Balconies and overhangs: designing out your own daylight

The single largest daylight obstruction in most new flats is not the building across the street. It is the balcony belonging to the flat above. A projecting slab one and a half metres deep sits directly over the window head, cutting out the part of the sky that delivers most of the useful light. Recess that balcony into the facade and the loss is worse again, because the reveal blocks light from the sides as well.

Designers who model this at massing stage fix it cheaply. Those who model it at planning stage end up arguing that a room falling short of target illuminance is acceptable in context.

How a balcony above a window reduces daylight

Daylight entering a window comes overwhelmingly from the upper part of the visible sky. A horizontal slab above the head of the window removes exactly that portion, and it does so before any external obstruction is considered.

The geometry is unforgiving. A balcony 1.5 m deep above a window 2.1 m high leaves a room whose usable sky angle has been cut sharply, and no amount of extra glazing width at low level recovers it. Widening the window helps far less than raising the head, because the light being lost is coming from above.

Two secondary effects compound it. The soffit is usually a dark grey or exposed concrete, so it reflects little back into the room. And the balustrade, if solid or opaque, blocks the lower sky and the ground-reflected component too.

The rooms most affected are those with a single aspect facing into a courtyard or a narrow street, which is also where the balconies tend to be deepest.

Recessed against projecting balconies

Projecting balconies sit outside the building line, with the slab above the window and open sides. Light still reaches the window obliquely from left and right.

Recessed or inset balconies sit within the building envelope. The slab above, the two flanking walls and often a solid balustrade form a box in front of the window. The daylight reduction is severe, and rooms behind inset balconies are the ones that fail assessment most often.

Semi-recessed balconies split the difference. Where an inset form is required for wind, weather protection or facade composition, the workable compromise is a shallow recess with the balcony floor stepped down, a light-coloured soffit, and a glazed or open balustrade.

There is a real trade-off here and it is worth stating honestly. Inset balconies read better in some architectural languages, weather better and often perform better in wind. That does not make the daylight consequence disappear, so the choice belongs on the drawing at concept stage with numbers next to it.

Modelling the effect

BRE Report 209, Site Layout Planning for Daylight and Sunlight, in its 2022 edition, brings interior daylight assessment into line with BS EN 17037. Assessment is now normally climate-based, using illuminance rather than the older average daylight factor.

The UK national annex to BS EN 17037 sets minimum target illuminances by room type, with lower values for bedrooms, higher for living rooms and higher again for kitchens, to be achieved over half the assessment area for at least half of daylight hours. A combined living, kitchen and dining space is assessed against the kitchen target, which is why open plan layouts behind deep balconies fail so regularly.

For modelling to be useful it has to include the balcony as built rather than as a line on a plan. That means the slab thickness, the soffit finish and its reflectance, the balustrade type and its transmittance, and any privacy screen at the party line. A model that omits the screens gives an answer nobody can rely on.

Run it at concept stage on a single representative bay. One test bay modelled early tells you more than a full building modelled after the layouts are frozen.

Design responses that recover daylight

The moves that work are geometric, and most of them cost nothing if made early.

1. Reduce balcony depth to the minimum that still delivers usable outdoor space, and gain the area by widening rather than deepening.
2. Raise the window head as close to the slab soffit as the structure allows, and consider a full-height glazed door leaf.
3. Cut the balcony back in front of the window and keep the depth where it sits against a solid panel of facade.
4. Specify a light-coloured, high-reflectance soffit rather than exposed dark concrete.
5. Use clear or open balustrades instead of solid panels, with privacy handled by a partial screen at the party line only.
6. Where an inset balcony is unavoidable, chamfer or splay the reveals so the flanking walls do not form a straight-sided box.
7. Place balconies on the rooms that tolerate the loss, such as bedrooms, and keep the main living space clear.

Point seven is often the whole answer on a constrained plan. Moving the balcony from the living room to the adjacent bedroom changes which target illuminance applies and can turn a failing unit into a passing one.

Trade-offs with amenity space standards

The reason nobody simply deletes the balconies is that policy requires private outdoor space. In London, Policy D6 of the London Plan sets a minimum of 5 m² of private outdoor space for one and two person dwellings, with an addition for each further occupant, and a minimum depth and width for that space to be usable. Many local plans outside London carry equivalent standards in a housing or design policy.

That creates a genuine tension: policy wants outdoor space, and outdoor space above a window removes daylight from the room below. There is no way to satisfy both by wishing.

The resolutions that officers accept are: wider and shallower balconies that keep the area while reducing the overhang, a mix of balcony types across the building with communal roof terrace provision making up the shortfall where the plan allows it, and winter gardens where the enclosure is glazed and counted differently. Check the local policy wording before assuming a winter garden counts, because authorities differ.

Where both a daylight standard and an amenity standard sit in the same plan, neither is subordinate. The application has to show the balance was struck deliberately.

Presenting the trade-off to a case officer

Officers are used to receiving a daylight and sunlight assessment that reports a percentage of compliant rooms and says nothing about why the others fall short. That invites a request for further information.

A better structure sets out the constraint, the options tested and the reason for the chosen design. Report the number of units assessed, the number meeting the target, and for those that do not, the room type, the shortfall and the specific cause. Where the cause is a balcony, say so, and show the alternative that was tested and why it was rejected.

Where sunlight to amenity areas is also at issue, an overshadowing study with shadow plots at 21 March and the solstices does that work more directly than a written argument.

Put the reasoning in the design and access statement as a design decision rather than burying it in a technical appendix. A scheme where 8 per cent of rooms fall below target for a stated and tested reason reads very differently from one where the same rooms fail without explanation.

And keep the assessment consistent with the drawings. Balcony depths that change between the model and the general arrangement plans undo the whole argument.

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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.