their location, aspect and the reason they underperform. Where a room fails, say what it is: a single-aspect ground-floor living room under an inset balcony on the north side of a court eighteen metres wide.

That level of candour does better at committee than a summary table showing 87 per cent compliance, because the officer can see that the failures are concentrated and explain why the scheme is still acceptable. It also tells you which four units to redesign.

Sunlight to the courtyard itself is a separate test. The BRE guidance looks for a proportion of an amenity area receiving a period of sun on 21 March, and a deep court with buildings on all four sides frequently fails it. An Overshadowing Study sets that out, and the results usually point to opening a corner of the block rather than to anything subtler.

If you are still establishing whether the assessment is needed, start with when a daylight and sunlight assessment is required, and the Daylight and Sunlight Assessment page sets out what the document covers. Councils usually expect a suitably qualified author for this work, and professional review is strongly advised.

PlanScope can produce an AI-assisted draft of the supporting narrative and flag the inputs that must come from the modeller, with missing site data marked rather than invented. It does not run the daylight model. Reports should be reviewed before submission, and PlanScope does not guarantee planning approval. For specialist technical reports, qualified professional input may be required. Enter your postcode on PlanScope to see which reports a courtyard scheme is likely to need. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.