Submitting a daylight model: files, assumptions and checks

A peer reviewer's job is to reproduce your result, and they will ask for whatever they need to do it. Supply it in the first submission and the review takes a fortnight. Supply a set of PDF tables and a rendered image and it takes three months, most of which is spent exchanging emails about the height of a building two streets away.

What a reviewer needs is narrower than a full project archive and wider than most submissions provide. The model file, the context source, the reflectance and transmittance values, the room and window schedules, and a clear statement of which drawing revision the model was built from.

What a reviewer needs to reproduce your result

Six things, and the absence of any one of them stalls the review:

1. The analysis model in a format the reviewer's software can read, with the assessment surfaces and reference planes intact.
2. The source and date of the surrounding context, with a statement of how heights were derived.
3. The material properties used: surface reflectances by element and glazing visible light transmittance.
4. Room schedules identifying every assessed room by unit, floor, room type and area, with the working plane height.
5. Window schedules with net glazed areas and any obstruction modelled, such as a balcony, reveal or brise soleil.
6. The drawing revision and date the model was built from, for both the proposed scheme and the existing buildings.

That last item is the one that causes the most wasted effort in practice. A reviewer comparing a model against a later drawing revision will find discrepancies that are real but irrelevant, and the correspondence that follows is entirely avoidable.

Context model extent and accuracy

The context model has to extend far enough that buildings outside it cannot affect the result, which is a function of height and separation rather than a fixed radius. A tall building 200m away can obstruct a low window; a two-storey terrace 60m away cannot.

State the extent and justify it. Then state how the context was built, because the three common sources have very different accuracy.

Photogrammetric or laser-scanned city models are accurate in plan and reasonable in height, and come with a survey date. Mapping-derived extrusions using building height attributes are convenient and can be wrong by a storey, particularly on pitched roofs where the attribute may record eaves or ridge inconsistently. Manual modelling from drawings and photographs is accurate where the drawings exist and guesswork where they do not.

Mixed sources are normal and acceptable, provided the report says which buildings came from where. What is not acceptable is a single sentence saying the context was modelled from available data, because it leaves the reviewer no basis to check anything.

Two details worth getting right. Pitched roofs modelled as flat boxes at ridge height overstate obstruction, sometimes materially. And boundary walls, garden outbuildings and mature vegetation affect ground-floor windows and are routinely omitted.

Surface reflectance and glazing transmittance

These are the inputs a reviewer checks first when a result looks favourable, and they are the easiest to overclaim.

For internal surfaces, state the reflectance used for ceilings, walls and floors separately, and use values that reflect a normal finished interior rather than a best case. A model assuming very high reflectance throughout every room is assuming a decorating scheme nobody has committed to, and a reviewer will rerun it at more typical values.

For glazing, use the visible light transmittance of the specified unit, not a generic figure. This matters more than people expect: a solar control double-glazed unit can transmit noticeably less visible light than a clear one, and a triple-glazed unit less again. Where the specification is not fixed, model the lowest transmittance the design is likely to end up with, because the overheating strategy pushes in that direction. The tension between the two is set out in daylight, glazing area and Part O overheating.

For external surfaces, state the reflectance assumed for the proposed facades and for the context. Light-coloured surfaces in a courtyard can make a real difference to reflected daylight, and an assumption of a bright white render that becomes grey brick at reserved matters is a problem.

Room and window schedules

The schedule is the backbone of the submission and it should be readable without the model.

For each assessed room: unit reference, floor level, room type as assessed, floor area, working plane height, and the result against each criterion. For each window serving it: net glazed area, head and cill heights, orientation, and any obstruction modelled.

Two conventions to state explicitly. How room type was determined, since a room shown as a study on the drawing may be assessed as a habitable room and the criteria differ. And how net glazed area was derived, because frame allowances vary and a 15 per cent frame assumption against a 10 per cent one changes results across an entire scheme.

Where a room is open plan, say how it was treated and which criterion was applied. The realistic targets on a constrained site are covered in daylight in a dense urban block.

Version control between drawings and model

Most disputes at review stage are version disputes wearing technical clothing.

Fix the position with a model register: a short table listing every model run, its date, the proposed drawing revision used, the context source and date, and what changed from the previous run. Put it at the front of the appendices.

Then keep the numbers in the report tied to a single run. A report that draws its summary table from one run and its appendix schedules from another is a report that will be rejected, and it happens routinely when a design change lands mid-assessment.

Where a design change arrives after the assessment is complete, the honest options are to rerun, or to state clearly which elements changed and why the change cannot affect the result. The second is acceptable for a change to internal partitions and unacceptable for a change to massing.

Supplying model files and outputs

Supply the native model and an exported neutral format where the software allows, because the reviewer may not hold the same licence.

Include the raw outputs as well as the summary. Grid-based results, vertical sky component values at each window, and the numerical results behind any diagram. A reviewer who has to read values off a colour-coded image will ask for the numbers, and the request adds a fortnight.

Diagrams help but do not substitute. Where you supply a rendered view, include the scale, the orientation and the date and time for any sunlight image, because a sunlight diagram without a date is decoration.

And keep file sizes workable. A 4GB model that the reviewer's machine cannot open is functionally the same as no model at all. Split it if necessary and say how the parts relate.

Responding to a peer review

Reviews come back with three kinds of comment, and they need different treatment.

Factual corrections, where the reviewer has found a wrong height, a missing obstruction or a transposed area. Accept these, correct them, rerun and report the revised figures even where they worsen the result. A reviewer who catches an error and then finds the revised result presented without comment will check everything else.

Methodological disagreements, where the reviewer would have used a different convention. Set out your reasoning with the source, and where the difference is material, run the alternative and report both. Two numbers with an explanation is a stronger position than one number defended.

Requests for information you should have supplied. Send it without arguing, and take the point for the next submission.

The Daylight and Sunlight Assessment that survives review is the one written for the reviewer rather than for the committee. Assume they are sceptical, competent and short of time, and give them everything needed to arrive at your answer independently.

Enter your postcode on PlanScope to see which daylight and amenity reports your scheme is likely to need.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified daylight consultant 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.