BS EN 17037 and illuminance targets for new flats

BS EN 17037 assesses daylight in new flats by illuminance rather than by obstruction geometry. It asks how much daylight actually reaches the working plane inside each room, for what fraction of the year, using the local climate. That is a different question from the one the site layout guidance asks about existing neighbouring windows, and the two are routinely confused in submissions.

For dwellings in England, the UK National Annex sets minimum target illuminance values by room type: 100 lux for bedrooms, 150 lux for living rooms and 200 lux for kitchens, achieved over at least half the assessment area for at least half the daylight hours in the year.

What the standard measures

The standard sets recommendations for daylight provision, view out, sunlight exposure and protection from glare. Planning submissions usually deal with the first, occasionally the third, and almost never the second and fourth, although councils in dense boroughs are starting to ask about view out.

Daylight provision is expressed as a target illuminance achieved over a fraction of the reference plane for a fraction of the daylight hours. The reference plane sits at a defined height above floor level with a border excluded at the room perimeter, so it is not the whole floor area of the room.

Two methods are permitted. Climate-based daylight modelling using hourly illuminance data for the location, which is the method the standard prefers. Or a daylight factor calculation, where the target illuminance is converted into a target daylight factor using the median external diffuse illuminance for the location. The second is simpler and produces a single number per room.

Target illuminance and the fraction of hours

The arithmetic matters because the two fractions do different work.

Half the assessment area is a spatial test. It asks whether daylight penetrates far enough into the room, which is where deep plan flats and single aspect layouts fail. Half the daylight hours is a temporal test. It asks whether the level is reached often enough through the year, which is where heavily obstructed windows fail.

A room can pass one and fail the other, and reporting only a single averaged figure hides that. Report both.

The UK National Annex targets are minima for dwellings. Where a local plan or borough guidance sets a higher expectation, that applies instead, and some London boroughs have adopted their own internal daylight standards. Check the adopted plan rather than assuming the national annex is the ceiling.

Climate-based modelling against static daylight factors

The choice between the two methods is worth making deliberately.

Climate-based modelling uses a weather file for the location, accounts for orientation and for the actual pattern of sky conditions through the year, and produces results that reflect a south-facing room differently from a north-facing one. It is the method that shows the value of orientation decisions.

The daylight factor approach assumes a standard overcast sky. It takes no account of orientation, so a north-facing and a south-facing room with identical geometry return identical results. It is quicker, it is easier for a case officer to check, and it is defensible where the assessment is comparing options rather than certifying a final design.

Say which you used, name the software and version, and state the weather file. An assessment that reports lux values without saying which method produced them cannot be checked.

Room by room reporting

Report every habitable room, not a sample, and give each one its own line.

Include the room reference matching the submitted plans, the use assumed, the floor area, the assessment area after the perimeter border is excluded, the glazing area and its visible light transmittance, the target illuminance applied, the result against both fractions, and a pass or fail.

Room use assumptions carry more weight here than anywhere else, because the target changes with the use. A room labelled as a study rather than a bedroom is assessed differently, and a flexible living and dining space needs a decision about which target applies. Make the assumption explicit rather than letting the software default decide it. The wider effect of these assumptions is set out in room use assumptions and how they change results.

Two inputs quietly drive the answer. Visible light transmittance of the glazing, which drops sharply with solar control coatings, and the reflectance values assumed for internal surfaces. Both should be stated and both should match what will actually be built.

Single aspect and dual aspect outcomes

Single aspect flats fail the spatial test more often than any other configuration, and the failures cluster in the same places.

North-facing single aspect units with deep plans. Units behind a recessed balcony, where the balcony above and the side walls cut the sky visible from the window by a large margin. Ground floor units facing a courtyard. Kitchens in a single aspect layout, because the 200 lux target is the hardest of the three and the kitchen is usually furthest from the window.

Balcony treatment is the design lever people reach for last. Moving from a fully recessed balcony to a partially inset or projecting one can change a failing bedroom into a passing one without altering the plan area, and testing that variant early is cheaper than arguing about it later.

Dual aspect units rarely fail on daylight provision. Where they do, it is usually the internal room nearest the core rather than either of the external faces.

How councils use the results

Authorities read the results in two ways, and it helps to know which you are dealing with.

Some treat the national annex targets as a standard to be met, and a scheme with a material proportion of failing rooms will be asked to redesign. Others treat the results as one consideration in an overall judgement about living conditions, alongside room sizes, outlook, privacy and amenity space.

Either way, present the shortfall honestly. A table that shows twelve per cent of rooms below target, with those rooms identified on a plan and an explanation of why, is far more persuasive than a summary paragraph asserting compliance. Officers cross-check, and a claimed compliance that falls apart under checking damages everything else in the submission.

Where the context is dense and the standard targets are hard to reach, the argument for flexibility has to be made explicitly and on evidence about the surrounding area. It is a separate argument from the assessment itself and belongs in its own section.

Presenting the assessment

Structure it so that a case officer can find the answer in ninety seconds.

Lead with a summary table of rooms assessed, rooms meeting the target and rooms below it, by unit type. Follow with the methodology statement covering standard, method, software, weather file, reference plane height and border, reflectances and glazing transmittance. Then the room by room schedule. Then plans with results shown on them, colour coded, at a scale that prints legibly.

Keep the internal daylight assessment separate from the impact on neighbouring properties. They use different methods, different standards and different reference points, and combining them into one document is the most common reason an officer misreads the results. A daylight and sunlight assessment covering both should still keep the two parts in separate sections, and sunlight to external amenity space belongs with the overshadowing study.

Where daylight results push towards larger glazing, check the effect on overheating before committing, because the two assessments pull in opposite directions and the overheating risk assessment should be run on the same model.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it.