Worked internal daylight check for a lower ground flat
A worked internal daylight check for a lower ground floor flat lit from a light well, with every input stated, the result against BS EN 17037 and average daylight factor, and the design changes that matter.
A lower ground floor flat lit from a light well will usually fail an internal daylight check unless the room is shallow, the glazing is wide and the surfaces are pale. The measure that decides it is target illuminance under BS EN 17037, or, where the authority still asks for it, average daylight factor. This is a worked internal daylight check for one room, with every input stated so the arithmetic can be followed and argued with. The figures are illustrative.
Internal daylight is a different question from neighbour amenity. BRE 209 tells you what the new building does to the windows next door. BS EN 17037 tells you whether the new flat is a decent place to live.
Choosing the assessment measure
Three measures are in circulation and councils ask for different ones.
BS EN 17037:2018 with its UK National Annex is the current standard. It works in target illuminance: a space meets the recommendation where the target level is achieved over a stated fraction of the reference plane for at least half the daylight hours. The National Annex gives dwelling values of 100 lux for bedrooms, 150 lux for living rooms and 200 lux for kitchens, which are the numbers most London boroughs quote back at you.
Average daylight factor, from the withdrawn BS 8206-2, is the older measure and remains in use because it can be calculated by hand. The familiar criteria are 1 per cent for bedrooms, 1.5 per cent for living rooms and 2 per cent for kitchens.
Climate-based daylight modelling sits behind the illuminance approach and needs software and a weather file. For a single room on a small scheme the hand calculation is often enough to show whether the design is close or hopeless, and that is what this example does.
Ask the authority which measure it validates against before you model anything. Some validation lists still name average daylight factor, and submitting only an illuminance result will get a request for the other one.
Room geometry and glazing inputs
The room in this example is a combined living, kitchen and dining space in a lower ground floor flat, opening onto a light well at the front.
- Room 4.5 m wide, 6.0 m deep, 2.5 m floor to ceiling.
- One window opening 2.4 m wide by 1.8 m high, giving 4.32 m2 of glazed area net of frames.
- Light well 2.0 m wide, with the retaining wall 2.4 m high above the finished floor level of the room.
- Double glazing with a diffuse transmittance taken as 0.68 after allowing for the frame and for dirt.
Two of those inputs do most of the work. Room depth of 6.0 m against a single window on one wall is a lot of depth, and the 2.0 m light well width against a 2.4 m wall height is a tight ratio.
Measure the glazed area from the glass, not from the structural opening. Overstating it by including the frame is the single most common error in submitted calculations, and a reviewer will spot it from the window schedule.
The effect of the external obstruction
The visible sky angle from the centre of the window, measured in the vertical plane, is what converts a large window into a small one. With a light well 2.0 m wide and a wall 2.4 m above the room floor, the angle of visible sky measured from the centre of the window comes to roughly 25 degrees in this example.
That figure is the honest killer in basement schemes. A room at ground level with the same window in an unobstructed elevation would see something in the region of 65 degrees, so the same glazing delivers a fraction of the daylight.
Three things change the angle: widening the light well, lowering the retaining wall, or raising the window head. Only the first two usually have any real effect, because the head is generally limited by the structure above.
Where the light well is bounded by a neighbour's wall or a pavement above, the geometry is fixed and the answer is a different plan rather than a different window.
Surface reflectances and being honest about them
The calculation is sensitive to the area-weighted mean reflectance of the room surfaces, and that sensitivity is regularly abused. A submission assuming 0.8 on the walls, 0.9 on the ceiling and 0.5 on the floor is describing a room nobody will ever furnish.
Use defensible values. A ceiling at 0.70, walls at 0.50 and a floor at 0.20 gives an area-weighted mean in the region of 0.5 for a room of these proportions, and that is what this example uses. If the design does depend on pale finishes, say so and offer a condition securing the internal finishes, which councils do accept on basement and rear infill flats.
State the reflectances you used in the report. A result quoted without them cannot be checked and will be treated as unverified.
Results against the target criteria
Average daylight factor for this room, using the standard formula with diffuse transmittance, glazed area, visible sky angle, total internal surface area and mean reflectance, works out at approximately 0.9 per cent. The total internal surface area of the room is about 106.5 m2 and the reflectance term reduces the denominator by a quarter.
