Average daylight factor and target illuminance compared

Average daylight factor and target illuminance answer different questions, and a scheme can pass one while failing the other. Average daylight factor measures how much of an overcast sky's light reaches a room, as a percentage, with no reference to orientation or climate. Target illuminance measures how many lux actually fall on a working plane across a real year in a real location. When an architect asks which one the council wants, the honest answer is that it depends on the authority, and on whether its housing policy has caught up with the current standard.

The split matters because the two methods can rank the same two flats differently. A north-facing room and a south-facing room with identical geometry have the same average daylight factor and very different annual illuminance.

What average daylight factor calculates

Average daylight factor is the ratio of internal illuminance to the simultaneous unobstructed external horizontal illuminance under the CIE standard overcast sky, averaged across the working plane and expressed as a percentage. The usual calculation applies the published formula relating net glazed area and visible sky angle to total internal surface area and average surface reflectance, with a transmittance factor for the glass.

Four inputs therefore drive the result: the net glazed area, the angle of visible sky at the window, the total internal surface area of the room, and the area-weighted average reflectance of the surfaces. Nothing in that list involves the sun, the compass or the weather.

The targets most often quoted come from the withdrawn British Standard on daylighting and are carried into the current site layout guidance: 2 per cent for kitchens, 1.5 per cent for living rooms and 1 per cent for bedrooms. Where a room combines functions, as most open plan flats do, the most demanding target is normally applied, which is why a kitchen-living-dining space is usually tested at 2 per cent.

Its virtue is that it is quick, inexpensive to calculate and easy for a case officer to check by hand. That is a serious practical advantage and explains why so many housing design guides still use it.

What target illuminance measures instead

The current European standard adopted in the United Kingdom, BS EN 17037 on daylight in buildings, replaces the single percentage with an illuminance criterion. Daylight provision is assessed by whether a target illuminance is achieved across a stated fraction of the reference plane for a stated fraction of the daylight hours, using either climate-based modelling or the standard's simplified daylight factor route.

The headline recommendation sets a target illuminance across half the reference plane together with a lower minimum across almost all of it, for at least half the daylight hours in the year. The United Kingdom national annex sets lower recommended targets for dwellings than the main body of the standard, differentiated by room type, with bedrooms lowest and kitchens highest. Verify the current figures against the published standard before writing them into a report, because the annex has been amended.

Two consequences follow. The output is a percentage of hours and area rather than a single number, so the reporting is more detailed. And because the calculation uses a climate file, orientation, local obstruction geometry and the actual sky conditions for the site all affect the answer.

Climate-based modelling and its inputs

Climate-based daylight modelling runs an hourly simulation across a full year using a weather file for the nearest representative location. The model needs:

• Room and building geometry, including reveal depths, balconies, overhangs and external obstructions modelled in three dimensions.
• Surface reflectances for walls, floor, ceiling, external surfaces and the ground.
• Glazing visible light transmittance for the specified product, not a generic value.
• A maintenance or dirt factor where one is applied.
• The reference plane height and the grid spacing.
• The daylight hours definition used.

Each of those is a place where an optimistic assumption can lift the result. Reflectance is the usual culprit: assuming white walls and a pale floor throughout is defensible for a show flat and less so for the building as handed over. A reviewer checking an illuminance assessment should go to the reflectance schedule and the glazing specification first.

Which measure a council asks for

Practice is mixed, and this is the part of the subject where a report needs to be explicit rather than assume.

Many adopted housing standards and design guides were written when the percentage method was the only one in general use, so they specify average daylight factor values by room type. Where a policy does that, the assessment has to report those values whatever else it contains. Other authorities, and the current site layout guidance itself, point towards the illuminance approach for new interiors while keeping the vertical sky component and no-sky line methods for assessing the effect of a proposal on existing neighbouring windows.

The safest approach on a scheme of any size is to agree the method at pre-application, in writing, and to report the percentage method as well where the adopted policy references it. A daylight and sunlight assessment that answers the policy as written and then adds the illuminance analysis is hard to criticise. One that substitutes the newer method without explanation invites a consultation response asking for the older one.

Thresholds used for each room type

Treat the right-hand column as a structure rather than as numbers to quote, and take the current figures from the standard itself. Where a use is particularly sensitive to daylight, the thresholds in sector guidance may be more demanding than either, which matters for schools, hospitals and care homes.

Where the two methods disagree

Orientation is the obvious case. The percentage method cannot distinguish a north-facing from a south-facing room, so a scheme optimised to it can produce a block where half the flats are noticeably darker in use than the calculations suggest.

Deep plan rooms are the second. A long room with a single window can reach an acceptable average percentage because the average is taken across the whole plane while the back of the room is close to dark. The illuminance method, reported as the fraction of the plane meeting the target, exposes that immediately. The no-sky line check does the same job within the older framework.

Obstruction geometry is the third. Balconies, deep reveals, recessed windows and tight courtyards all reduce the sky angle, and the two methods penalise them by different amounts. On courtyard schemes where the lower floors are the problem, running both methods often gives a clearer picture than either alone, and the same is true for single aspect flats.

High interior reflectance is where the disagreement can be manufactured. Both methods respond to reflectance, but the climate-based model is more sensitive to it, so the assumptions deserve to be stated on the face of the report.

Choosing a method and justifying it

Pick the method the adopted policy requires, add the other where it helps, and state the inputs. A short methodology section covering the standard applied, the software, the weather file, the reflectances, the glazing transmittance, the reference plane and the obstruction model lets a reviewer test the work instead of doubting it.

Where the results are marginal, say which rooms fail and by how much, and deal with them in the planning balance rather than averaging them away. An assessment that identifies six flats below target and explains the design reasons reads as professional. One that reports a scheme-wide average and nothing else reads as evasive, and officers have seen enough of both to tell the difference. Overshadowing of amenity space is a separate question with its own method, covered by an overshadowing study.

This is general guidance, not legal advice. Daylight modelling needs a suitably qualified author, and councils normally expect professional input on contested schemes. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.