Courtyard geometry and daylight in perimeter blocks

Courtyard daylight in a perimeter block is decided by geometry long before anyone chooses a glazing specification. If the courtyard is narrower than the buildings around it are tall, the lower floors facing into it will struggle, and no amount of pale render will rescue them. Test the courtyard daylight perimeter block relationship in the massing study, while moving a facade still costs a drawing rather than a redesign.

The assessment framework is the BRE guidance on site layout planning for daylight and sunlight, read with BS EN 17037 and its UK National Annex. Both are design tools. Neither is a policy threshold, though local plans and the London Plan treat them as the reference point.

Width to height ratios that work

Start with the obstruction angle. Measured from the centre of the lowest window on one side of a courtyard to the parapet opposite, an angle of 25 degrees or less is the point at which the BRE guidance suggests daylight is likely to be adequate without further calculation. That is a screening tool rather than a standard, and it is useful precisely because it can be read off a section in seconds.

Run the arithmetic on a square courtyard. With a six storey block at roughly 18 m to parapet and a ground floor window at 1.5 m above the courtyard, a 25 degree obstruction angle needs a courtyard roughly 35 m wide. At a 1:1 width to height ratio, the angle from that window is close to 45 degrees, so ground and first floor units facing the courtyard will need modelling and some will fail. This worked example is illustrative, and the real number depends on window head heights, balconies and the level of the courtyard deck.

The consequence is a design rule most teams arrive at eventually. Courtyards get wider as blocks get taller, and where the site will not allow that, the lower floors have to be planned for something other than habitable rooms.

Re-entrant corners and the units that fail

Re-entrant corners are the reliable failure point. A window set into the inside corner of an L or a U sees sky through a narrow wedge, and a single aspect one bedroom flat in that position will produce the worst numbers in the whole scheme.

Three moves help, in rough order of effectiveness:

1. Chamfer or splay the corner so the window sees a wider arc of sky.
2. Pull the two wings apart by widening the re-entrant, even by a metre or two.
3. Move the plan around so the corner carries circulation, storage or a dual aspect living space rather than a bedroom.

Balconies make all of this worse. A recessed balcony above a window can remove more daylight than the opposite building does, and inset balconies in a re-entrant corner combine both effects. Where the design needs balconies in those positions, model them in the base case rather than adding them after the daylight results look acceptable.

Reflectance of courtyard walls and floor

The standard assumption for external obstructions in the BRE method is a reflectance of 20 per cent, which represents ordinary brick and glazing. Courtyards are one of the few situations where arguing for a higher figure is reasonable, because the designer controls every surface the light bounces off.

Pale render, light brick, a light coloured courtyard deck and generous glazing on the opposite facade all raise the effective reflectance. If the model uses a higher figure, say so explicitly, state the value, and secure the finishes by condition. An assessment that quietly assumes 40 per cent reflectance without a materials schedule attached invites a reviewer to rerun it at 20 per cent.

Soft landscape cuts the other way. Trees in a courtyard are good for amenity and for the sunlight reaching the amenity space calculation they do nothing for, so model mature canopy where it sits in front of windows.

Setbacks, chamfers and stepped massing

Where the courtyard cannot be widened, step the upper floors back. Setting back the top two storeys on the courtyard side lifts the obstruction angle for every window below and usually costs less area than it appears, because the setback floors gain terraces.

Other geometry that earns its place:

• Opening one corner of the block entirely, which lets low sun into the courtyard and gives the deepest units a longer view of sky.
• Raising the courtyard deck so ground floor windows sit higher relative to the opposite parapet.
• Widening the courtyard at the ends and accepting a tighter middle, which concentrates the failures into fewer units.

Each of these is a massing decision. They belong in the design review before planning, not in the response to a consultee comment.

Deck level is the move people forget. Where the courtyard sits over undercroft parking or a podium, raising it by even half a storey improves every ground floor window facing into it, and it also removes the awkward relationship between a private terrace and a shared route. The cost is a longer ramp or a lift, so it has to be decided early.

Ground floor uses in the darkest positions

Some positions in a courtyard will never deliver good daylight to a habitable room. Planning the building around that is an honest response, and case officers recognise it.

Put cycle stores, plant, bin stores, entrance lobbies, managed workspace or a residents' amenity room where the sky view is worst. Where a ground floor flat has to sit in a poor position, give it dual aspect, raise the ceiling, widen the window and accept a larger unit. A wheelchair accessible unit placed in the darkest corner of a courtyard because it needed level access is the version of this that draws criticism.

Testing early and reporting the residual failures

Run a coarse test at massing stage using vertical sky component on a simple block model, before the facade design starts. Full internal modelling comes later, against the illuminance targets in the National Annex to BS EN 17037 for bedrooms, living rooms and kitchens, and the results should be reported room by room.

The reporting matters as much as the modelling. A Daylight and Sunlight Assessment that lists only the percentage of rooms meeting the target is doing half the job. Say how many rooms fall short, by how much, which rooms they are, what was tried, and why the design is acceptable in the round. Officers and inspectors accept residual shortfalls in dense urban schemes where the reasoning is transparent and reject schemes that appear to hide them.

Two further points to cover. Explain how the courtyard performs for amenity as well as for windows, drawing on an Overshadowing Study of the courtyard surface through the year. And keep the daylight case separate from the privacy case, because the distances between facing windows are a different judgement with a different history.

Get the section right and most of the rest follows. Draft the supporting documents once the massing is settled: see which reports your scheme is likely to need by entering the postcode on PlanScope.

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