Against a 1.5 per cent criterion for a living space, that is a clear fail. Against 2 per cent for a kitchen, which this room also is, it is a comfortable fail.
| Input | Value used | Effect if improved |
|---|---|---|
| Glazed area | 4.32 m2 | Roughly proportional |
| Visible sky angle | 25 degrees | Roughly proportional |
| Room depth | 6.0 m | Increases surface area, dilutes the result |
| Mean reflectance | 0.50 | Non-linear, gains fall away above 0.6 |
| Transmittance | 0.68 | Small, and limited by glazing choice |
Read down that table before touching the model. It tells you that doubling the light well width or doubling the glazing will each move the answer far more than repainting the walls.
Design changes that actually move the number
In order of effect on a room like this one:
- Reduce the depth of the habitable room. Splitting a 6.0 m deep space into a 4.0 m daylit living area with utility and circulation behind it changes the result more than anything else.
- Widen the light well. Going from 2.0 m to 3.0 m raises the visible sky angle materially, and it also improves the outlook, which is a separate policy consideration.
- Widen the glazing, ideally full width, with a slim frame and a high head.
- Add a rooflight where any part of the room sits under an external area, which brings in light from directly overhead where the sky is brightest.
- Lighten the internal finishes, and secure them by condition if the case depends on them.
A single storey rear extension over part of the room is the change that is usually proposed and usually makes things worse, because it removes the only unobstructed sky the room had.
Where daylight becomes a refusal reason and the scheme is otherwise sound, the case has to be argued on the standard's own terms rather than on assertion, and an appeal statement of case for a householder refusal shows how that evidence is structured.
Reporting the check
The report should let a reviewer redo the sum. State the measure and the edition of the standard, the room use and dimensions, the glazed area net of frames, the transmittance and its basis, the visible sky angle and how it was derived, the reflectances by surface, the total internal surface area, the result, and the criterion applied.
Add a plan and a section with the light well geometry dimensioned. Add a sentence on overheating, because glazing changes that improve daylight can push a south-facing room the wrong way under Approved Document O, and an Overheating Risk Assessment may be needed alongside.
Where the room fails, say so and set out the mitigation. A report that reaches a failing result honestly, with a design response, is treated far better than one that reaches a pass on reflectances nobody believes. The same discipline applies to any worked calculation submitted with an application, and a worked greenfield run-off rate for a 0.6 hectare site follows the same pattern of stating every input.
A full Daylight and Sunlight Assessment is expected from a suitably qualified author, and professional review is strongly advised where the result is contested. You can draft the assessment narrative and the Design and Access Statement in PlanScope, then send them for review before submission.
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.
Frequently asked questions
Which standard applies to internal daylight in a new flat?
BS EN 17037:2018 with its UK National Annex is the current standard and works in target illuminance, with dwelling values of 100 lux for bedrooms, 150 lux for living rooms and 200 lux for kitchens. Average daylight factor from the withdrawn BS 8206-2 is still named on some validation lists, so check which measure your authority assesses against.
How is internal daylight different from a BRE 209 assessment?
BRE 209 deals with the effect of a new building on daylight and sunlight reaching neighbouring windows, using vertical sky component and annual probable sunlight hours. Internal daylight asks whether rooms inside the new building receive enough light. A scheme can pass one and fail the other, and most planning submissions need both questions answered.
What reflectance values are defensible?
A ceiling at 0.70, walls at 0.50 and a floor at 0.20 gives an area-weighted mean around 0.5 for a normally proportioned room, and that is accepted. Values above 0.8 on walls describe a room nobody furnishes. If the design depends on pale finishes, state that and offer a condition securing the internal finishes.
What improves daylight in a basement flat the most?
Reducing the depth of the habitable room and widening the light well, in that order. Both change the geometry rather than the surface treatment, and the visible sky angle is roughly proportional to the result. Wider glazing with a high head helps, a rooflight under an external area helps, and repainting the walls helps least.
Do daylight improvements create an overheating problem?
They can. Larger glazing areas on south and west elevations raise solar gain, and a scheme that fixes daylight can then fail the overheating check under Approved Document O. Test the two together rather than in sequence, and where glazing has grown, expect an overheating assessment to be asked for alongside the daylight work.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.