Presenting vertical sky component results

A daylight report lives or dies on how its vertical sky component results are presented. The modelling itself takes an afternoon. Making the output checkable by a case officer, by a neighbour's consultant and eventually by an inspector takes deliberate structure: every window referenced to a drawing, every room identified by use, existing and proposed values in adjacent columns, and the proportional change shown beside the absolute figure. A sound assessment with a sloppy table reads as evasive, and that impression is hard to recover once an objector has pointed it out.

What the vertical sky component measures

Vertical sky component is the ratio of direct sky illuminance reaching a reference point on a vertical plane to the illuminance on an unobstructed horizontal plane under a standard overcast sky, expressed as a percentage. The reference point sits at the centre of the window, on the outside face of the wall. An entirely unobstructed vertical wall gives roughly 40 per cent, which is why the 27 per cent figure in BRE 209 (2022) Site Layout Planning for Daylight and Sunlight is a benchmark rather than a pass mark: it corresponds approximately to an obstruction subtending 25 degrees above the horizontal at that point.

The measure describes the sky available to the window. It says nothing about the room behind it, which is both its strength and the source of most presentation problems. You can calculate it from massing alone, with no internal information at all. So a table of bare percentages is easy to produce and almost impossible for a reader to interrogate.

That is the gap the reporting has to close. The rest of a daylight and sunlight assessment carries the interpretation, but the sky component table is where a reader starts.

Numbering windows and rooms so results can be checked

Build the reference scheme before you run a single calculation, and build it around the receptor, not around your model tree. A convention that survives scrutiny reads property, floor, then window: 14 Ashwell Road / First / W03. Keep ground, first and second rather than L0, L1, L2, because officers and residents read floors, not levels.

Every window on every assessed elevation gets a reference, including the ones that pass comfortably. Omitting compliant windows to shorten the table is the single fastest way to attract an allegation of selective reporting, and it saves a page. The window location plan must use the same labels, elevation by elevation, at a scale where the numbers are legible when printed at A3.

Say where the geometry came from. Measured survey, point cloud, aerial photogrammetry, Ordnance Survey footprints with heights estimated from photographs: each carries a different tolerance, and a reader who knows which one you used can judge how much weight a result near a threshold deserves. Where window positions were assumed rather than surveyed, mark those references with a flag in the table and explain the assumption once.

And do not recycle references between revisions. If the scheme changes and a window drops out of the assessment, retire its number and keep a short revision note.

Reporting existing and proposed values together

The table needs four numbers per window and two identifiers. Existing vertical sky component, proposed vertical sky component, the absolute loss in percentage points, and the ratio of proposed to existing. Then the room served and that room's use. Nothing else belongs in the same table.

That is an illustrative example, not a real property. Report to one decimal place and calculate the ratio from the unrounded values, because a ratio derived from whole numbers can flip either side of 0.80 for windows that matter.

Where the existing value is already below 27 per cent, which is normal in dense urban blocks, say so in the row and again in the text. The absolute benchmark has effectively been exhausted by the existing context, and the proportional test is then carrying the whole judgement.

Reading proportional change alongside the absolute value

BRE 209 treats a reduction as noticeable where the proposed sky component is both below 27 per cent and less than 0.8 times its former value. Both limbs have to be met. A window falling from 30.0 to 26.2 is below the benchmark but retains 0.87 of its former value, so the guidance does not describe that loss as noticeable. A window falling from 20.0 to 15.0 sits below 27 per cent and at 0.75 of its former value, and both limbs are met.

Reports that print the ratio alone, or the absolute value alone, invite exactly the challenge they were trying to avoid. Print both, then state the conclusion for each window in words in the commentary rather than leaving it to a colour-coded cell.

Be plain about the status of the numbers. BRE 209 describes its figures as guidance to be applied flexibly, with the context of the site taken into account, and a good report quotes that flexibility rather than hiding behind it. Many local plans in dense boroughs point to alternative targets or to a context-based approach, and where they do, the policy wording governs how you frame the result.

Identifying room uses

BRE 209 gives more weight to main habitable rooms, so the room-use column does real work. Living rooms, dining rooms and kitchens carry the most weight, bedrooms less, and bathrooms, cloakrooms, stores and circulation space are normally left out of the amenity judgement altogether.

Getting room uses right means evidence. Internal inspection where the occupier grants access, historic floor plans on the council's planning register, or plans submitted with an earlier application on the same property. Where none of that exists, state the assumption you made and the basis for it, and keep the assumption conservative. A report that quietly labels a front room a bedroom, to the benefit of the applicant, will not survive a third-party review.

Dual-aspect rooms need a sentence of their own. A room served by three windows, one of which loses sky while the other two are unaffected, is not in the same position as a single-aspect room behind one window, and the no-sky-line analysis is the test that shows the difference. Rooms lit only from above sit outside this method entirely.

Summarising by property rather than by window

Officers and members think in dwellings, not windows. So the front of the results chapter carries a property-level summary and the window-by-window table goes to an appendix.

For each affected property give the address, the number of windows assessed, how many meet the BRE numeric guidance, how many do not, the worst ratio recorded, and one sentence naming the room uses behind the non-compliant windows. Then a scheme-level paragraph: how many properties were assessed, how many are affected at all, and where the residual effects concentrate.

This is the same discipline that makes a TVIA viewpoint schedule usable, and the same reason a thermal comfort chapter leads with dwellings rather than rooms when reporting CIBSE TM59 results. Aggregate first, then let the reader drill down.

Presentation faults that invite challenge

The faults that come up again and again in third-party reviews are mechanical rather than technical.

• Window sets that cover only the properties the objectors named, rather than every neighbouring window with a view of the site.
• Proposed values with no existing baseline, which makes every proportional test unverifiable.
• No statement of which baseline the existing case used: as-built surroundings, a consented but unbuilt neighbouring scheme, or both run as sensitivity tests.
• Sky component and no-sky-line results merged into one column under a single heading.
• An undated model with no version reference, so a revised scheme cannot be compared with the one the officer read.
• Sunlight results for windows facing within 90 degrees of due north, where the guidance does not expect the annual sunlight tests to be applied.

A short methodology note that answers all six before anyone asks costs about 300 words. If shadowing on gardens and amenity space is also in issue, keep that in a separate overshadowing study chapter with its own assumptions, rather than folding sun-on-ground results into the window tables.

Councils normally expect a daylight assessment to be written by someone experienced in the BRE 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, so the modeller fills the numbers. 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. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.