Using BS EN 17037 targets alongside the BRE guidance
The illuminance based approach in BS EN 17037 and the obstruction measures in the BRE guidance answer different questions. This explains how to use both in one assessment and report them to an officer who expects one method.
BS EN 17037 tells you whether rooms in the building you are proposing will receive enough daylight. The Building Research Establishment guidance on site layout planning tells you what your building does to daylight and sunlight reaching other people's windows and gardens. Both belong in a planning assessment, and they answer different questions.
Confusion arises because the 2022 edition of the BRE guidance adopted the illuminance based approach from the standard for assessing interior daylight, replacing the average daylight factor method that most older supplementary planning documents still cite.
What the standard measures
BS EN 17037 assesses daylight provision by illuminance on a horizontal reference plane inside the room, typically taken at desk height. The criterion is whether a target illuminance is achieved over a defined fraction of the assessed area for a defined proportion of daylight hours across the year, using climate data for the location.
The UK National Annex sets minimum target illuminance values by room type, with lower values for bedrooms, higher for living rooms and higher again for kitchens, to be met over half the assessed area for at least half of daylight hours. Check the current published values rather than quoting from memory, because they are set out in the annex and are easy to misremember.
The standard also deals with sunlight exposure, view out and protection from glare, which the BRE guidance does not cover for new interiors. View out is the criterion most often left out of UK submissions, and it is the one a design team can influence cheaply at sketch stage by widening a window rather than deepening a reveal.
How it differs from the daylight factor approach
Average daylight factor expressed daylight as a ratio between internal and external illuminance under a standard overcast sky. It was simple, it could be hand calculated, and it took no account of orientation, of local climate or of the distribution of light within the room.
The illuminance approach uses annual climate data, so a north facing room and a south facing room with identical geometry give different results. It also reports distribution across the room rather than a single average, which shows up deep plan rooms that an average would flatter.
| Average daylight factor | BS EN 17037 illuminance | |
|---|---|---|
| Basis | Ratio to an overcast sky, no orientation | Annual climate data for the location |
| Output | One figure per room | Illuminance across a grid, reported as area and time met |
| Orientation | Ignored | Reflected in the result |
| Deep rooms | Can appear acceptable | Shortfall at the back of the room is visible |
| Still cited by | Many older local supplementary documents | The 2022 BRE guidance and newer policy |
The practical consequence is that a scheme assessed under the old method and the new one can produce different answers for the same room, and an officer holding a supplementary document from 2014 may expect the older figure.
Which assessment answers which question
Keep the two jobs separate in your head and in the report.
Effects on neighbours are assessed with the BRE measures: vertical sky component and the no sky line for daylight, annual probable sunlight hours for sunlight, and shadow studies for gardens and amenity space. These are exterior or geometric measures and they have not been replaced.
Daylight inside the proposed building is assessed with the illuminance criteria. This is where the standard applies, and it is the part of the assessment that has changed.
Sunlight to proposed amenity space is a third question again, usually answered with a shadow study rather than either method, and an Overshadowing Study is the right document where amenity space rather than windows is the issue.
Running both without contradicting yourself
Three assumptions have to be consistent across the whole assessment, and most contradictions come from getting one of them wrong.
Room use. A space modelled as a bedroom for the illuminance test and described as a living room in the neighbour assessment is an obvious inconsistency, and the effect of these assumptions is covered in room use assumptions and how they change results.
The baseline massing. Surrounding buildings modelled in one analysis should appear in the other, at the same heights, from the same survey.
Surface and glazing properties. Reflectances and glazing transmittance drive the illuminance results, and using optimistic values that the specification will not deliver is the fastest way to lose credibility. State the values used and where they came from.
Declare the climate file, the reference plane height, the grid spacing and the maintenance factor. An officer or a third party reviewer will ask, and having the answers in an appendix saves a round of correspondence.
Reporting to an officer who expects one method
Lead with the method the local policy names, then give the other.
If the supplementary document asks for average daylight factor, report it, then present the illuminance results as the current approach adopted by the 2022 BRE guidance and explain what they add. Do not simply refuse to provide the older figure because it is superseded. Officers are constrained by their adopted documents.
If the policy is silent, use the illuminance criteria and say why.
Where the two methods disagree about a particular room, say so plainly and explain which result better describes the condition. A report that quietly presents only the flattering method reads as advocacy, and reviewers check.
Presentation matters as much as method here, and the practical format for a committee audience is set out in presenting daylight results a committee can read.
Choosing the method at the outset
Decide before the massing is fixed, because the tests reward different things. Illuminance criteria reward window area, room proportion and orientation. Obstruction measures reward setback, angle and separation distance.
Run a coarse test at concept stage. A simple obstruction angle check will tell you whether a courtyard is going to work long before anyone models it properly, and the approach is described in obstruction angles and the early massing test.
Then agree the method with the case officer at pre-application stage and record the agreement in writing. Ask two specific questions: which document the authority applies to interior daylight, and whether it has adopted local targets that differ from the national ones. A one line email answer to each is worth a great deal later.
One honest caveat. Neither approach tells you whether a room will feel bright. Both are proxies, tuned to different aspects of the problem, and a room that passes on illuminance can still be gloomy if the window is high, narrow and north facing behind a deep reveal. Where a layout is marginal, look at the section drawing rather than the spreadsheet. Method disputes discovered at week eleven of a thirteen week determination are expensive, and the Daylight and Sunlight Assessment is not a document you want to rerun under time pressure.
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. For specialist technical reports, qualified professional input may be required.
Frequently asked questions
Has BS EN 17037 replaced the BRE guidance?
No. The 2022 edition of the BRE site layout guidance adopted the illuminance based approach of the standard for assessing daylight inside new rooms, but its measures for effects on neighbours, including vertical sky component, the no sky line and annual probable sunlight hours, remain in use and are unchanged in purpose.
What target illuminance does the UK National Annex recommend?
It sets minimum target illuminance values by room type, lowest for bedrooms and highest for kitchens, to be achieved over half the assessed area for at least half of daylight hours across the year. Check the published annex for the current figures rather than relying on a quoted number, since they are frequently misremembered.
Which method should I use if the local policy names the old one?
Provide the method the adopted policy names, then present the illuminance results as the current approach and explain what they add. Officers are bound by their adopted documents, so refusing to supply the older figure creates an unnecessary argument. Say clearly where the two methods reach different conclusions and why.
Why do the two approaches give different answers?
Average daylight factor uses a standard overcast sky and ignores orientation, producing one figure for the room. The illuminance approach uses annual climate data and reports how light is distributed across the room. Deep plan rooms and north facing rooms are the cases where the difference shows up most clearly.
What assumptions should the report declare?
The climate file used, the reference plane height, the grid spacing, the maintenance factor, internal surface reflectances, glazing transmittance, and the room uses assumed. Reviewers ask for all of these, and putting them in an appendix avoids a round of correspondence part way through determination.
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.