Balconies and overhangs: the daylight cost to the flat below
Balcony geometry usually costs more daylight than massing does. Inset balconies with solid returns are the worst arrangement, and the fixes only work at concept design.
A balcony above a window takes daylight out of the room below it, and on a residential scheme that effect is usually larger than anything the massing does. A 1.5 m deep soffit directly over the head of a window removes most of the sky the room could otherwise see, and the loss shows up in every metric an assessor reports. Design teams discover this late, when the daylight results come back and the only remaining fix is to reduce the balconies that the amenity space policy requires.
Why an overhang costs so much daylight
Daylight into a room comes overwhelmingly from the upper part of the window, because that is the part that sees sky rather than ground and opposing buildings. An overhang cuts precisely that band.
The effect compounds with depth. A shallow projection clips the highest sky angles. A deep one removes the whole visible sky from the window head down, leaving the room lit by reflected light from the balcony soffit and the ground. Reflected light is far weaker, and it is why rooms under deep balconies feel gloomy even on bright days.
Side walls matter too. A balcony enclosed on both sides by solid returns is closer to a recessed alcove than a canopy, and it restricts light arriving obliquely as well as from above. The most common poor outcome is a fully inset balcony with solid party walls to either side and a solid soffit above.
Recessed against projecting forms
The two families behave differently, and each has a daylight cost paid by a different room.
| Balcony form | Room most affected | Daylight effect | Other consequences |
|---|---|---|---|
| Fully inset within the facade | The room behind the balcony | Largest, because the room sits in a recess | Simple weathering, no projection over boundary |
| Projecting cantilever | The room below on the storey beneath | Moderate, concentrated at the window head | Thermal bridging, overlooking, structure |
| Partially inset with one open side | The room behind, less severely | Reduced, with oblique light retained | Reasonable compromise on most schemes |
| Juliet balcony with no floor | None | None | Provides no usable amenity space |
Fully inset balconies are popular because they keep the elevation clean and avoid projecting structure over a boundary. They are also the worst arrangement for the flat that has to look through them. Where a scheme uses them at every level on a single elevation, expect the daylight results to fail across that whole facade rather than at scattered receptors.
Modelling balconies properly
Assessments that omit balconies overstate daylight to the flats behind them, and the error is discovered at the worst point in the process.
Model the soffit, the side returns, the balustrade and its light transmittance. A glass balustrade is not fully transparent and a perforated metal one is not fully open, so use the manufacturer's transmittance figure rather than assuming.
BRE 209 also gives the useful technique for separating two questions. Where an existing neighbouring window sits under a balcony, calculate the results with and without the balcony in both the existing and proposed cases. That isolates the effect of your development from the effect of the neighbour's own building, and it often shows that a large apparent reduction is mostly caused by an overhang that has been there for 40 years. Present both sets of numbers rather than only the helpful one.
For daylight within your own scheme, the assessment turns on internal illuminance rather than on sky component, and the balcony geometry feeds straight into it. Test the worst-case units early, meaning ground floor flats with a single aspect under a deep balcony, and test them at concept design rather than at technical design.
Design moves that recover daylight
Most of these cost nothing at the right stage and are impossible later.
- Raise the window head. A higher floor to ceiling height, with the head above the balcony soffit line where the balcony belongs to the flat above, restores the sky band directly.
- Reduce depth where you can. Amenity policy usually sets a minimum depth, so the room to move is in width rather than depth.
- Open one side. A balcony inset on three sides performs far worse than one with an open flank.
- Step the balcony sideways from the main window so the living room glazing is clear and the overhang sits over a secondary opening.
- Use a light-coloured, high-reflectance soffit finish, which recovers a modest amount of reflected light.
- Choose balustrades with high transmittance and avoid solid upstands at the base.
- Consider a winter garden with fully openable glazed screens, which changes how the space is counted and assessed. Confirm the approach with the case officer, because treatment varies.
None of these is a single fix. Together they typically move a facade from failing across the board to failing at a handful of receptors, which is a different conversation at committee.
The trade-off with private amenity space
Balconies exist because policy requires private outdoor space, and in London the London Plan sets a minimum area for private outdoor space with a minimum depth of 1.5 m. That depth is exactly the dimension that causes the daylight problem, which is the tension at the centre of this issue.
Outside London, standards vary between local plans and design guides, and some authorities set no numerical requirement at all. Read the adopted policy before assuming a balcony is compulsory, because a scheme with generous communal amenity space and a well-designed roof terrace can sometimes justify smaller private balconies for the units where daylight is tightest.
Where the policy is fixed, the honest answer is that a deep balcony on a single-aspect north-facing flat is poor accommodation whatever the compliance position. Say so internally at concept design. It is a far cheaper conversation than the one at planning committee.
What to put in the submission
The design evidence should show the officer that the trade-off was made deliberately rather than discovered late.
Set out in the Design and Access Statement the balcony strategy, the depths chosen, why they were chosen, and what was done to protect daylight to the affected rooms. Include a typical bay section showing the window head against the soffit line, because that single drawing answers most of the question.
Then present the results. A Daylight and Sunlight Assessment reporting internal daylight for the worst-affected units, alongside the with and without balcony comparison for neighbouring windows, is more persuasive than a summary table of passes. Where amenity space is also assessed for sun on ground, an Overshadowing Study covers the balcony and terrace areas themselves.
An illustrative example: a scheme with 1.5 m deep balconies fully inset on the south elevation moves to partially inset balconies open on one flank, with the window head raised 200 mm, and recovers daylight in the majority of the affected living rooms without losing any amenity area. The change was made at concept design. Made at technical design, it would have altered the structural grid.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission, and daylight modelling should be prepared or checked by a suitably qualified consultant. We do not guarantee planning approval. Enter your postcode on PlanScope to see which amenity documents a residential scheme is likely to need.
Frequently asked questions
Why does a balcony reduce daylight so much?
Daylight comes mostly from the upper part of a window, which sees sky rather than ground and opposing buildings. An overhang cuts exactly that band, leaving the room lit by weaker reflected light from the soffit and ground. A deep soffit with solid side returns removes oblique light as well as light from above.
Which balcony form performs best for daylight?
A partially inset balcony open on one flank, with the window head raised above the soffit line. Fully inset balconies enclosed on three sides are the worst arrangement because the room sits in a recess. Juliet balconies cause no daylight loss at all but provide no usable amenity space.
Should balconies be included in the daylight model?
Always. Omitting them overstates daylight to the rooms behind, and the error surfaces at the worst point in the process. Model the soffit, the side returns and the balustrade using the manufacturer's light transmittance figure rather than assuming glass is fully transparent or perforated metal fully open.
How do I separate my scheme's effect from an existing balcony?
Calculate results with and without the balcony in both the existing and proposed cases, which is the approach BRE 209 describes. That isolates the effect of your development from the effect of a neighbour's own overhang, and it often shows most of an apparent reduction was already there. Present both sets of figures.
Can I make balconies shallower to protect daylight?
Only within the amenity space policy that applies. The London Plan sets a minimum private outdoor space area with a minimum depth of 1.5 m, and other authorities set their own standards or none at all. Where depth is fixed, the room to move is in width, orientation and the position of the balcony relative to the main window.
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.