Climate based daylight modelling against the BRE method

Climate based daylight modelling planning work answers a different question from the traditional sky component tests, and conflating the two is the most common error in daylight submissions. The BRE 209 neighbour tests measure what a proposal takes away from existing windows. Climate based daylight modelling measures how much daylight the rooms inside the new building will actually receive, hour by hour, across a year of real weather data. One is an impact assessment. The other is a design quality assessment.

Both belong in most applications on urban sites, and the 2022 edition of BRE 209 is explicit that internal daylight should now be assessed using the illuminance-based approach of BS EN 17037 rather than the average daylight factor method practitioners used for decades.

What each method measures

The neighbour tests are geometric. Vertical sky component measures the proportion of unobstructed sky visible from the centre of a window, expressed as a percentage, and the guidance treats 27 per cent as the reference value, with a retained value of at least 0.8 times the former figure where the 27 per cent threshold is not met. The no sky line test looks at how much of the room's working plane can see any sky at all. Annual probable sunlight hours looks at sunlight rather than skylight, with reference values of 25 per cent of annual hours including at least 5 per cent in the winter months.

None of those produces a lux figure, and none of them accounts for orientation, glazing specification or internal reflectance. That is a feature rather than a defect. A test designed to compare before and after at a neighbour's window should not depend on what that neighbour has painted their walls.

Climate based daylight modelling takes building geometry, surface reflectances, glazing transmittance and an hourly climate file, and calculates illuminance on a grid at working plane height for every daylit hour of the year. The output is the proportion of the room achieving a target illuminance for at least half the daylight hours.

Illuminance targets against sky component ratios

BS EN 17037 sets levels of daylight provision, and its UK national annex applies reduced target illuminances for dwellings, recognising that a flat is not an office. The annex works to room-specific targets, with bedrooms at the lowest figure, living rooms higher and kitchens higher still, achieved over at least half the assessment area for at least half the daylight hours.

Note the last row. BS EN 17037 offers a simplified daylight factor route as well as full modelling, and for a simple room with modest obstruction it gives the same answer for far less effort. Reserve the hourly modelling for the cases where it changes the result.

Where authorities accept the newer method

Since the 2022 edition of BRE 209 endorsed it, the illuminance approach has become the expected method for internal daylight in new residential schemes, and some local guidance now requires it outright. Officers in authorities dealing with high density housing are generally familiar with it. Officers in districts dealing mainly with householder applications may not be, and a report full of unexplained illuminance grids will slow determination.

Two practical consequences. First, write the report so that a reader who has only ever seen daylight factor percentages can follow it, with a plain statement of what the target is and what the result means. Second, do not use the newer method for the neighbour assessment. The guidance's impact tests remain the sky component and sunlight hours tests, and substituting an illuminance comparison at a neighbour's window invites an objection that the wrong test has been applied. How local policies actually use the guidance is set out in local plan amenity policies and how they use BRE 209.

Inputs a modelling study needs

A climate based model is only as good as the assumptions, and the assumptions are what an objector's consultant will go after.

• Geometry modelled to the window reveal, including frame width, cill depth, balcony soffits and any brise soleil. A model built from centre-line glazing areas overstates daylight.
• Surface reflectances, stated rather than left at software defaults. The conventional values are around 0.7 for ceilings, 0.5 for walls, 0.2 for floors and 0.2 for external ground and obstructions.
• Glazing visible light transmittance for the specified unit build-up, not for the glass type, with a maintenance factor applied.
• The climate file, named, for the nearest representative location.
• Grid spacing and the offset from the room perimeter, both stated, because a grid that starts at the wall drags the result down and one offset too far flatters it.
• The assessment area definition for each room type, since a kitchen within an open plan living space is assessed differently from a separate kitchen.

List all of these in a table at the front of the report. Every argument about a daylight result comes back to one of them, and a report that states them openly is much harder to attack than one that buries them in an appendix.

Reporting both methods together

The structure that works keeps the two assessments physically separate.

1. Existing conditions and the surrounding windows identified, with a plan and a photographic record.
2. Neighbour impact: vertical sky component, no sky line and annual probable sunlight hours results in a table, window by window, with the retained ratios.
3. Internal daylight to the proposal: BS EN 17037 daylight provision by room, with the recommendation level being targeted stated at the outset.
4. Where rooms fall short, the mitigation considered and the reason the residual position is acceptable.
5. The assumptions table.

Do not average results across a block to produce a headline compliance percentage. Officers and inspectors look at the rooms that fail, and a scheme where 94 per cent of rooms pass is judged on the other 6 per cent. Where those rooms are single aspect, the options are the ones described in daylight to single aspect flats and how to fix them.

Where the results diverge and why

The methods disagree in predictable ways, and knowing which way to expect is useful at design stage.

Orientation is the main one. The old average daylight factor method ignored it entirely, so a north-facing and a south-facing room with identical geometry scored identically. Climate based modelling does not, so south and west facing rooms improve and north facing rooms get worse relative to the old method.

Deep plans get worse under the newer method. A room 7 metres deep with a single window at one end may have scraped an average daylight factor target while leaving the back third effectively unlit, and an illuminance grid exposes that immediately.

External obstruction geometry matters more. A tall building directly opposite reduces illuminance across the whole year in a way that a single sky component figure averages away.

And there is a sting. Rooms that improve most under climate based modelling, the south and west facing ones with generous glazing, are the rooms most likely to fail an Overheating Risk Assessment under Approved Document O. Optimising one in isolation makes the other harder, which is why the two studies should be run by the same team on the same geometry.

Agreeing the method with the officer

Do this at pre-application and get it in writing. A one-page method statement is enough: which standard and edition, which recommendation level is being targeted, the software, the climate file, the reflectances, the grid, the glazing transmittance, and the confirmation that neighbour impact will be assessed on the guidance's own tests.

The reason this matters is practical. Redoing a daylight study because the authority expected a different method costs weeks, and on a scheme with a committee date it can cost a cycle. Where an objector has already engaged a consultant, an agreed method also removes the easiest line of attack, which is that the applicant chose the test that suited the answer. The evidence that actually carries weight in that argument is covered in objecting on daylight grounds.

A Daylight and Sunlight Assessment that states its method, its assumptions and its failures openly is worth more at committee than one with better numbers and no audit trail.

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This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. Councils usually expect daylight and sunlight work to be prepared by a suitably qualified author, and 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.