Assessing daylight inside your own proposed flats

A daylight assessment of proposed flats tests the accommodation you are asking permission to build, and on a dense urban scheme it is a harder argument to win than the effect on the neighbours. The neighbour case is about a change from an existing baseline, so a small loss is defensible. The internal case has no baseline. Either the flats meet the target illuminance or they do not, and an officer reading a schedule showing a third of habitable rooms below target has to decide whether to recommend approval of homes that are known to be dark.

Run this analysis at concept stage rather than at submission. By the time the façades are drawn, the only remaining lever is glazing, and glazing is the lever that creates an overheating problem.

Why the scheme's own daylight matters at planning stage

There is no national numeric standard for daylight in new dwellings written into policy. The requirement arrives through adopted local plan policies on residential amenity and quality, and through design guidance, which point to BRE 209 and, since the 2022 edition, to the illuminance approach it took from BS EN 17037. The UK National Annex targets for dwellings are 100 lux in bedrooms, 150 lux in living rooms and 200 lux in kitchens, achieved over half the reference plane for at least half of daylight hours.

That leaves a policy judgement rather than a pass mark, which cuts both ways. A scheme with a handful of marginal rooms in an accepted urban context will normally be approved. A scheme where the bottom two floors of a courtyard fail across the board is a different conversation, and in authorities with their own standards it may be refused on that ground alone. How context-based targets work where they exist is set out under alternative daylight targets in dense urban areas.

The commercial point is that this analysis controls unit numbers. Every scheme has a floor level below which flats in the courtyard cannot be made acceptable, and finding that level in month two is worth more than finding it at committee.

Which rooms to test and which to leave out

Test every habitable room: living rooms, dining rooms, kitchens and bedrooms. Leave out bathrooms, cloakrooms, stores, plant spaces and circulation, and say in the methodology that you have done so.

Open-plan living, kitchen and dining rooms need a decision, and it needs to be stated rather than buried. The constituent spaces have different targets, and the defensible approaches are either to assess the whole room against the living room target while reporting the illuminance in the kitchen zone separately, or to divide the room into zones and assess each against its own target. Whichever you choose, apply it consistently across the scheme and explain it once.

Two further points on the room schedule. Studio flats have a single room performing several functions, so state which target you applied and why. And where a room is served by more than one window, or by a window and a rooflight, the schedule should say so, because reviewers check whether a dual-aspect room has been credited correctly.

Number the rooms to a convention that carries the block, the core, the floor and the flat, and use the same references in the drawings, the schedule and the daylight and sunlight assessment text. On a 200-unit scheme the referencing is what makes the report usable.

Courtyards, light wells and re-entrant corners

Courtyard blocks fail in predictable places, and the pattern is geometric.

The lower floors go first, because the sky visible from a window is cut by the opposite elevation. The corners go before the middle of each side, because a window in a corner sees obstruction on two sides. And single-aspect flats facing into the courtyard have nowhere else to draw light from.

Three dimensions control the outcome: the clear width of the courtyard, the height of the enclosing elevations, and the reflectance of those elevations. The third is the one designers forget. A courtyard faced in dark brick performs materially worse than the same geometry in a pale material, so the material choice is a daylight decision as well as an appearance one, and the model has to use the reflectance of the actual specification.

Re-entrant corners and narrow light wells behave the same way in miniature. Where a plan has a slot between two wings, windows facing across that slot obstruct each other, and the flats at the closed end of the slot are the worst in the building. The remedies are widening the slot, chamfering or splaying the corner, putting non-habitable rooms on the affected elevations, or accepting that those units are single-bed flats with the bedroom on the better aspect.

Balconies over the windows below

A private balcony is a planning requirement in most authorities and a daylight problem in every one. The soffit of the balcony above acts as an overhang across the head of the window below, cutting exactly the part of the sky that contributes most to the illuminance in the room.

The effect is largest on lower floors and on windows already obstructed, so it compounds the courtyard problem rather than sitting alongside it. The mechanics are covered under balconies, overhangs and the daylight loss below.

Six design responses, roughly in order of how much they recover.

• Reduce the balcony depth. The depth is the single biggest variable, and a reduction of a few hundred millimetres recovers more than any glazing change.
• Offset the balcony from the window below, so the overhang sits over a solid panel rather than over glass.
• Increase the head height of the window under the balcony, or raise the soffit relative to the window head.
• Use a projecting balcony with a slender edge detail rather than a deep upstand.
• Specify a reflective or pale soffit finish, which returns some light into the room.
• Reserve inset balconies for the upper floors, where the daylight margin is greater.

Glass balustrades help far less than people expect, because the obstruction that matters is the solid soffit above rather than the guarding in front.

Reporting failures with the design reason

A report that lists failing rooms with no explanation invites a refusal. A report that explains each failure, shows what was tested and states why the residual position is acceptable gives the officer the material to recommend approval.

For each room below target, give four things. The cause, named specifically: balcony overhang, courtyard depth, single aspect on an obstructed elevation, or deep plan. The margin by which it falls short. The design options tested and the reason each was rejected or adopted. And the mitigating circumstances for that particular unit, such as a dual-aspect living room elsewhere in the flat, a generous private balcony, or the room being the second bedroom rather than the main living space.

Then aggregate honestly. State the number and percentage of habitable rooms meeting the target, the number falling short by a small margin, and the number falling well short, with the last group identified by block and floor. Do not present a single average. An average of 160 lux across a scheme tells the officer nothing about the eight flats at 60 lux.

Acknowledge one uncertainty plainly. The weight given to internal daylight varies between authorities and between inspectors, because there is no numeric policy threshold, and a report that pretends otherwise reads as advocacy rather than assessment.

Amending massing against amending glazing

When rooms fail, there are two routes, and they have very different consequences.

Changing the massing works: widening the courtyard, dropping a storey on the obstructing wing, chamfering the corners, or reducing the depth of the plan. It also changes unit numbers, the neighbour assessment, the townscape case and the viability, so it has to happen early.

Changing the glazing is quicker and it works in the model. Larger windows, higher heads, better transmittance and paler interior finishes all move the illuminance result. The cost arrives in the overheating risk assessment under Part O and in the Part L fabric calculation, and a scheme that solves daylight with glass and then adds external shading has to remodel the daylight with the shading in place.

The practical sequence that avoids circular work: test the massing with a coarse daylight model at concept stage, fix the geometry, then optimise glazing with the thermal modeller in parallel rather than in sequence. The relationship between the two analyses is one of the things the 2022 edition of BRE 209 made more visible, as set out under the BRE 209 changes for new interiors.

How the results read in an officer report

Officers lift tables. So give them one worth lifting: a schedule by block and floor showing habitable rooms tested, rooms meeting the target, rooms below it, and the worst value in each group, with a short note on the cause.

Follow it with three or four sentences of conclusion in plain language, naming the number of units affected and the reason the residual position is acceptable in this context, with the local policy wording quoted. That paragraph is what appears in the committee report, and writing it yourself is more reliable than hoping the officer summarises the appendix correctly.

Councils normally expect this work to be done by someone experienced in the current method, and professional review before submission is strongly advised. PlanScope drafts the report structure, the methodology narrative and the policy framing, and marks every figure it cannot source as to be confirmed. Enter your postcode to see which reports your site is likely to need.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.