Daylight in new homes: BS EN 17037 illuminance targets
BS EN 17037 asks whether the homes you are building have enough daylight of their own, using climate-based modelling against lux targets. Here are the UK National Annex figures, the model inputs that matter and the geometries that cause failures.
BS EN 17037 daylight assessment answers a different question from the one BRE 209 answers. BRE 209 asks whether your scheme takes light from the neighbours. BS EN 17037 asks whether the homes you are building will have enough daylight of their own. Councils increasingly want both, and an application that submits only the first is answering half the question.
The UK National Annex to the standard sets the numbers that matter for dwellings: 100 lux in bedrooms, 150 lux in living rooms and 200 lux in kitchens, achieved over at least half the space for at least half the daylight hours.
Why new dwellings are assessed differently
The neighbour tests are comparative. Vertical sky component and the no-sky line measure a change against an existing condition, so a room that was always dark can pass simply because the proposal does not make it much darker.
Provision inside a new dwelling has no before. There is only the question of whether the room, as designed, receives enough daylight. That requires an absolute target rather than a ratio, and it requires modelling of the actual room with its actual window under the actual sky conditions of the site's location.
The consequence for design is direct. A single-aspect flat behind a 2 m deep balcony, facing north, into a courtyard, can comply with every neighbour test ever devised and still be a poor home. The standard exists to catch exactly that.
Target illuminance, and the fraction of space and time
The method is climate-based. Rather than a single daylight factor, it asks how often a target illuminance is met across a reference plane, using an hourly climate file for the location across a full year.
Two fractions define the result. The fraction of the reference plane that reaches the target, and the fraction of daylight hours over which it does so. The standard's general recommendation is a target illuminance achieved over half the reference plane for at least half of daylight hours, with a lower minimum target illuminance achieved over almost all of the plane.
For dwellings the UK National Annex gives room-specific figures, and these are the ones British councils cite:
| Room type | Target illuminance | Fraction of space | Fraction of daylight hours |
|---|---|---|---|
| Bedroom | 100 lux | 50% | 50% |
| Living room | 150 lux | 50% | 50% |
| Kitchen | 200 lux | 50% | 50% |
Where a room serves more than one function, the higher target applies. A kitchen-living-dining room, which is most new flats, is assessed against the kitchen figure across the whole space unless the assessment justifies zoning it, and the justification has to be about how the space is used rather than about what passes.
Climate-based modelling and its inputs
The model is only as good as what goes into it, and reviewers check the inputs before the results.
- The climate file for the nearest appropriate location, named in the report.
- Room geometry from the proposed drawings, including floor to ceiling height and any change in ceiling level.
- Window geometry with frame factors, reveal depth, transoms and glazing bars modelled rather than approximated.
- Visible light transmittance for the specified glazing, which drops sharply with solar control coatings and triple glazing.
- A maintenance factor for dirt on the glass.
- Internal surface reflectances, with the values used stated.
- External obstructions in three dimensions, including the rest of your own scheme and its reflectance.
Visible light transmittance is where daylight and overheating pull against each other. Part O drives designers towards solar control glass and smaller openings, and every step in that direction reduces the daylight the same room receives. Say so in the report rather than presenting each in isolation, because a case officer reading an Overheating Risk Assessment and a daylight assessment that appear to describe different buildings will ask.
Balconies, overhangs and deep plans
Three geometries account for most failures.
A balcony above a window removes the upper sky, which is the brightest part. A 1.5 m deep balcony over a living room window can take a compliant room below the target on its own, and the effect compounds on lower floors where the obstruction opposite is also higher in the field of view.
Deep plans fail at the back. A room 8 m deep lit from one end will meet the target near the glazing and fail beyond the midpoint, so the fraction of space calculation decides it. Widening the window helps less than shortening the room.
Recessed windows and deep reveals, often introduced for solar shading or for elevational modelling, cut the visible sky at exactly the angles that matter. Courtyards do the same thing at building scale, and a narrow courtyard with dark brick on all four sides performs far worse than the same geometry with a pale render.
The design responses are known: shallower plans, dual aspect where possible, balconies set to one side of the window rather than over it, higher window heads, and lighter surfaces in courtyards. Most of them cost nothing if they are decided at the massing stage.
Where councils set their own thresholds
National policy does not set a daylight standard for new homes. Authorities pick it up through design policy, and in London the London Plan's housing design policies require daylight and sunlight to be assessed, with Greater London Authority guidance pointing at the current standard.
That leaves variation. Some boroughs expect full compliance with the National Annex targets and treat shortfalls as a design failure. Some accept a proportion of rooms falling short where the scheme is dense, urban and otherwise well designed. A few still ask for average daylight factor figures from the older approach, which is a different metric and cannot simply be converted.
Ask at pre-application which method and which thresholds the authority applies, and record the answer. Doing the wrong assessment well is an expensive mistake.
Reporting so an officer can use it
Case officers are not daylight specialists. Write for someone who has twenty minutes and needs to know whether the scheme is acceptable.
Lead with a table of every habitable room in the scheme, by unit type and floor, giving the room use, the target, the percentage of the reference plane meeting it and the percentage of daylight hours. Sort it so the failures are visible rather than buried. State the total number of rooms assessed and the number falling short.
Then explain the failures. Which rooms, why, what was tried, and what the design does about it. Where rooms fall short and the scheme is still put forward, make the case openly on the basis of the site context and the wider quality of the homes.
Keep the assumptions in one place: climate file, reflectances, transmittance, maintenance factor and the software used. And keep this assessment distinct from the neighbour work in a Daylight and Sunlight Assessment, because merging the two produces a document nobody can follow.
Browse the 66 report types to see which daylight, overheating and design documents your scheme is likely to need.
This is general guidance and 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.
Frequently asked questions
What are the UK daylight targets for new dwellings?
The UK National Annex to BS EN 17037 gives 100 lux for bedrooms, 150 lux for living rooms and 200 lux for kitchens, achieved over at least half the reference plane for at least half of daylight hours. Where a room serves more than one function, the higher target applies.
How does BS EN 17037 differ from BRE 209?
BRE 209 tests the effect of a proposal on existing neighbouring windows, comparing a before and after position. BS EN 17037 tests daylight provision inside the proposed dwellings against absolute targets. They answer different questions, and many councils now expect both to be submitted.
What is climate-based daylight modelling?
Modelling that uses an hourly climate file for the site's location across a full year, rather than a single daylight factor, to work out how often a target illuminance is met across a reference plane. It needs accurate room and window geometry, glazing transmittance, reflectances and external obstructions.
Why do balconies cause daylight failures?
A balcony above a window removes the upper part of the sky, which is the brightest. A deep balcony over a living room window can take an otherwise compliant room below the target on its own, and the effect is worse on lower floors where opposite buildings also sit high in the field of view.
Do daylight and overheating requirements conflict?
Often. Solar control glazing and smaller openings reduce overheating risk under Part O and also reduce the daylight the same room receives. Report the interaction rather than presenting the two assessments in isolation, because a reviewer comparing them will notice if they describe different buildings.
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.