Balconies and self-shading in a daylight model

An inset balcony sitting above a window will take more daylight off that window than most neighbouring buildings ever could. A 2 metre deep recess above a living room window can halve the vertical sky component at that window before any external obstruction is considered, and on a single aspect north-facing flat it can take the room below any sensible target. This is self-shading, and it is caused by your own design rather than by anything on the other side of the boundary.

The reason it matters in a planning submission is that a case officer reading a daylight and sunlight assessment sees a list of rooms that fail. Unless the report separates the loss caused by the balcony from the loss caused by the surroundings, the scheme looks worse than it is and the mitigation discussion heads in the wrong direction.

How a balcony reduces sky visibility

The vertical sky component measures the proportion of sky visible at the centre of a window on its outside plane, against an unobstructed vertical surface. Anything that blocks the upper part of that hemisphere reduces it, and the balcony slab directly above the window blocks the part of the sky that contributes most.

Daylight distribution, the no sky line, behaves differently. It maps the area of the working plane inside the room that can see some patch of sky, so the deeper the overhang the further the no sky line retreats towards the window, leaving the back of the room in permanent gloom even on a bright day.

Sunlight is affected too. Annual probable sunlight hours at a window under a balcony drop sharply, and the winter proportion drops faster still, because low winter sun arrives at exactly the angle the slab intercepts.

Inset against projecting balconies

A projecting balcony cantilevers out from the facade, so the slab above it is also projecting and the window beneath sits broadly under open sky. Losses are real but modest.

An inset balcony is cut into the building envelope. The window serving the room behind now sits at the back of a recess with walls on two or three sides and a soffit above. That geometry is the problem, and the deeper and narrower the recess, the worse it gets.

Architects reach for inset balconies because they keep the elevation flat, they are easier to detail for thermal bridging and water, and they do not project over a boundary. Those are good reasons. They are just expensive in daylight terms, and the cost should be known at concept stage rather than discovered at the planning submission.

Modelling the overhang correctly

Three errors recur.

The first is omitting the balcony from the model altogether, usually because it was drawn as a line on the elevation rather than as geometry. The results then look fine and fall apart when the authority's consultant rebuilds the model.

The second is treating a glazed balustrade as if it were not there. A clear glass balustrade transmits most of the light reaching it, but it is not invisible, and the honest approach is either to model its transmittance explicitly or to model it as opaque and say so. Picking whichever assumption gives the better number, without stating it, is the quickest way to lose an officer's confidence.

The third is inconsistent treatment between the existing and proposed cases when assessing effects on neighbours. If the neighbouring window has its own balcony above it, that balcony exists in both scenarios and belongs in both models.

BRE 209 anticipates the self-shading problem directly. Where a room's shortfall is caused by its own balcony, the recommended approach is to report the result twice, once as designed and once with the balcony removed, so that the contribution of the external obstruction can be seen on its own. That is not a trick to improve the numbers. It is the only way to attribute cause, and officers who read these reports regularly expect to see it.

Soffit and wall reflectance

Light reaching a recessed window has usually bounced off the balcony soffit, the side walls of the recess and the floor of the balcony. Surface reflectance therefore does real work, and a dark soffit in exposed concrete or a dark grey render loses light that a pale finish would return.

Standard internal reflectance assumptions used in daylight modelling are conservative, and external surfaces are often modelled at a low default value. If you intend to rely on a light balcony soffit, model it at the actual specified reflectance, state the value, and then secure the finish by condition. A reflectance assumption that is not in the materials schedule will be treated as optimistic.

Design changes that recover daylight

The fixes are architectural rather than analytical. But they only work if they are tested before the elevations are signed off, and they fall in roughly this order of effectiveness.

1. Make the balcony projecting instead of inset, accepting the detailing and boundary consequences.
2. Reduce the depth of the recess, since the relationship between depth and loss is steeper than most designers expect.
3. Move the balcony so it sits in front of a secondary window, a circulation space or a kitchen rather than the main living room window.
4. Raise the window head towards the soffit, or push the head above the soffit line where the floor zone allows.
5. Widen the balcony opening and remove one of the side walls, which helps daylight distribution more than it helps the sky component.
6. Specify a pale soffit and pale recess walls, and secure them.
7. Use a glazed balustrade with a stated transmittance rather than a solid parapet.

Illustrative example, not a real scheme: a 1.8 metre deep inset balcony with walls on both sides, serving a single aspect living room on the second floor of a six storey block, is remodelled at 1.2 metres deep with one side opened to the facade and the soffit specified pale. The daylight distribution in the room behind improves enough to bring it back within the target, with no change to the building footprint and a loss of about 1.4 square metres of private amenity space per flat. Whether that trade is worth making is a judgement about the scheme, not about the model.

Where the flats are single aspect to begin with, the balcony is often the final straw rather than the only problem, and the wider set of remedies in daylight to single aspect flats is worth reading alongside this.

Reporting self-shaded rooms

Say plainly which rooms are self-shaded, give both sets of figures, and explain the design reason for the balcony. Do not bury the balcony rooms in an appendix table of 400 windows.

A short section that lists the self-shaded rooms, states the depth and configuration of each balcony, gives the result with and without it, and sets out which of the mitigation options were tested and why they were or were not adopted will answer most of what an officer wants to know. Where a local plan amenity policy sets its own targets, address those directly as well, because policies vary in how they apply the BRE guidance and some set absolute floors rather than relative tests, a point covered in how local plan amenity policies use BRE 209.

The same discipline applies to the external spaces. Sun on the balcony itself, and on any communal amenity area below, belongs in the overshadowing study rather than being folded into the window results.

And if the answer is that the scheme accepts some self-shading because the balconies are worth more to residents than the marginal daylight, say that. An honest trade-off argued in the daylight and sunlight assessment carries further than a set of numbers that quietly omitted the geometry causing them.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. Daylight and sunlight modelling is specialist work and councils usually expect a suitably qualified author, so professional review is strongly advised.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.