Daylight through an enclosed balcony or conservatory

An enclosed balcony puts a second sheet of glass between the sky and the living room, and most daylight assessments handle it badly. Daylight through an enclosed balcony has to account for two layers of transmittance, the obstruction formed by the balcony soffit and side walls, and the awkward question of which space is actually being assessed. Get any of those wrong and the reported figures describe a building that does not exist, in either direction.

Why a second glazed layer changes the assessment

An open balcony already reduces daylight to the room behind it. The soffit above cuts the upper part of the visible sky, which is the brightest part, and the side returns narrow the view. Enclose it and three further effects appear.

The outer glazing removes a proportion of the transmitted light at every angle. The inner glazing removes a proportion of what is left. And the enclosed space itself becomes an intermediate room with its own surfaces, reflecting some light inward and absorbing the rest, which means the calculation is no longer a single window looking at a sky.

A common error is to model the enclosure as though it were simply a deeper reveal. It behaves more like a light well: geometrically restrictive, with losses that compound.

Choosing the reference plane

The first decision is where to measure, and it has to be stated explicitly in the report.

For the geometric tests, the outer plane of the enclosure is the sensible reference for the sky the assembly can see, because that is where the obstruction geometry is resolved. The vertical sky component measured at the inner window will already be reduced by the balcony structure, which mixes the effect of the existing balcony with the effect of any new obstruction being assessed.

That mixing is the reason BRE guidance advises separating the two. Calculate the neighbour's condition both with and without the balcony, so the change attributable to your proposal can be seen on its own. Without that pair of figures, an existing balcony can make a modest new obstruction look severe, or can mask a real loss because the baseline was already low. The same logic underpins the retained ratio approach described in the 0.8 rule for retained daylight, which compares before and after rather than absolute values.

State in the report: where the reference plane sits, whether the balcony structure is included in the obstruction model, and whether the with and without pair has been calculated.

Transmittance losses through two layers

The arithmetic here is worth doing by hand once, because the result surprises people. This is an illustrative calculation with assumed values rather than a result for any real assembly.

Clear double glazing has a diffuse visible transmittance in the region of 0.68. Two layers in series give roughly 0.68 multiplied by 0.68, so about 0.46. Apply a frame factor to each layer, say 0.75 for the outer screen and 0.8 for the inner window, and the effective transmission of the assembly falls to around 0.28 of the unobstructed condition. Add a maintenance factor for soiling on two sets of surfaces, one of which is harder to clean, and less than a quarter of the incident diffuse light reaches the room.

Two refinements matter in practice. Frameless or minimally framed sliding glass systems on the outer layer perform better than a conventional framed screen, and the difference is large enough to be worth specifying. And if either layer carries a solar control coating, which the overheating strategy may require, transmittance drops further and the daylight case weakens again.

None of this means enclosed balconies cannot work. It means they have to be designed for daylight deliberately, with generous outer opening area, light coloured soffits and returns, and a room depth that does not rely on borrowed light.

Winter gardens used for acoustic mitigation

The enclosure is often not an amenity decision at all. On noise exposed facades, a sealed glazed balcony lets the habitable room window open for ventilation while the outer layer attenuates traffic or railway noise, which is a recognised mitigation approach.

That creates a direct conflict between two technical cases. The Noise Impact Assessment wants the outer screen closed to achieve internal levels. The daylight assessment reports the room behind two layers of glass. And the overheating strategy has to deal with a glazed unventilated volume on a sunny elevation, which is why the enclosure belongs in the thermal model rather than being treated as external space in an Overheating Risk Assessment.

Resolve it as a single design decision, not three separate reports. The usual moves are to make the outer screen largely openable so it can be modelled in both conditions, to keep the enclosure shallow, to raise the head height of the inner window, and to avoid single aspect rooms on the affected facade altogether where the layout allows.

Then report the acoustic case with the screen closed and the daylight case in the condition the occupant will actually use, and say which condition each figure describes. An assessment that quietly reports daylight with the screen open and noise with it closed is describing two different buildings.

Which room is actually being assessed

Definitions do real work here.

A winter garden or enclosed balcony is not normally a habitable room. It is unheated, it is not counted in the gross internal area for space standards in most authorities, and it is not the space whose daylight the standards are concerned with. So the assessment target is the living room or bedroom behind it, reached through the enclosure.

The mirror image arises on the neighbour side. Where an existing house has a conservatory in front of a kitchen or dining room, the conservatory itself is usually excluded from assessment, because the tests focus on habitable rooms. But the room behind it may still need assessing, and that assessment has to pass light through the conservatory. Reports often stop at the conservatory and declare no habitable room affected, which is a shortcut an alert objector will identify.

Where a scheme relies on the enclosure as private amenity space, check the local policy wording. Some authorities accept a winter garden towards amenity provision only where it meets stated dimensions, has a specified proportion of openable area, or is additional to a minimum of open balcony or terrace. That is a policy question rather than a technical one, and it is worth resolving at pre-application stage.

Policy resistance to habitable rooms behind glazing

Officers and inspectors tend to be sceptical of layouts where the only window to a habitable room opens into an enclosed space, and the scepticism is reasonable.

Three concerns recur. Daylight to the room is materially reduced, as the arithmetic above shows. Outlook is mediated by a second layer, which affects the quality of the living environment rather than any measurable metric. And purge ventilation and summer comfort depend on the occupant opening an outer screen that may be left closed for noise or security reasons.

The stronger schemes answer all three rather than only the daylight one. Dual aspect layouts, shallower rooms, a proportion of the facade left as open balcony, and generous openable area in the screen all help. Where a single aspect room behind an enclosure is unavoidable, say so and explain the compensating features rather than presenting the layout as unremarkable. Officers respond better to an acknowledged constraint than to an assessment that reads as though the enclosure were free.

Internal daylight comparisons between the proposed flats and neighbouring properties often expose this, and the framing set out in daylight inside new flats compared with neighbours is a useful way to present it honestly.

Reporting both conditions

The reporting convention that keeps everyone out of trouble is to present two sets of results for every affected room.

1. The condition with the enclosure in place and closed, modelling both glazed layers with stated transmittance and frame factors.
2. The condition with the outer screen open or absent, so the effect of the enclosure itself is visible.
3. For neighbour assessments, the with and without balcony pair, so the change caused by the new obstruction is separated from the existing geometry.
4. The assumptions schedule: transmittance for each layer, frame factors, maintenance factor, surface reflectances inside the enclosure, and the reference plane used.
5. A note of which condition each figure in the summary table describes, on the same page as the table.

That is more work than a single set of numbers, and it is what makes the assessment defensible when a consultee asks how the enclosure was treated.

PlanScope drafts a Daylight and Sunlight Assessment structured to the tests councils apply, with the modelling inputs listed as fields to complete rather than assumed, so the specialist supplies the geometry and the results. Councils normally expect a suitably qualified author for this kind of assessment. Reports should be reviewed before submission, and PlanScope does not guarantee planning approval.

This is general guidance, not legal advice. Check the current BRE guidance and your authority's amenity policies before relying on it.