Daylight inside the proposed development: assessing new flats

Internal daylight in a new residential scheme is assessed against a target illuminance achieved over a stated fraction of the room for a stated fraction of daylight hours. It is a different exercise from the neighbour impact tests, it uses different software settings, and a scheme can pass one comfortably while failing the other. On a block of flats, the daylight inside new development assessment is usually the one that changes the plan, because the variables it depends on, room depth and window head height, are the ones the architect controls.

The current framework is BS EN 17037, with a UK national annex that sets lower target illuminances for dwellings than the main standard's general levels, reflecting UK practice and climate. Many authorities still ask for average daylight factor figures alongside it, so in practice you often report both.

Why internal daylight is assessed separately

Neighbour assessment asks what your building does to somebody else's window. Internal assessment asks whether the homes you are creating are decent places to live. The policy hooks are different: neighbour impact runs through amenity policies, while internal daylight runs through housing quality and design policies, and in London through the housing standards in the London Plan.

The practical consequence is that the two assessments have different consequences when they fail. A marginal neighbour result is weighed against the benefits of the scheme. A room that receives very little daylight is a permanent defect in a home that will exist for a century, and officers treat it accordingly.

The Daylight and Sunlight Assessment should present both, clearly separated, with the internal results given room by room rather than as a headline percentage of compliant rooms.

Target illuminance and the fraction of the room

The method is climate-based. A model of the building is run against a local climate file to calculate the illuminance on a reference plane, usually at 0.85 metres above floor level in dwellings, across the daylight hours of the year.

Compliance is then expressed as two conditions: a target illuminance achieved across at least half the reference plane for at least half the daylight hours, and a lower minimum target achieved across almost the whole plane for the same period. The UK national annex sets dwelling targets that differ by room use, with bedrooms lowest, living rooms higher and kitchens highest, on the basis that a kitchen is a task space.

Two things to check in any report. Which target set has been used, because the main standard's general levels and the UK dwelling levels are materially different numbers. And whether the reference plane has been set with the correct border offset from the walls, because that alone can move a result across the line.

Where average daylight factor is also reported, it is a different calculation with different assumptions, and our note on average daylight factor and where it still applies explains when it is still the relevant measure. The relationship between the two frameworks is covered in BS EN 17037 daylight targets against BRE guidance.

Room depth, window head height and aspect

Three geometric variables decide almost everything.

Window head height sets how far daylight penetrates. Raising the head of a window by 300 millimetres does more for the back of a room than widening it by a metre, because the depth of useful daylight is governed by the sightline from the working plane over the head. Ceiling heights therefore matter for daylight, not just for perceived quality, and the first casualty of a value engineering exercise that shaves floor-to-floor heights is usually the daylight result.

Room depth is the other half of the same relationship. Beyond roughly two to two and a half times the window head height above the working plane, the back of a room contributes little. Deep open plan kitchen-living-dining rooms served by a single window are the most common failing room type in flatted schemes, and they fail on the kitchen target because the kitchen is usually at the back.

Aspect determines the total available. A dual aspect unit has more window wall and more sky access, and it also solves ventilation and overheating questions at the same time.

Single aspect and north facing units

Single aspect flats are not automatically unacceptable, and it is worth being precise about that because objections often overstate the position. What is difficult is a deep single aspect unit facing north, with a balcony over the window, in a courtyard with high facing elevations. Each factor is survivable, and the combination is not.

Policy in London discourages single aspect north facing dwellings specifically, and many authorities outside London have adopted similar wording in design guidance. Where a scheme includes them, the assessment should say so plainly, and the design case has to explain why that unit type exists in that position.

The fix at layout stage is usually to move the single aspect units to the south and west elevations, put dual aspect units on the corners, and reserve the north elevation for circulation, cores and non-habitable space. That conversation costs far less at concept design than at planning committee.

Balconies, recesses and light shelves

A balcony above a window shades it, and an inset balcony recesses the window so that the effective aperture sits deep inside the building line. The loss can be substantial, and it lands on exactly the units a marketing brochure calls the best in the block.

The options, in descending order of effectiveness, are to project the balcony rather than inset it so the window plane stays at the facade, to reduce the balcony depth, to cut the balcony back at one edge so the window has an open corner, to use an open balustrade with a lighter soffit finish, and to raise the window head within the recess. Light shelves, which are frequently proposed, redistribute daylight deeper into a room but do not add much to total illuminance, so they are a comfort measure rather than a compliance one.

Soffit and balustrade reflectance assumptions should be stated in the model. A dark soffit modelled as light is a familiar way of making a difficult unit pass on paper.

Reporting compliance room by room

The format officers find usable is a table listing every habitable room by unit reference, floor, orientation, room use, the aperture arrangement, and the result against the target. Compliance summaries expressed as a single percentage hide the pattern, and the pattern is what matters: if the failures cluster in one core on the north side, that is a design issue, whereas isolated failures spread across the block usually reflect specific obstructions.

Where units fall short, say so and explain the balance. Officers deal with imperfect schemes constantly, and a report that admits a shortfall and sets out what was done about it is more persuasive than one that reports full compliance through generous assumptions. Standards for shared and co-living accommodation differ again, as our note on amenity standards for bedrooms in shared and co-living sets out.

Design changes that recover a failing unit

Test the sensitive units early, in outline, before the floor plates are fixed. A single test model of the worst unit type at concept stage costs a fraction of a redesign. And check the interaction with overheating at the same time, because the glazing change that rescues a daylight result can create a Part O problem on a south or west elevation.

Illustrative example. A north facing single aspect one bedroom flat, 7 metres deep with an inset 1.8 metre balcony, fails the kitchen target across the rear half of the room. Projecting the balcony, raising the window head by 250 millimetres and reducing the room depth to 6 metres by taking the wardrobe wall forward brings it back inside the target. The figures are illustrative and every model differs.

Daylight modelling should be carried out by a suitably qualified assessor. PlanScope can draft the surrounding planning documents and flags where technical inputs are missing rather than inventing them. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance, not legal advice.