Daylight in new flats and the target illuminance approach

Daylight in new flats is now assessed against illuminance targets rather than average daylight factor. The 2022 edition of BRE 209, Site Layout Planning for Daylight and Sunlight, adopted the approach in BS EN 17037:2018, which asks whether a stated illuminance is achieved over a proportion of the room for a proportion of the daylight hours in a year. That is a climate-based calculation run in software against a weather file, not a single static figure.

For a developer this changes what the report says and what the officer argues about. The old question was whether a room hit a daylight factor number. The new question is how much of the room reaches a target illuminance, for how much of the year, and whether the modelling assumptions behind that answer are defensible.

What replaced the average daylight factor

Average daylight factor expressed daylight as a ratio of internal to external illuminance under an overcast sky. It was quick, it ignored orientation entirely, and it treated a north-facing room and a south-facing room as identical.

BS EN 17037 replaced the ratio with target illuminance sustained over time. The standard's own recommendation is 300 lux over half the reference plane for at least half of daylight hours, with a minimum of 100 lux over 95 per cent of the area. The UK National Annex sets lower recommended values by room type, in the region of 100 lux for bedrooms, 150 lux for living rooms and 200 lux for kitchens, achieved over half the area for at least half of daylight hours.

Those National Annex values are what most English assessments use for residential schemes, and they are what a Daylight and Sunlight Assessment should state explicitly at the front. A report that gives results without naming the target it applied is not reviewable.

Reference planes, room types and combined spaces

The calculation grid sits on a horizontal reference plane, normally 0.85 metres above finished floor level and set back from the walls, covering the usable area of the room rather than the space behind furniture.

Combined kitchen, living and dining rooms are the recurring argument. The three functions carry different targets, and BRE 209 leaves judgement about how to treat a single space serving all three. Some assessors apply the kitchen target across the whole room, some assess the kitchen zone separately, and some apply the living room target on the basis that the space reads as a living room. All three appear in submitted reports. Agree the approach with the case officer at pre-application, because the difference decides compliance in a lot of flats.

Bathrooms, halls, stores and circulation are outside the assessment. So are rooms whose only opening is onto an enclosed balcony, which have to be modelled with the balcony glazing in place rather than as if the opening were external.

Sunlight is assessed separately and should not be dropped. BS EN 17037 sets an exposure target measured on a nominated date, and BRE 209 retains its own guidance on annual and winter probable sunlight hours for existing dwellings. A report that covers daylight in detail and gives sunlight one line will come back.

Balconies decide more cases than glazing area

An overhanging balcony above a window removes sky from the upper part of the view, which is the part doing the work. On a typical apartment block the recessed or projecting balcony is the single most common cause of a failing living room, ahead of window size, glazing specification and orientation.

Design responses that actually shift the result:

• Reduce balcony depth, or set the window forward of the slab edge
• Use glass or open metal balustrades rather than solid panels
• Increase window head height, since the top of the window contributes most
• Cut back the balcony over the window and keep the projection to one side
• Avoid stacking a deep balcony over a single-aspect north-facing living room

Single-aspect north-facing flats behind deep balconies fail on daylight and on sunlight together, and no amount of internal reflectance modelling rescues them.

Modelling assumptions an officer will check

The results depend on inputs, and a report that does not declare them invites a challenge. Five inputs matter.

Surface reflectances, usually taken as high for ceilings, medium for walls and low for floors where finishes are unknown. Assuming unrealistically pale finishes throughout inflates the result.

Glazing visible light transmittance. Clear double glazing sits well above a heavily coated solar control unit, and specifying performance glass late in design can quietly break the daylight case.

The maintenance or dirt factor applied to glazing.

Whether external obstructions were modelled from survey data, from consented schemes, or from what happens to be standing today. Consented but unbuilt neighbours should be modelled and the report should say whether they were.

The weather file used, and the definition of daylight hours applied to the calculation.

Where a scheme sits close to an existing building, the assessment of effects on that neighbour uses a different set of tests entirely, based on vertical sky component and sunlight availability, and a separate Overshadowing Study may be needed for amenity space. Do not let the two sets of numbers get mixed up in one table.

Reading the results room by room

A usable report gives a schedule of every habitable room, its use, its aspect, the target applied, the percentage of the reference plane meeting that target, and a pass or fail. Anything less than room-by-room reporting is a summary, and officers ask for the schedule anyway.

Expect some failures. A dense urban scheme with any depth to it will not achieve full compliance in every room, and BRE 209 is guidance rather than a standard to be applied mechanically. The argument that works is a proportionate one: how many rooms fail, by how much, which uses are affected, whether the failing rooms are bedrooms rather than living rooms, and what design measures were tried and rejected.

The argument that does not work is asserting that the site is constrained, without showing the alternatives tested. Officers see that version constantly.

Illustrative example: a six-storey block with 42 flats, of which nine are single-aspect and north-facing. Modelling shows those nine living rooms reaching the 150 lux target over roughly a third of the reference plane. Trimming the balcony depth on that elevation and raising window heads by 200 millimetres lifts most of them, and the Design and Access Statement records the change as a design response. That record is what makes the residual failures defensible.

What to submit and when

Model at the point the massing is fixed and before the planning drawings are frozen, not after. Retro-fitting daylight compliance into a consented envelope means either losing units or accepting failures.

Submit the assessment with the application, not in response to a validation query, and include the room schedule, the assumptions table, the plans showing modelled obstructions and the target values used. Where the scheme is in London, check the borough's own requirements and the Greater London Authority housing design guidance, both of which reference the BS EN 17037 approach.

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.