BS EN 17037 daylight targets for new homes explained

BS EN 17037 changed how internal daylight in new homes is assessed, replacing the average daylight factor targets most architects learned with an illuminance-based method. The standard asks whether a target illuminance is achieved across a proportion of the room for at least half the daylight hours, using climate data for the location rather than a single overcast sky assumption. The UK National Annex sets the figures that matter for dwellings: 100 lux for bedrooms, 150 lux for living rooms and 200 lux for kitchens, achieved over half the assessment area.

For a scheme where the council asks for internal daylight evidence, that is the method to use, and the older daylight factor route survives as an alternative rather than as the default.

What BS EN 17037 sets out to measure

Four separate things, and only the first is what people mean when they say "the daylight assessment".

Daylight provision, which is the illuminance criterion described above. It uses hourly climate data for the location, so a scheme in Newcastle and the same scheme in Plymouth do not produce the same result.

Sunlight exposure, assessed on a reference date, with the minimum recommendation being 1.5 hours of direct sunlight to a habitable room. Higher levels sit above that.

View out, assessed on horizontal sight angle, outside distance and the layers of the view visible from the room.

Protection from glare, which in practice is handled through the design of shading rather than through a numerical planning submission.

Councils asking for an assessment usually want the first, sometimes the second, and rarely the other two. Ask which before you commission the modelling, because the cost difference is real.

Illuminance targets against daylight factor methods

The two methods answer the same question in different currencies.

The daylight factor method is simpler, faster and insensitive to orientation and location, which is both its attraction and its weakness. A north-facing living room and a south-facing one with identical glazing produce the same average daylight factor and very different lived experience.

The illuminance method captures that. It also captures the effect of an obstructing building on the actual hours of useful light rather than on a sky-view proxy.

Where a report presents daylight factor results, check whether it is using average or median values, because they are not the same and the equivalences in BRE guidance are expressed in median terms.

How the standard treats different room types

Assessment is room by room, over a reference plane at working height, with a border excluded around the perimeter.

The room's use determines the target, so the room schedule matters. A room labelled "study" on the planning drawing and "bedroom 3" on the sales brochure will be assessed against whichever label the assessor was given, and an objector will use the other one.

Open plan kitchen, living and dining spaces are the recurring difficulty. The standard assesses a space, and a single space with three functions has to be assigned a target. The defensible approach is to assess against the most demanding use present, state that you have done so, and report the result for the whole space rather than carving it into notional zones. Carving it up invites the accusation that the zones were drawn to pass.

Note: a room with a single window under a deep balcony is the classic failure. If the scheme depends on those units, model them early rather than at planning submission.

Choosing between the standard and older methods

Three considerations decide it.

What the council asks for. Some validation lists name BS EN 17037, some name a daylight factor target, and some ask only for "an assessment of internal daylight". Read the list.

What the scheme needs to prove. Where the design is generous and the context open, either method passes and the simpler one is enough. Where the design is tight, the illuminance method usually gives a fairer answer and is worth the extra modelling.

What the objectors will use. On a contested scheme, presenting the method the standard now prescribes is a stronger position than presenting a superseded one, even where the superseded one gives a better number.

Presenting both is legitimate where they agree. Presenting both where they disagree, without explaining why, is not.

What a council validation list expects

For a new residential scheme, expect some combination of:

• A room by room table of results against the stated targets.
• A statement of the method, the software, the climate data file and the standard version used.
• The assumptions schedule: surface reflectances, glazing visible light transmittance, maintenance factor, and whether obstructing buildings were modelled from survey or mapping.
• Plans showing the assessed rooms and the reference plane.
• A list of any rooms failing the target, with the reason and the design response.

That last item is the one that separates a report from a submission. A Daylight and Sunlight Assessment that reports six failing units and explains why they are acceptable in this context does better than one that quietly omits them.

Where the scheme is in London, check the borough's housing design guidance alongside the London Plan, because the internal daylight expectation is often expressed there rather than in the validation list.

Reporting results for a residential scheme

Write the summary for a planning officer, not for a lighting engineer.

Lead with the proportion of habitable rooms meeting the target. Follow with the failures, grouped by cause: single aspect north-facing, deep plan, balcony overhang, obstruction from the adjoining building. Then set out what was changed in the design in response, and what could not be changed and why.

Keep the technical appendix complete and separate. The officer will not read it; the objector's consultant will.

One honest uncertainty: the interaction between daylight provision and overheating is genuine and awkward. Larger glazed areas improve daylight results and worsen the position under Approved Document O, and an Overheating Risk Assessment modelled after the daylight case is settled will sometimes push the glazing back down. Run the two together.

Illustrative example: a nine-unit block where two north-facing single-aspect flats failed the bedroom target under the illuminance method and passed under average daylight factor. The design response was to widen the window and reduce the balcony projection on those two units, which brought them within the target and cost a revised elevation. This example is illustrative.

Model daylight at the stage where the plan can still move. After planning submission, every failing room is an argument rather than a design decision.

PlanScope drafts the Design and Access Statement and supporting documents that sit around a specialist daylight assessment, structured to UK planning policy, with modelled results left to a qualified assessor. Councils expect a suitably qualified author for daylight work, and professional review is strongly advised. Browse the full report list.

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.