Balconies and overhangs: the light they take from below
A balcony above a window removes sky from the part of the hemisphere that matters most. Here is how to model it, and which design levers recover the loss.
A balcony above a window is an obstruction, and in daylight terms it behaves like a very close overhang: it cuts the upper part of the sky visible from the window head, which is the part contributing most of the light. Balcony daylight obstruction is the reason flats that model well in a scheme with no balconies fail vertical sky component and no-sky line tests once the amenity space is added. The effect is largest for the flat directly below and for windows set close to the balcony soffit.
Test it before the elevation is fixed. Changing a balcony from projecting to inset after planning is a facade redesign, and after construction it is nothing at all.
How a balcony above obstructs the window below
The vertical sky component measures the proportion of sky visible at the centre of the outside face of a window, expressed as a percentage of the unobstructed hemisphere. The Building Research Establishment guide Site Layout Planning for Daylight and Sunlight, second edition 2011 and revised in 2022, uses 27 per cent as the value for an unobstructed vertical wall and treats a reduction to less than 0.8 times the former value as likely to be noticeable.
A balcony above the window removes sky from the top of that hemisphere. Because sky luminance is higher towards the zenith, a slab projecting 1.8 metres can remove a larger share of the useful daylight than a building of the same angular size further away. Inside the room, the no-sky line moves towards the window, so the rear of the room sees no sky at all and the daylight distribution test fails even where the window head is generous.
Annual probable sunlight hours behave the same way. A south-facing window under a deep balcony can lose most of its summer sun while keeping its winter sun, because winter sun arrives at a low altitude and slides under the slab.
Recessed, inset and projecting balconies compared
The three arrangements behave differently and the terms get used loosely, so define them on the drawing.
| Type | Relationship to the facade | Effect on the window below |
|---|---|---|
| Projecting | Slab cantilevers beyond the facade line | Largest loss, because the full depth overhangs the window head |
| Inset or recessed | Balcony sits within the building envelope | Loss to the window in the recess, little or none to the flat below |
| Winter garden | Enclosed, glazed amenity space | Behaves as an extra glazed layer, needs transmittance modelling |
| Juliet | No floor plate, guarding only | Negligible daylight effect |
Inset balconies solve the problem for the flat below by moving it inside the same flat, which is usually the better trade because the occupier who loses the light is the occupier who gains the balcony. That is a design judgement worth making explicitly in the Design and Access Statement rather than leaving the reviewer to work it out.
Modelling the obstruction correctly
Three modelling errors turn up repeatedly in submitted assessments.
The first is omitting balconies entirely from the massing model and reporting results for a smooth facade. The second is modelling the balcony slab but ignoring the guarding, which for a solid or opaque balustrade is a real obstruction to the window below at low angles. The third is assessing the window from the facade plane when the balcony recess means the reference point should sit at the outer face of the recess.
The BRE guide anticipates the problem and suggests reporting results both with and without balconies where the effect is material, so that the contribution of the balcony can be seen separately from the contribution of the building massing. Do that. It gives the case officer a way to understand what is causing a shortfall, and it gives the design team a lever.
Where an assessment is testing effects on neighbours rather than on the scheme's own units, the same logic applies to any projecting element, and the guidance on responding to a neighbour's daylight objection covers how those results are usually presented.
Glass balustrades and what the model should assume
A clear glass balustrade transmits light, a fritted or etched one transmits less, and a solid metal or masonry one transmits none. Modelling software will happily treat every balustrade as opaque unless told otherwise, which understates performance on schemes using clear glass.
The defensible approach is to model the balustrade with its actual visible light transmittance, state the value used, and cross-refer to the specification in the materials schedule. Then condition it. If the daylight case depends on clear glass balustrades and the contractor value-engineers them to perforated metal after consent, the assessment no longer describes the building. Councils are increasingly alert to this and will ask for the balustrade specification to be secured by condition where the margin is thin.
Maintenance matters too. A balustrade that is clear on day one and permanently grubby by year three is a real reduction, which is one reason some assessors apply a conservative transmittance rather than the manufacturer's figure.
Setting balcony depth against daylight results
Depth is the main variable within the designer's control, and the relationship is not linear. Reducing a 2.0 metre balcony to 1.5 metres often recovers more daylight than the 25 per cent reduction in depth would suggest, because the removed strip is the part nearest the building where the obstruction angle is steepest.
Other levers are available before depth is cut. Raising the window head, reducing the floor-to-soffit dimension above the window, stepping the balcony sideways so it sits over a solid pier rather than over glazing, and moving from a continuous balcony band to discrete balconies all help. On deep plans, a window head raised by 300 millimetres can move the no-sky line further than any balcony change.
Illustrative example: a seven storey block with continuous 2.1 metre projecting balconies on the south elevation might show the lower three floors failing the daylight distribution target, while the same block with 1.4 metre balconies inset by 600 millimetres brings all but the ground floor within target. Figures of that kind depend entirely on the geometry and should never be assumed from another scheme.
Coordinating with the daylight consultant
The sequence that works is a massing test at concept stage, a facade test once the window pattern is drawn, and a final run on the submission drawings. The first two are quick and low in cost, because they need only a block model. The last is the one that goes into the application.
Ask the consultant for the failing windows to be listed by unit and orientation rather than as a headline percentage. A scheme where the shortfalls fall on single-aspect north-facing studios is a different planning argument from one where they fall on family flats, and the officer will make that distinction whether or not your report does.
A formal Daylight and Sunlight Assessment is normally prepared by a specialist, and councils expect a suitably qualified author. Where overshadowing of amenity space is also in issue, an Overshadowing Study covers the external ground plane rather than the windows.
What to test before the elevation is fixed
Run these checks while changes are still drawings.
- Model the scheme with and without balconies and compare vertical sky component at every window on the lowest three residential floors.
- Record the balustrade transmittance assumption and check it against the materials schedule.
- Check the no-sky line in the deepest plan type on each elevation, not just the typical one.
- Test one reduced balcony depth and one inset option so the design team has a costed alternative before the officer asks.
- Note which windows rely on the balcony being clear-glazed, and flag those for a condition.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.
Frequently asked questions
Should balconies be included in a daylight model?
Yes. Omitting them produces results for a building that is not being built. The BRE guide suggests reporting results both with and without balconies where the effect is material, so the contribution of the balcony can be separated from the contribution of the massing. Case officers increasingly ask for both sets of figures.
Do inset balconies perform better than projecting ones?
For the flat below, yes, because the slab does not overhang the window head. The loss moves into the flat that has the balcony, which is usually the fairer outcome. Inset balconies cost more in floor area and can complicate the structural grid, so the decision belongs in the design narrative rather than the daylight appendix.
How is a glass balustrade treated in the assessment?
It should be modelled with its actual visible light transmittance, and the value stated in the report. Clear glass transmits most of the light, fritted or etched glass less, and solid guarding none. Where the result depends on clear glazing, expect the authority to secure the specification by condition.
Is there a maximum balcony depth for daylight purposes?
No fixed limit exists in national policy or in the BRE guidance. Performance depends on window head height, room depth, orientation and the obstruction opposite. Depths beyond about two metres tend to cause problems on lower floors, but that is a rule of thumb rather than a standard, and each scheme needs modelling.
Can a scheme be refused on internal daylight to its own flats?
Yes. Poor internal daylight to proposed units is a material consideration and appears regularly in refusal reasons, particularly for single-aspect units on lower floors. Many local plans reference the BRE guidance or set their own internal standards, so check the adopted policy and any design guide before fixing the elevation.
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.