Daylight to rooms behind deep balconies and access decks

A balcony that projects two metres or more over the window below removes the brightest part of the sky from that room, and the daylight result for the room behind a deep balcony can fail where the same room with a recessed or glazed balcony would pass. The obstruction is your own building, not the neighbour's, which means it is entirely within the design team's control and entirely your problem to solve. Access decks on refurbishment schemes do the same thing along an entire elevation.

Sky luminance under the overcast sky used in daylight calculation is highest towards the zenith. A soffit overhead cuts off exactly that part of the sky vault, so a modest projection costs more daylight than its area suggests.

How an overhang removes sky from a window

Stand at the back of a living room and look out. What you can see of the sky, measured from a point on the working plane, is what drives the daylight result. A balcony soffit above the window head replaces a band of bright sky with a dark surface, and unless that surface is light and reflective it contributes almost nothing back.

Two consequences follow. The room's daylight falls, concentrated at the rear where the sky view was already marginal. And the distribution worsens, because the window now behaves as if it were set deep in a reveal, so the no-sky area at the back of the room grows.

Single-aspect flats suffer most, since there is no second window to compensate. There is a fuller treatment of that problem in daylight to single aspect flats.

Depth, soffit height and the daylight penalty

Three dimensions matter: how far the balcony projects, how high the soffit sits above the window head, and how wide the balcony is relative to the window. Projection does the most damage. Raising the soffit, by setting the balcony structure above the window head rather than level with the ceiling, recovers some of the loss.

Those directions are reliable. The magnitude depends on room depth, window size, floor-to-ceiling height and the external obstruction beyond, so the only honest answer for a specific scheme is a modelled one.

Access decks on refurbishment schemes

Post-war blocks with external access decks present the problem at its worst. The deck is continuous, so every room on that elevation sits under an overhang, and the rooms are usually kitchens or bedrooms with windows onto the deck. Residents then keep curtains or blinds closed for privacy, which removes whatever daylight survived the overhang.

On a refurbishment or infill scheme, the deck is often retained for cost reasons and the daylight consequence goes unexamined. If the scheme involves re-planning flats, moving habitable rooms away from the deck elevation does more for daylight than any glazing specification.

Measuring the obstruction in the model

The common error is modelling the facade without its balconies. A flat elevation with correct window sizes produces results that look acceptable and bear no relation to the building being applied for. Officers and consultees have become alert to it.

What a credible Daylight and Sunlight Assessment states:

• That balconies, soffits, balustrades and access decks are included in the geometry, with a view of the model to show it
• The reflectance values used for soffits, balustrades and internal surfaces
• The transmittance assumed for glazing and for any glass balustrade, including a maintenance allowance
• Room-by-room results with room use identified, not a single building-wide average
• Which rooms fall below target and why, with the specific reason for each

BRE 209 in its 2022 edition aligns with the illuminance-based approach in BS EN 17037, while many adopted local plan policies still refer to the older average daylight factor targets of 1 per cent for bedrooms, 1.5 per cent for living rooms and 2 per cent for kitchens. Report against whichever the authority's policy actually cites, and say which you have used. Local policy wording varies more than people expect, and there is a separate piece on how local plan amenity policies use BRE 209.

Design fixes that work

In rough order of how much daylight they recover for the money:

1. Reduce the projection. A 1.5 metre balcony is usable and costs far less daylight than a 2.5 metre one.
2. Raise the soffit above the window head, which usually means a structural change rather than a finish change.
3. Cut a notch or void through the balcony in front of the window, so part of the sky view is restored.
4. Use clear glass balustrades and a high reflectance soffit finish.
5. Serve the balcony from a secondary room, placing the living room window clear of the overhang.
6. Set the balcony into the facade as an inset or wintergarden, accepting the loss of internal area in exchange for a working daylight result.

Options three and five are the ones that get rejected for reasons that have nothing to do with daylight, usually elevation rhythm or saleable area. That is a legitimate trade, as long as it is made knowingly rather than discovered at validation.

Where the balcony should be relocated

Private amenity space is a policy requirement in most local plans and in the London Plan, so removing balconies is rarely an option. Moving them is.

Corner balconies serve two rooms and overhang less of each. Recessed balconies at the end of a block take the overhang off the principal windows. Shared terraces at roof or podium level can discharge part of the amenity requirement, though many policies insist on a minimum of private space per dwelling as well, so check the wording before assuming a trade is available.

Where overshadowing of neighbouring amenity space is also in issue, an Overshadowing Study deals with a different question and should not be offered in place of the daylight work.

Evidence for the application

Submit the daylight report with the geometry visible, the targets named, the room schedule complete and the failures explained. A short section on what was changed during design, and what the alternatives cost in amenity or area, answers the question an officer will otherwise ask at week six.

One honest difficulty: there is no settled national rule on how much daylight a new flat must receive, and authorities apply the guidance with varying strictness. Treat BRE 209 as guidance on method and read the local policy for the standard.

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