Daylight inside new flats compared with neighbours

Daylight new dwellings versus neighbours is two assessments in one report, using different measures, different targets and different guidance. Inside your proposed flats the question is whether the rooms receive enough daylight to be decent places to live, measured now against BS EN 17037 and its UK National Annex. Next door the question is how much daylight and sunlight the existing windows lose, measured with vertical sky component, no sky line and annual probable sunlight hours.

Confusing the two is the most common error in a submitted daylight report, and it is obvious to a reviewer within a page. A scheme that achieves excellent internal daylight can still cause unacceptable loss to a neighbour, and a scheme that barely touches the neighbours can produce flats nobody should be asked to live in.

Two questions inside one report

The neighbour assessment is a comparison. It measures the existing condition, models the proposed condition, and reports the change. The target values are ratios as much as absolutes, which is why a window that already receives very little daylight can pass a test that a well lit window fails.

The internal assessment is absolute. It measures the daylight the proposed room receives, against a target, with no before and after. There is no existing condition to compare with because the room does not exist yet.

That difference drives everything else. Neighbour results are expressed as percentages of former value. Internal results are expressed as illuminance in lux, or as a percentage of the room area meeting a target for a percentage of the year. They are not comparable and should never appear in the same table.

Measures used inside the proposed flats

BRE 209 in its 2022 edition points internal assessment at BS EN 17037 and the UK National Annex rather than at the older average daylight factor approach.

The method is climate based. A model of the room is run against a year of local climate data, and the output is the illuminance achieved across the working plane for a proportion of daylight hours. The National Annex sets recommended minimum target illuminance values for dwellings, commonly taken as 100 lux for bedrooms, 150 lux for living rooms and 200 lux for kitchens, to be achieved over at least half the room area for at least half the daylight hours.

Older reports still quote average daylight factors of 1 per cent, 1.5 per cent and 2 per cent for the same three room types, drawn from guidance that has since been withdrawn. Councils vary in how strictly they insist on the newer method. Ask before you model, because rerunning a scheme of 200 flats by a different method is not a small exercise.

Two modelling decisions change the answer more than the geometry does. Assumed room use, because a space called a study scores differently from the same space called a bedroom, which is the subject of our note on room use assumptions and how they change results. And assumed surface reflectances, because a model run with optimistic internal finishes produces numbers the finished building will not deliver.

Sunlight inside the flats is assessed separately again, and amenity space gets its own test based on the area receiving at least two hours of sun on 21 March.

Measures used for existing neighbouring windows

The neighbour tests are the familiar ones and they have not changed in substance.

The orientation rule applies to the sunlight test only. Windows facing more than 90 degrees from due south need no sunlight assessment, because they receive little direct sun in any case. Daylight tests still apply to them in full.

Two practical points. No sky line requires an internal layout for the neighbouring property, which you often do not have, and the approaches to that problem are covered in our note on assessing daylight to a property you cannot access. And the 0.8 rule is a proportional test, so a heavily obstructed window in a dense urban street can lose very little in absolute terms and still fail.

Why a scheme passes one and fails the other

The two assessments respond to different parts of the design.

Internal daylight is driven by window head height, room depth, glazing area, balcony overhangs and the distance to whatever faces the window. Neighbour loss is driven by the height and proximity of the new massing relative to existing windows, and by the angle it subtends from them.

So a building with generous glazing and shallow floor plates can perform well internally while its bulk causes a serious loss next door. And a set back, low rise scheme that protects every neighbouring window can still produce deep single aspect flats with balconies that cut out most of the sky.

Balconies deserve their own mention. A recessed balcony above a window is one of the most effective ways of ruining internal daylight, and it is usually introduced late for amenity or fire reasons. Model the balcony as built rather than as an outline, including any privacy screen, and check the room below rather than the room behind.

Single aspect units and what policy expects

Policy treats single aspect as a design quality question rather than a numerical one. The London Plan sets out expectations for housing quality and discourages single aspect dwellings that are north facing or exposed to noise, and many local plans outside London carry similar wording.

That means a north facing single aspect flat needs a positive case, not an apology. What usually makes it acceptable is a combination of things: a generous floor to ceiling height, a window head close to the ceiling, a shallower plan depth than the standard, no deep balcony overhang, and an internal daylight result that meets the target rather than approaching it.

Where the unit mix contains a run of such flats, expect the officer to ask for the results unit by unit rather than as an average. Averages hide the worst case, and the worst case is what gets refused.

Reporting both sets of results

Structure the report so the two assessments never blur. A workable order is neighbour daylight, neighbour sunlight, overshadowing of neighbouring amenity space, then internal daylight and sunlight, then internal amenity space, with a separate summary table for each.

For the neighbour section, report every window assessed, not a selection, and give both the absolute values and the ratios. For the internal section, report every habitable room rather than a sample, and state the room use assumed for each.

Where results fall short, say so in the report and explain why, rather than leaving the reviewer to find it. A Daylight and Sunlight Assessment that identifies its own three failing windows with a reasoned justification is treated very differently from one that buries them in an appendix. Councils usually expect a suitably qualified author for this work and professional review is strongly advised.

Where the scheme also affects gardens, play areas or amenity decks, an Overshadowing Study covers the two hour sunlight test on 21 March across those spaces.

Where officers look first

In our experience the sequence is predictable. The officer opens the summary table for neighbour loss, scans for anything below 0.8, and reads the justification for those windows. Then the internal results for the worst orientation. Then the drawings, to check that the model matches the scheme that was actually submitted.

That last check catches more reports than any methodological argument. A model built from an earlier revision, or one that omits a plant enclosure or a later balcony, undermines everything else in the document.

Run the assessment early enough to change the design. A daylight report produced two weeks before submission can only describe the problem, and the same discipline applies to sensitive uses covered in our note on daylight targets for schools and nurseries.

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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.