Balcony overhangs and the room behind them
An inset balcony can take more light from a room than the building opposite. How the with and without convention works, and which design changes actually recover the loss.
An inset balcony can remove more daylight from the room behind it than the building on the opposite side of the street does. That is the uncomfortable finding behind the balcony daylight impact on the room behind, and it is why the established convention is to run the calculation twice: once with the balcony in place and once without it, so the effect of the proposal can be separated from the effect of the applicant's own design.
Get this wrong at planning stage and the scheme carries a daylight shortfall that no amount of negotiation about the neighbouring massing will fix, because the obstruction is your own soffit.
Why the soffit costs so much light
A window under a balcony loses the upper part of its view of the sky, and the upper part is where most of the useful light comes from. Sky luminance is greatest away from the horizon, so an overhang that blocks a modest slice of the visible hemisphere removes a disproportionate share of the illuminance.
The loss compounds with depth. A two metre deep recess with a solid soffit and a solid balustrade produces a window that is, in daylight terms, set back inside a short tunnel. Add a north orientation and single aspect and the room is unlikely to reach the target illuminance in the national annex to BS EN 17037, whatever the glazing area.
Vertical sky component behaves the same way for neighbours. Where an existing window has a balcony above it, the balcony is already suppressing the figure, so a proposal that reduces it further starts from a low base and the relative loss looks worse than the change in the reader's actual experience of the room.
Calculating with and without
BRE guidance on site layout planning for daylight and sunlight is explicit about balconies, and the practice that follows from it is to present both results side by side.
| Case modelled | What it tells the officer |
|---|---|
| Existing window with its balcony, before and after the proposal | The real change the occupier will experience |
| Same window with the balcony removed from the model | Whether the proposal alone would breach the guidance |
| Proposed flats with their balconies | The scheme as it will be built |
| Proposed flats without balconies | Whether the design, not the context, causes the shortfall |
The pair of runs answers the question officers actually ask, which is who caused the problem. Where the without-balcony figures comply and the with-balcony figures do not, the honest conclusion is that the balcony design is the constraint. Where both fail, the massing needs to change.
That logic runs alongside the broader question of the baseline being used, and the same care applies to the mirror image baseline and existing obstructions when the neighbouring building is itself an outlier.
Depth, width and the finish
Three variables move the numbers, and only one of them costs money.
Depth is the dominant one. Reducing a balcony from 2.0 metres to 1.5 metres recovers a meaningful part of the loss, and below about 1.5 metres the balcony stops functioning as usable outdoor space, which is where the conversation with the client gets difficult.
Width matters because a balcony that is wider than the room behind it, with open sides, lets light in laterally. A narrow slot recess the same width as the window is the worst arrangement for the same projection.
Surface finish is the low cost variable. A white or pale soffit with high reflectance returns light into the room, and a glazed or perforated balustrade instead of a solid upstand makes a measurable difference to the lower part of the window. Specify the reflectance value in the assessment, because a model run with a pale soffit and a scheme built with dark anodised aluminium is a discrepancy someone will find.
Dual aspect as the answer
The structural fix is layout rather than detailing. A dual aspect flat with the balcony on one elevation and clear glazing on the other keeps the room above target even where the balcony side performs poorly.
Single aspect north-facing flats with deep inset balconies are the combination that fails, and London Plan policy on housing quality discourages exactly that arrangement. If the plan form forces single aspect, put the balcony outside the line of the facade rather than inside it, or move it to serve a secondary space.
Corner units are worth protecting in the layout for this reason. They absorb a balcony without a daylight penalty, and they are also where the room use assumptions matter most, since how a room is labelled changes the target applied to it.
Juliet balconies, winter gardens and the alternatives
A Juliet balcony has no soffit and no daylight cost. It also provides no usable outdoor space, so it satisfies the assessment and not the amenity standard, and many councils will not accept it as private outdoor space at all.
Winter gardens are the response to noise-affected facades, and they carry a daylight penalty of their own because light passes through a second layer of glass before it reaches the room. Model the actual glazing specification rather than assuming clear single glazing, and state the transmittance you used.
Projecting balconies avoid the overhang problem for the room they serve, but they cast shade on the flat below and on any shared amenity space at ground level, which is a point that surfaces in courtyard blocks and sunlight to shared amenity space. The balcony has to go somewhere. The question is whose light pays for it.
Reporting it so an officer can use it
State the convention in the methodology section, in one sentence, before any results appear. Give the with and without figures in the same table rather than in separate appendices, and put the balcony depth, the soffit reflectance and the balustrade type in the assumptions.
Then write the conclusion in words a planning committee can follow. Our note on presenting daylight results a committee can read covers the format, and the short version is that a table of percentages without a sentence explaining what the occupier will notice is not an assessment.
Do the obstruction test early. Running obstruction angles at the massing stage takes an afternoon and tells you whether the balcony strategy survives contact with the site.
What recovers the loss
- Cut the balcony depth and increase its width, keeping the area roughly constant.
- Open the sides of the recess so light enters laterally rather than only through the front plane.
- Specify a pale, high reflectance soffit and record the value.
- Replace solid balustrades with glazed or perforated panels on the upper part.
- Increase head height at the window and raise the ceiling line, which recovers the sky view the soffit took.
- Move single aspect units off the north elevation, or give them a second aspect.
Test each change in the model rather than assuming it works. The gains are additive and modest individually, and two of them together usually close a marginal case.
A Daylight and Sunlight Assessment prepared this way is defensible at appeal, because the balcony question has been answered before anyone asks it. PlanScope drafts the document structure and the policy narrative in minutes, with the modelled figures flagged for the assessor to supply. 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.
Frequently asked questions
Why are daylight results reported with and without the balcony?
Because the pair of runs separates the effect of the proposal from the effect of the balcony itself. If the figures comply once the balcony is removed from the model, the shortfall is caused by the applicant's own design rather than by the neighbouring massing, and the officer needs to know which. The convention follows established BRE guidance.
How deep can an inset balcony be before daylight suffers?
There is no fixed limit, because the outcome depends on room depth, window head height, orientation and the finishes. As a rule, depth is the dominant variable and reductions from around two metres to 1.5 metres recover a useful amount. Below 1.5 metres the space stops being usable, which is where the design trade-off bites.
Does a pale soffit really make a difference?
Yes, and it is the least costly change available. A high reflectance soffit returns light into the room, and a glazed or perforated balustrade improves the lower part of the window. Record the reflectance value used in the model, because a scheme built with a dark soffit will not match the assessment that supported the permission.
Do Juliet balconies solve the problem?
They remove the daylight penalty because there is no overhang, but they provide no usable outdoor space and many councils will not count them towards private amenity space standards. They are a reasonable answer for a secondary room or where external space is provided elsewhere on the plot, not a general substitute.
Are winter gardens treated differently?
They are usually proposed to deal with noise on an exposed facade, and they carry their own daylight cost because light passes through an extra layer of glass. Model the specified glazing transmittance rather than assuming clear single glazing, and state the assumption, since the difference between specifications is large enough to change the result.
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.