Target illuminance and the current daylight standard
BS EN 17037 measures daylight as illuminance achieved for a fraction of the year, not as a single ratio. Here is how the target and minimum criteria work, what the UK National Annex asks for, and how to report both methods.
BS EN 17037 measures daylight as illuminance on a horizontal plane inside the room, achieved for a stated fraction of the year, rather than as a single ratio between inside and outside. That change of unit is what catches people out. A target illuminance of 200 lux over half the room for at least half the daylight hours is a different kind of statement from an average daylight factor of 2 per cent, and the two are not interchangeable in an argument with a case officer.
The standard covers four things: daylight provision, sunlight exposure, view out and protection from glare. Most planning submissions engage the first two.
What the standard measures and over what period
Daylight provision is assessed on a reference plane, usually taken at working plane height above the floor, with a border adjacent to the walls excluded from the grid. The assessment period is the daylight hours across a full year, using hourly climate data for the location.
Two criteria run together. A target illuminance has to be met across at least half the reference plane for at least half the daylight hours, and a minimum target illuminance has to be met across at least 95 per cent of the plane for the same fraction of the hours. A room can pass the first and fail the second where daylight is concentrated near the window and falls away sharply at the back, which is exactly the geometry of a deep single-aspect flat.
The standard sets three recommendation levels, described as minimum, medium and high, each with its own pair of illuminance values. Stating which level you have assessed against is not optional, because the difference between them is large.
Target and minimum values by room type
For dwellings the UK National Annex gives values by room, and those values are lower than the main body of the standard's minimum recommendation. They follow the room-type distinction that UK practice has used for decades, with bedrooms treated least demandingly and kitchens most.
| Room in a dwelling | Target illuminance over 50 per cent of the plane |
|---|---|
| Bedroom | 100 lux |
| Living room | 150 lux |
| Kitchen | 200 lux |
Read the National Annex itself before relying on those figures in a report, because it also sets the corresponding minimum values and explains how combined spaces are treated. Where a room serves more than one function, the more demanding value applies to the relevant part, and a kitchen and living space in one room is the case that arises on nearly every apartment scheme.
For non-residential rooms the National Annex does not give the same room-by-room table, so the recommendation levels in the standard apply, and the assessed level should be justified against the use of the space.
Climate-based modelling and what it needs
Meeting the standard properly means climate-based daylight modelling. The model needs the room geometry, surface reflectances, glazing transmittance including any frame and any external obstruction, and an hourly climate file for the location.
Reflectances are where results move most. Using default light surfaces for a room that will be finished in dark timber produces a pass that the finished flat will not deliver, and a reviewer who asks for the reflectance schedule will find it. State the values used for ceiling, walls and floor, the glazing transmittance, and the maintenance factor if one is applied.
External obstruction has to include what is consented rather than only what is built. Modelling a light well against a neighbouring site with an unimplemented permission for four more storeys gives an answer with a short shelf life. Our note on daylight inside new flats compared with neighbours covers how the two halves of that argument interact.
Where the older daylight factor is still asked for
Average daylight factor targets of 1 per cent for bedrooms, 1.5 per cent for living rooms and 2 per cent for kitchens come from BS 8206-2, which has been withdrawn. They persist in local plan policies, in validation lists and in officers' expectations, because they are quick to calculate and easy to compare.
The standard itself allows a daylight factor route, converting a target illuminance into a daylight factor using the median external diffuse horizontal illuminance for the location. And that route is legitimate for early design. But it loses the seasonal and orientation information that the illuminance method carries, which matters most for north-facing rooms. Our note on daylight to a north facing habitable room sets out why.
BRE 209 remains the reference for effects on existing neighbouring buildings, using vertical sky component and the annual and winter probable sunlight hours tests. It is not a competitor to BS EN 17037, because the two answer different questions. One asks whether the new rooms are adequately lit, the other asks what the new building does to the old one.
Reconciling the two approaches in one report
Where a policy asks for daylight factor and you have modelled illuminance, report both and say which you regard as governing. The order that reads best is a short statement of method, the illuminance results room by room against the National Annex values, the daylight factor results for the same rooms, and then a paragraph explaining any divergence.
Divergence usually has a physical cause. A room with a large north-facing window can meet a daylight factor target while sitting below the illuminance target, because diffuse sky light is counted the same way at all orientations in the factor method. Saying that plainly is more persuasive than presenting only the favourable number.
Sunlight exposure under the standard is assessed as hours of direct sun on a chosen date between 1 February and 21 March, with a minimum recommendation of one and a half hours. That is a different test from the annual and winter sunlight measures used for neighbouring windows, and from the amenity space test on 21 March.
What an authority actually reads first
Case officers rarely read the modelling chapter. They read the summary table and look for rooms that fail, then they look for the explanation.
So build the report around a single table listing every assessed room, its use, the criterion applied, the result, and pass or fail. Put the failures in it rather than leaving them to be discovered. Then, for each failure, give the reason, the design change considered, and why the remaining shortfall is acceptable given what the scheme delivers.
A few practical points that improve the reception of the work:
- Name the software and the climate file, with the version and the weather station.
- Give the reflectance and transmittance schedule in the appendix.
- State the recommendation level assessed against, and do not mix levels between rooms without explaining why.
- Show the modelled obstruction, including consented but unbuilt neighbouring schemes.
- Keep the neighbour effects in a separate section so the two questions do not blur.
The Daylight and Sunlight Assessment page sets out what a PlanScope draft covers and where a reviewer has to add modelling output, because illuminance results come from software rather than from text. For amenity areas and sun on the ground, an Overshadowing Study is the separate piece of work.
This is general guidance, not legal advice. Daylight modelling is specialist work and councils expect a suitably qualified author, so professional review is strongly advised. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
What is target illuminance in BS EN 17037?
It is the illuminance level that has to be achieved on the reference plane inside the room for at least half the daylight hours across the year, over at least half the plane's area. A second criterion, the minimum target illuminance, has to be met over at least 95 per cent of the plane for the same fraction of hours.
What values does the UK National Annex give for homes?
For dwellings it sets values by room type, with bedrooms treated least demandingly and kitchens most. The commonly cited targets are 100 lux for bedrooms, 150 lux for living rooms and 200 lux for kitchens over half the reference plane. Read the Annex itself for the corresponding minimum values and the treatment of combined spaces.
Is average daylight factor still acceptable?
Many local plan policies and validation lists still refer to daylight factor, and the standard permits a factor route by converting the illuminance target using median external diffuse illuminance for the location. The older 1, 1.5 and 2 per cent targets come from the withdrawn BS 8206-2. Report both methods where a policy asks for the factor.
Does BS EN 17037 replace BRE 209?
No. BS EN 17037 assesses whether the rooms in the new development receive adequate daylight and sunlight. BRE 209 assesses what the new building does to daylight and sunlight at existing neighbouring windows and amenity areas, using vertical sky component and probable sunlight hours. Most planning submissions need both, in separate sections.
What inputs will a reviewer ask to see?
Room geometry, surface reflectances for ceiling, walls and floor, glazing transmittance, any maintenance factor, the modelled external obstruction including consented but unbuilt schemes, the software and version, and the climate file with its weather station. Optimistic reflectances are the most common reason a set of results is challenged.
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.