Daylight modelling software and what officers accept

No local planning authority in England has a list of approved daylight modelling software. Officers accept a method, not a tool. What gets a daylight assessment through is a stated methodology that follows the BRE guidance, inputs a third party can check, and results presented so that a reviewer can reproduce them. The same numbers produced in two different ray tracing engines will be accepted or rejected on the strength of the assumptions behind them.

That is good news for anyone choosing software and bad news for anyone hoping the choice will do the arguing for them.

Why the method matters more than the tool

The substantive guidance is BRE 209, Site Layout Planning for Daylight and Sunlight, in its 2022 third edition, which brought the British approach closer to BS EN 17037 and introduced climate based daylight modelling alongside the older geometric tests. Councils refer to it in local plan policy and in supplementary guidance. They do not name products.

Where a tool matters is in what it can do. A package that only calculates vertical sky component and annual probable sunlight hours cannot produce target illuminance results, and a scheme in a borough whose guidance expects the newer metrics will need the newer method. Conversely, a full climate based model of a single rear extension is disproportionate, and an officer will read it as obfuscation.

So the first decision is which tests apply to this scheme, and the second is which software can run them. Not the other way round.

The inputs that decide the result

Daylight results are mostly determined before any calculation runs. Six inputs carry the outcome.

• The existing obstruction geometry, including the heights and positions of surrounding buildings and their parapets
• The window schedule, with the glazed area, the reveal depth and the head height of each opening
• The room each window serves, and the use assumed for that room
• Internal surface reflectances
• Glazing visible light transmittance and any maintenance and framing allowances
• For climate based work, the climate file and the analysis period

Get the obstruction heights wrong and everything downstream is wrong. Survey them where you can, use a measured topographical survey rather than mapping data where the numbers are close to a threshold, and state the datum. Consented but unbuilt schemes on adjoining land are a judgement call: model them as a sensitivity test and say you have, because a reviewer who finds a consented tower missing from your baseline will stop reading.

Room use is the input most often disputed, and the article on room use assumptions in a daylight assessment sets out how to evidence it rather than assert it.

Reflectance and transmittance assumptions

Internal reflectance affects the average daylight factor and the target illuminance results, and it does not affect vertical sky component or the no sky line at all. Knowing which test is sensitive to what saves a good deal of pointless argument.

Where reflectance is used, state the values applied to ceiling, walls and floor and the area weighted average that follows. Typical modelling defaults sit around 0.7 for ceilings, 0.5 for walls and 0.2 for floors, and a submission claiming higher figures needs a reason. Assessments of existing neighbouring rooms are a particular trap, because you cannot know how somebody has decorated. Use the guidance defaults for third party rooms and say so.

Transmittance follows the glazing specification. Report the visible light transmittance of the specified unit, not the centre pane figure for a single sheet, and apply the maintenance and framing factors the guidance describes. A model run at 0.8 transmittance where the specification is a triple glazed unit at 0.6 will be picked apart.

One honest uncertainty worth stating in the report: where the glazing specification is not fixed at planning stage, model the worst case you would accept as a condition, and offer that condition.

Model geometry and how much detail to build

Build what affects the result and no more.

Balconies, recessed reveals, brise soleil, projecting bays and overhangs all cut daylight and all belong in the model. Internal partitions matter where they define the room being assessed. Furniture does not. Deciduous trees are normally excluded from the geometry under the BRE approach, though a dense evergreen screen close to a window is worth a sensitivity run.

Level of detail on surrounding buildings should be proportionate. A block 60 metres away can be a massing box. The building 6 metres opposite needs its parapet, its recesses and its actual roof form, because those are what define the visible sky angle at the window you are assessing.

Two geometry errors recur. Mirrored or rotated plans, which produce results for the wrong elevation and are hard to spot once rendered. And models built to the outside face of the wall, so window positions sit forward of where they will be, flattering the reveal.

Check the model against a section drawn by hand through the worst affected window. If the visible sky angle on the drawing and the angle in the model differ, the model is wrong.

Presenting results so a reviewer can check them

The report is the deliverable, and this is where most assessments lose officers. Reviewers are looking for the ability to audit, and the article on what a case officer checks in a daylight report first covers the order they read in.

Include, as a minimum:

1. A methodology section naming the guidance edition, the tests applied, the software and its version, and every assumption with its source.
2. Keyed floor plans of every assessed property, with room references and room uses, and the window serving each room identified.
3. Results tables by room and by window, showing the existing value, the proposed value and the ratio, with the relevant guidance reference in the column heading rather than in a footnote.
4. Sections or elevations showing obstruction angles at the worst affected windows.
5. A short list of rooms that do not meet the guidance, with the reason and the planning judgement, in the body of the report rather than buried in an appendix.

Do not present only percentages of rooms passing. An officer needs to know which rooms, in which property, on which floor. And offer the model file to the council's reviewer. Refusing that request is read as an admission.

Errors reviewers find most often

Wrong room use, first and always. A kitchen treated as a non-habitable space when it is the only living space in a one bedroom flat.

Then the transposition errors: obstruction heights taken from a drawing at a different datum, window head heights scaled from an elevation rather than read from a schedule, and floor to floor heights assumed rather than taken from the section.

After that, a set of reasoning failures. Treating the numerical targets in BRE 209 as standards to be met rather than guidance to be applied with judgement, which the guidance itself warns against. Presenting a 0.8 times former value comparison without stating the absolute values. Omitting winter sunlight results where the test applies. Assessing only the worst window in a room that has two.

And one that comes up in extensions: assessing the neighbour's principal window while ignoring a second window to the same room, which changes the daylight distribution result. The article on rear extensions and neighbour amenity in terraced streets works through that geometry.

Agreeing the method before you model

A short methodology note sent to the case officer before the modelling starts is the least costly risk reduction available on a daylight job.

Set out the properties and rooms to be assessed, the tests to be applied and why, the software, the surface reflectances and transmittance, the treatment of consented schemes, and the sensitivity tests you propose. Ask the officer to confirm or comment. Where the council uses an external reviewer, ask for that reviewer's preferences at the same time, because they will have them.

On sensitive schemes it is worth putting the note in as part of a pre-application submission alongside the Daylight and Sunlight Assessment scope and, where overshadowing of gardens or amenity space is in play, the Overshadowing Study method. Agreement on method turns a later disagreement into a disagreement about judgement, which is a much better place to be arguing from.

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.