Climate-based daylight modelling (CBDM) predicts how much real daylight a room receives across a whole year, using local weather data rather than a single overcast sky. Instead of one static figure, it tells you what the light is actually doing in January, in June, at 9am and at 4pm — and that is why an increasing number of UK planning authorities now ask for it alongside the traditional external tests.
If you have commissioned a daylight report and seen terms like sDA, daylight autonomy or useful daylight illuminance on the front page, this guide explains what they mean, how they are calculated, and where they sit within BS EN 17037 and the 2022 BRE guidance.
What is climate-based daylight modelling?
Traditional internal daylight assessment in the UK relied on the average daylight factor (ADF) — a ratio of indoor to outdoor light measured under a standardised, permanently overcast CIE sky. It is simple and repeatable, but it ignores orientation, season and the path of the sun entirely. A north-facing room and a south-facing room with the same window area score the same ADF, which is plainly not how daylight behaves.
Climate-based daylight modelling takes a different approach. It uses a Typical Meteorological Year (TMY) weather file for the specific location — hour-by-hour records of sun position, cloud cover and sky brightness — and simulates daylight at thousands of points on the working plane across all 8,760 hours of the year. The output is a realistic, time-varying picture of illuminance, expressed in lux, that responds to which way the building faces and how the seasons change.
This performance-based method is closely tied to BS EN 17037, the European daylight standard that the UK adopted. The 2022 edition of the BRE's Site layout planning for daylight and sunlight (BR 209) was rewritten partly to align with it, encouraging designers to assess internal daylight using illuminance targets rather than ADF alone. For a refresher on the wider metrics, see our explainer on VSC, NSL and APSH and the article on the BRE 2022 daylight provision tests.
Spatial daylight autonomy (sDA) explained
Daylight autonomy (DA) is the percentage of occupied hours in a year during which a single point in a room receives at least a target level of daylight — commonly 300 lux — without needing the lights on. A point with 80% daylight autonomy is, in effect, daylit for four-fifths of the working day across the year.
Spatial daylight autonomy (sDA) scales that idea up from a point to a whole space. It reports the percentage of floor area that meets the daylight-autonomy threshold. The most common benchmark, written sDA 300/50%, asks: what proportion of the room receives at least 300 lux for at least 50% of occupied hours? A result of, say, sDA 300/50% = 65% means almost two-thirds of the floor is comfortably daylit for half the working year or more.
BS EN 17037 frames internal daylight provision around target illuminance reached across a fraction of the reference plane for a fraction of daylight hours, and CBDM is the natural engine for testing it. In some sectors the thresholds are firm: for new English schools, the Department for Education's output specification asks for sDA 300/50% of at least 50% across the working plane during occupied hours. In planning more broadly the targets are applied as best practice rather than hard law, but a scheme that demonstrably hits them is far easier to defend.
Useful daylight illuminance and the other metrics
CBDM produces a family of related figures, and reports often quote more than one:
- Useful daylight illuminance (UDI): the percentage of time daylight falls within a “useful” band — typically 100–300 lux as supplementary, 300–3,000 lux as autonomous — with anything above flagged as potential over-lighting and glare risk. UDI is valued because it captures too much light, not just too little.
- Annual sunlight exposure (ASE): the percentage of floor area receiving direct sun above a set threshold for too many hours, used as a glare and overheating warning that balances the daylight-maximising pull of sDA.
- Daylight factor (median): BS EN 17037 also allows a daylight-factor route, but expressed as a median target tied to the local climate rather than the old ADF.
Read together, sDA tells you whether there is enough daylight, UDI and ASE tell you whether there is too much in the wrong way, and the assessor balances the two. Maximising glazing to lift sDA can push ASE into glare territory, so good design tunes window size, orientation and shading rather than simply adding more glass.
When do UK planners ask for CBDM?
CBDM is most often requested for the internal daylight of new homes, schools and offices — particularly in dense urban schemes where deep floor plates, balconies and tight back-to-back distances make simple metrics unflattering. It is increasingly common to see a report that runs the external impact tests (VSC, APSH, overshadowing) on neighbouring properties and a CBDM study of the proposed interiors.
You are more likely to be asked for it when: a local validation checklist or design guide cites BS EN 17037; the scheme is a higher-density flatted development; rooms are single-aspect or north-facing; or an officer has queried the liveability of the proposed units. If your design is struggling against the simpler tests, a CBDM study can sometimes show the daylight is genuinely adequate in use even where a crude metric suggests otherwise — the kind of evidence that matters at appeal. For the converse situation, our guide to office-to-residential conversions covers the deep-plan problem in detail.
One practical caveat: CBDM is a modelling exercise, and the result depends on honest inputs — realistic reflectances, glazing transmittance, external obstructions and room use. A report that quietly assumes white walls and crystal-clear glass everywhere can flatter a scheme. A credible study states its assumptions plainly.
How Fortress Associates can help
At Fortress Associates we prepare daylight and sunlight reports to BRE BR 209 (2022) and BS EN 17037, including climate-based daylight modelling where your local authority or design demands it. We turn assessments around in 4–5 working days, ask for no advance payment, and work with architects, developers and homeowners across the UK. If you are not sure whether your scheme needs a CBDM study or a conventional assessment, that is exactly the kind of question we will answer before you commit. Learn more about our daylight report service, see the full list of what we offer on our services page, or get in touch with the details of your project for a clear, no-obligation steer.
Sources & further reading
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