Assessing daylight to a window serving two rooms

When a single window serves a combined kitchen, dining and living space, the daylight assessment has to treat that space as one room with the most demanding target of the three. That is the short answer to the daylight window serving two rooms problem, and it is why open plan layouts routinely fail an assessment that a two-room arrangement behind the same window would pass. The reference plane gets deeper, the back of the space sits further from the glazing, and the illuminance target rises to the kitchen value rather than the living room value.

Why open plan layouts complicate the assessment

BRE 209 (2022) and BS EN 17037 both assess daylight room by room. The room is the unit of assessment, so the way a floor plan is labelled changes the numbers without a single millimetre moving on the elevation.

Two effects compound. Depth is the first: a kitchen-living-dining space 8 metres deep with glazing at one end has a far larger proportion of its working plane beyond the no-sky line than a 4 metre living room with a separate kitchen behind it. The second is the target. The UK National Annex to BS EN 17037 sets different minimum target illuminances by room use, with kitchens the most demanding of the three habitable categories, so a combined space inherits the kitchen figure across its whole area.

Apply 200 lux across an 8 metre deep plan with single-aspect glazing and the result is usually a shortfall at the rear. That is a design problem rather than an assessment problem, and the honest thing to do in a report is to say so.

How the guidance treats combined spaces

The principle in BRE 209 is that where a room serves more than one purpose, the assessment uses the target for the most demanding of those purposes. It applies to the illuminance or daylight factor calculation and it applies equally to the older average daylight factor approach that many local plan policies still refer to.

For the assessment of existing buildings affected by new development, the same logic reaches the daylight distribution test. The no-sky line is plotted across the whole room, and the retained area is compared with the former area using the 0.8 factor. A combined space therefore has more area at risk of falling behind the no-sky line, and a given obstruction causes a larger proportional loss than it would in a subdivided plan.

The vertical sky component test is not affected by room labelling at all. It is measured at the centre of the window on the outside face, so the 27 per cent reference value and the 0.8 former-value comparison give the same answer whatever happens behind the glass. That asymmetry is worth stating explicitly in a report, because it explains why a scheme can pass on vertical sky component and fail on distribution.

Choosing the assessment area

The judgement call is whether the kitchen part of a combined space counts as a room serving a kitchen function, or as a kitchenette ancillary to a living space. BRE guidance accepts that a small galley or run of units without any dining or sitting function can be treated as ancillary, in which case the living room target applies to the space as a whole.

Four features push a plan towards being treated as a full kitchen rather than an ancillary one: a dining table or island with seating shown on the drawing, a floor area for the kitchen zone comparable with a standalone kitchen, a position at the rear of the space furthest from the window, and the absence of any other space in the dwelling capable of serving as a kitchen.

Illustrative example: a two-bedroom flat with a 6.5 metre deep open plan space, glazing across one end, and a 2 metre run of units with no table against the rear wall. Assessed as a living room with an ancillary kitchenette, the space meets the 150 lux target over the required proportion of the reference plane. Assessed as a kitchen-living-dining room at 200 lux it does not. The figures and the layout are illustrative rather than drawn from a real scheme, and the point is that the same geometry produces two defensible answers.

Where both readings are tenable, present both. A report that gives the result under each assumption, states which it regards as the correct characterisation and why, is far harder to attack than one that silently picks the favourable reading.

Borrowed light and internal glazing

Daylight through internal glazing from an adjoining space should not be counted as daylight to the room behind it. Both BRE 209 and BS EN 17037 work from windows in the external envelope, and the transmission losses, the obstruction from the intervening space and the dependence on that space's own glazing make borrowed light unreliable as a planning metric.

That does not make internal glazing worthless. A glazed screen between a combined space and an inner room does improve perceived brightness and the sense of outlook, and it is worth describing in the design and access material. It just should not appear in the calculation. The same caution applies to light wells, inner rooms and single-aspect layouts generally, which we look at in the article on daylight to single aspect flats and how to fix them.

Disclosing the assumption in the report

For a proposed scheme the room uses come from the architect's plans and there is no real ambiguity. For an assessment of neighbouring properties, which is where most objections arise, you cannot get inside, so every room use is inferred.

State the inference and its basis. A short schedule listing each assessed window, the assumed room use, and the source of that assumption covers it: internal layouts from a submitted or historic planning application, external observation of window type and position, published floor plans from a previous sale, or an assumption applied in the absence of information. Where an assumption is adverse to the applicant, say that too.

Three disclosures separate a report that survives scrutiny from one that does not. The room use schedule with sources. The treatment of any combined space and the target applied. And a sensitivity statement showing what changes if the alternative characterisation is adopted. This is the kind of methodological transparency that local plan amenity policies increasingly expect, as set out in our note on local plan amenity policies and how they use BRE 209.

A Daylight and Sunlight Assessment that hides its room use assumptions in an appendix invites exactly the challenge it is trying to avoid.

Challenges from objectors and how to answer them

The most common objection is that a room assessed as a kitchen is really a living room, or the reverse. It is usually made without evidence, and the answer is the schedule described above rather than a re-run of the model.

The second is that a large open plan space should be notionally subdivided so that the front portion passes. BRE guidance does not support that, because the room is the unit of assessment and a space with no physical separation is one room. Resist it even when it helps your client, because an inspector who spots it will discount the whole report.

The third is that the comparison should be against an unobstructed condition rather than the existing condition. For effects on existing buildings the baseline is the existing situation, and for a site that is already partly obstructed the retained values matter more than the absolute ones. Objectors with a professional adviser will know this; those without often do not, and the clearest way to deal with it is a short paragraph in the non-technical summary. The evidence that actually carries weight in an objection is covered in our piece on objecting on daylight grounds.

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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. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, including daylight modelling, qualified professional input may be required.