Balcony overshadowing of the flat below

Balcony overshadowing of the flat below is the most common self-inflicted daylight problem in new residential schemes, and it is also the easiest one to quantify. A projecting balcony sits directly over the head of the window beneath it and removes the part of the sky the window depended on. The room loses the highest, brightest part of its view of the sky, which is the part that does most of the work.

BRE 209 in its 2022 edition deals with this directly. Where a balcony overhangs a window, calculate the vertical sky component twice, once with the balcony and once without, and report both. That single step separates the effect of your own balcony from the effect of the massing, and it changes the conversation with the case officer from a bad number into a design choice you can explain.

Why projecting balconies cut daylight below

Daylight arriving at a window is weighted by angle. Light from high in the sky contributes far more illuminance on the working plane than light arriving near the horizontal, partly because of the cosine effect and partly because the sky is brighter overhead. A balcony soffit 2.4 metres above the window head of the flat below blocks precisely that high band.

The result in a vertical sky component calculation is a loss that often looks catastrophic compared with the same window on a balcony-free elevation. Read at face value it suggests the massing is wrong. Read with the with-and-without pair, it shows the balcony accounting for most of the reduction, which is a different and more tractable problem.

Where the scheme also has to deal with existing neighbours, keep the two assessments separate in the report. Mixing your own balcony losses into the neighbour section invites confusion, and a Daylight and Sunlight Assessment that reports internal and external effects in one table is hard for a committee to read.

Depth, projection and the light removed

Three geometric variables control the loss: how far the balcony projects, how far the soffit sits above the window head, and how wide the balcony is relative to the window.

Projection does the most damage. Going from 1.5 metres to 2.0 metres of projection removes a further band of sky at exactly the angles that matter. Raising the soffit helps, and you rarely have more than a couple of hundred millimetres of floor-to-floor to play with. Narrowing the balcony so that it does not extend past the window reveals helps at the edges, which is why a balcony centred on a wide sliding door performs worse than one offset to the side of it.

None of these is free. Cutting the projection below about 1.5 metres produces a balcony that takes a chair and not a table, and policy on usable private outdoor space will catch that. The London Plan sets minimum areas and a minimum depth and width for private outdoor space, and most local plans outside London borrow a similar standard, so a shallow balcony can fail one policy while fixing another.

Inset and semi-inset alternatives

Pulling the balcony into the building footprint removes the overhang problem entirely for the flat below, because the balcony sits within the same storey rather than over the storey beneath. It also removes the projecting structure, the thermal bridge at the slab edge and a chunk of the drainage detailing.

The cost is internal floor area and daylight to the inset flat itself. An inset balcony 1.8 metres deep puts the living room window 1.8 metres back behind a soffit and a side wall, and the room behind it loses light for the same reason the flat below used to. A semi-inset balcony, half within the footprint and half projecting, splits the difference and is usually the better answer on a deep plan.

Two rules of thumb are worth testing rather than assuming. Inset balconies suit corner positions, where light arrives from two directions. Projecting balconies suit single aspect flats where the window needs everything it can get and the balcony can be offset sideways.

Balustrade material and transparency

A solid balustrade in render or metal panel blocks light at the lower angles and, more importantly for the flat below, reflects almost nothing useful. Clear glass with minimal framing transmits most of what reaches it, and the difference at the window beneath is measurable.

Three details decide how much you keep. The proportion of the balustrade that is glazed rather than framed. Whether the glass is clear, low iron or fritted for privacy. And whether the soffit above is a light finish or bare dark concrete, because a light soffit bounces a useful amount of reflected light into the room beneath.

Privacy pulls the other way. Fritted or obscured glass to the lower part of the balustrade is a common compromise and it costs less light than a solid panel. Where overlooking between balconies is the concern, the geometry of the screen matters more than its opacity, as our note on roof terraces and neighbour privacy sets out.

Testing the affected rooms properly

Vertical sky component alone will not tell you whether the room works. It tells you about the sky available at the window. What matters to the resident is illuminance on the working plane across the room, which means a climate-based calculation to BS EN 17037 with the UK national annex targets, expressed as illuminance over a proportion of the reference plane for a proportion of daylight hours.

Run the assessment on the rooms most at risk rather than all of them: single aspect flats, rooms served by one window, rooms with the deepest balcony above, and any room where the window head is close to the ceiling. Where a room is served by two windows the arithmetic changes, and our note on a neighbour's room served by two windows explains why averaging across them misleads.

Report the assumptions in the open: surface reflectances, glazing transmittance, whether the balcony soffit was modelled with its real finish, and whether the balustrade was modelled as glass or as an opaque plane. An assessment that quietly models a clear balustrade when the elevation shows metal panels will not survive a reviewer.

Where amenity space requirements conflict

This is the honest tension in the whole subject. Policy wants private outdoor space of a usable size and shape. Policy also wants adequate daylight to habitable rooms. On a mid-rise single aspect flat with a 2.7 metre floor to floor, you cannot fully satisfy both with a projecting balcony over the window below.

Say so in the submission and show the trade-off. A short table of options tested, with the daylight result and the outdoor space area for each, is more persuasive than a claim that the chosen option is optimal. Officers deal with this weekly and they recognise an honest constraint.

Illustrative example: a seven storey block with single aspect flats and 1.8 metre deep projecting balconies. Moving to 1.5 metre semi-inset balconies with clear glass balustrades and light soffits recovers a usable amount of vertical sky component at the windows beneath, while keeping private outdoor space above the policy minimum. The scheme loses a small amount of internal area at each corner. Figures here are illustrative.

Design responses that keep both

Stagger the balconies vertically so that not every window has one directly above it. Offset them laterally from the main glazing. Use the balcony on the bedroom side and keep the living room window clear. Push floor to floor height up by 150mm on the worst storeys if the overall height allows it.

And consider a Juliet balcony on the storeys where the daylight result is marginal, with the full private outdoor space provided as a shared terrace elsewhere in the scheme. That is a policy argument rather than a technical one, and it is winnable where the communal provision is real.

Enter your postcode on PlanScope to see which reports a residential scheme at your site 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.