Why balconies cause daylight failures in new flats

Balcony geometry is the most common reason a flatted scheme fails its internal daylight assessment while the massing itself is fine. A projecting balcony sits directly above the window of the flat below and removes most of that window's view of the sky. An inset balcony pushes the window back into a deep reveal and does the same thing from the side. Either way the room behind loses daylight that no amount of glazing area recovers, because the problem is obstruction rather than aperture.

The number of rooms affected is usually small and concentrated, which is the useful part. Identify them at massing stage and the fix costs a design iteration. Identify them at planning stage and the fix costs units.

How an overhanging balcony cuts light to the room below

Daylight to a habitable room depends on how much sky the window can see. A balcony slab 2.4 metres above the window head of the flat below is a horizontal obstruction across the upper part of that view, and the upper part of the sky is where the useful light comes from.

The loss is worst nearest the window, which is exactly where the assessment measures. A room with a 1.8 metre deep balcony overhead can lose a third or more of its daylight against the same room with an open soffit above, and on lower floors of a courtyard block, where the opposite facade already cuts the sky angle, the two effects compound.

BRE 209, in its 2022 edition, is explicit that balconies are a recognised cause of low results and should be examined as a separate design variable. It also aligns the assessment with BS EN 17037, so the metric most consultants now report for new dwellings is a target illuminance achieved over a proportion of the reference plane, with the simplified median daylight factor route used where full climate-based modelling is not warranted.

Many authorities still ask for average daylight factor against the older BRE figures of 1, 1.5 and 2 per cent for bedrooms, living rooms and kitchens. State which edition and which metric you have used, every time. The figures are not interchangeable and a reader comparing them without that label will reach the wrong conclusion.

Recessed against projecting geometry

The two arrangements fail differently, and the distinction drives the remedy.

A projecting balcony obstructs the window below. The room losing light is not the room the balcony serves, which is why the problem is invisible on a single-flat plan and only appears in section. Remedies act on the slab: reduce the depth, chamfer or step the outer edge, or set the balcony to one side so it sits over the kitchen window rather than the living room glazing.

An inset balcony obstructs the room it belongs to. The window opening is set back by the depth of the recess, so its view of the sky is bounded by the reveals as well as the soffit. Here depth is everything. A 1.5 metre inset with a 2.7 metre opening behaves very differently from a 2.5 metre inset with the same opening, and past about 2 metres of depth the room behind rarely passes without an unusually generous aperture.

The hybrid, a partly inset and partly projecting balcony, is often the best answer on a tight facade. Take the first metre from the recess and cantilever the rest, and you halve the reveal depth while keeping the usable area. The obstruction angle check at massing stage is the right moment to test this, before floor plates are fixed.

Modelling the soffit, the balustrade and the reveal

Three modelling decisions change the results by more than most design changes do, so agree them with the consultant early and record them in the report.

The soffit is an opaque obstruction, and its reflectance matters. A dark soffit and a white soffit give measurably different results, and if the design relies on a high reflectance value, that finish has to appear on the drawings and ideally in a condition.

The balustrade is the contested one. Solid or perforated metal is modelled as opaque. Glass is sometimes modelled as transparent or partially transmitting, and that assumption is generous, because balcony glazing collects dust, salt and rain marks. BRE guidance is cautious about it. The defensible approach is to model glass balustrades as opaque and report a sensitivity case with them transparent, rather than the other way round.

The reveal depth has to be modelled as built, including the thickness of any external insulation and the position of the frame within the opening. A 250 millimetre difference in where the glass sits in a deep wall changes the result at the back of a small bedroom.

Design moves that recover the light

In rough order of effectiveness on a typical scheme:

1. Reduce balcony depth to the minimum the amenity policy allows, and recover the area by widening rather than deepening.
2. Raise the window head of the room below as close to the underside of the balcony as the structure permits. Head height buys more sky than sill height ever does.
3. Move the balcony laterally so it sits over a bathroom, kitchen or circulation window instead of the main living space.
4. Open the balcony floor. A partially open slab edge or an open grating section at the outer edge returns light to the window below, subject to privacy and debris considerations.
5. Light the soffit. Pale, high reflectance finishes, sometimes with a chamfered edge to catch more sky.
6. Put a corner window on the unobstructed flank of the room, which is often the only move that works in a deep inset.

What does not work is enlarging the window within the same obstructed opening. If the sky is blocked, more glass simply gives more view of the soffit.

Where a winter garden changes the assessment

A winter garden is enclosed, so the habitable room behind it is assessed through two layers of glazing and a frame. Transmittance drops, and the room typically performs worse than the same room with an open inset balcony of the same depth.

It also counts as internal floor area, which pulls it into the space standard schedule and the gross internal area calculation. Some schemes use that deliberately on noisy or polluted frontages, where the acoustic and air quality benefits outweigh the daylight penalty. If you take that route, say so explicitly and show the daylight consequence, because a reader who finds it in the appendix will assume you were hiding it.

The trade with amenity space and overheating

Private outdoor space policies set minimum areas and minimum depths, commonly around 1.5 metres of clear depth, which puts a floor under how shallow a balcony can be. So the balcony depth that fixes daylight sometimes breaches amenity policy, and the honest answer is to present the trade rather than to pretend it does not exist.

The overheating regime pulls the other way. Balconies shade the glazing below, and that shading helps with solar gains under Approved Document O and in dynamic modelling. Strip the balconies to rescue daylight and the Overheating Risk Assessment gets harder, sometimes to the point of needing external shading you have just removed. Run the two studies together on any south or west facing elevation with deep plan flats behind it.

There is a real uncertainty here that the profession has not settled: the daylight metrics and the overheating metrics are calibrated independently, and a scheme can be pushed to optimise one at measurable cost to the other with no method for weighing them against each other. Say which you prioritised and why.

Reporting balcony-affected rooms separately

Present the results in three groups: rooms meeting the target, rooms below target because of balcony geometry, and rooms below target because of external massing or context. That separation is what allows an officer to see which shortfalls are yours to fix and which are inherent to a dense urban site.

BRE guidance supports running the model with and without the balconies to isolate the balcony contribution, and that pair of numbers is the most persuasive thing you can put in front of a committee. It demonstrates that the design has been tested rather than defended. The way daylight results are presented for a committee matters as much as the modelling, because a table of 400 rooms with no grouping reads as an attempt to bury the failures.

Where a small number of rooms remain below target, say how many, which rooms, in which units, and what the occupant experience will be. A Daylight and Sunlight Assessment that names 11 bedrooms at 0.6 per cent and explains the dual aspect compensation is more credible than one claiming full compliance on a heroic balustrade assumption. Councils normally expect this work from a suitably qualified author, and professional review before submission is strongly advised.

Enter your postcode on PlanScope to see which daylight, overheating and amenity documents your scheme is likely to need.

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. We do not guarantee planning approval.