When two daylight reports disagree: how to resolve it

Two competent daylight consultants can assess the same scheme and produce results that differ by ten percentage points on vertical sky component, and neither of them has done anything wrong. The disagreement almost always comes from inputs rather than method: the geometry each model used, the rooms each assumed behind the windows, and the baseline each measured against. Resolving it is a matter of asking for the assumptions in writing and comparing them line by line, which is faster and costs less than commissioning a third report.

When an objector's consultant lands a contradictory assessment three days before committee, that is the drill.

Why two competent assessments reach different answers

BRE guidance sets out numerical targets, not a modelling protocol. The 2022 edition of BR 209 gives the vertical sky component, no-sky line and annual probable sunlight hours measures, and it says plainly that the figures are guidance to be applied flexibly according to context. Everything upstream of the numbers is left to the assessor's judgement.

That leaves four places for two reports to diverge, and in practice they account for nearly every dispute.

Geometry. One model built from a measured topographic survey, the other from Ordnance Survey mapping and photogrammetry. Building heights taken from a laser scan differ from heights derived from imagery, and a 400 mm error in a parapet height 8 m from a window changes the answer.

Window and room assumptions. Where internal layouts are not available, the assessor infers them. A room assumed to be a bedroom lit by one window behaves differently from the same volume treated as a living room with two.

Surface properties. Reflectance values for the obstruction and for internal surfaces, and whether balconies, brise soleil and window reveals were modelled or simplified away.

Baseline. The single largest source of divergence, and the subject of its own argument below.

Model geometry and survey accuracy

Ask for the source of every building in the model and the date of the survey. A report that cannot say where its geometry came from is not defensible at inquiry.

Three checks worth running on any assessment you receive:

• Does the model include buildings that were demolished, or exclude ones recently built? Urban sites change faster than the mapping.
• Are ground levels modelled, or has the assessor assumed a flat site? On a sloping street this alone can move results.
• Are the window positions and sill heights measured or assumed? Sill height drives the no-sky line result directly.

The Daylight and Sunlight Assessment should carry a geometry and assumptions schedule as an appendix. Where it does not, request one before engaging with the numbers.

Room layout and use assumptions

Neighbouring internal layouts are rarely available, and the assessor has to make a choice. That choice is a legitimate area of disagreement.

The strongest position is evidence: internal floor plans from a previous application on the neighbouring property, an approved conversion drawing, or a photographed layout supplied by the occupier. Where an objector's consultant has obtained plans that the applicant's team did not have, the applicant's report is not wrong, it is less well informed, and the honest response is to remodel rather than argue.

Room use matters because the guidance applies different expectations. A kitchen, a bathroom and a circulation space are not treated as main living spaces. An assessor who has classified a rear room as a habitable living room when it is a utility has overstated the impact, and the reverse is equally common.

Where the use is not known at all, the defensible approach is to test both and report both.

Baseline condition and permitted schemes

Two questions decide the baseline, and they are not the same question.

The first is what stands on the application site now. Where an existing building is to be demolished, the comparison should normally be against the existing built condition, not against a cleared site. A site cleared shortly before the application is made invites the argument that the baseline has been manufactured, and inspectors have been alive to that.

The second is what has already been permitted nearby. An unimplemented permission on an adjoining plot may be a material consideration, and a cumulative assessment showing the effect of the proposal alongside committed schemes is often requested. Whether the consented scheme forms the baseline or an additional scenario is a matter of judgement, and it should be stated rather than assumed.

Report both where they differ. An assessment that only presents the baseline most favourable to its client invites the inspector to prefer the other side's.

Requesting the underlying model

Ask for the model. Not the report, the model.

In practice you will be offered an assumptions schedule, a set of context plans and a table of results rather than a native file, and that is usually enough. What you need is the ability to reproduce a sample of the results. Pick three windows where the two reports differ most, and ask both assessors to show their working for those three.

A short technical note comparing the two sets of inputs, agreed between the consultants, resolves more disputes than a third opinion. It also produces something an officer can put in a committee report.

Where the difference survives that exercise, the remaining gap is usually a genuine question of judgement: which baseline, or which room use. That is a matter for the decision maker.

Commissioning an independent review

An independent review is worth the fee when the scheme is large, the objection is well resourced, and the officer has said the daylight evidence will decide the recommendation. It is not worth it when the disagreement is 2 percentage points on one window.

Brief the reviewer to audit rather than to reassess. The instruction should be to test both models against BRE guidance, identify where the inputs differ, and say which input is better supported by evidence. A reviewer asked to produce a third set of results usually produces a third answer, and the officer is no better off.

Councils sometimes appoint the reviewer and recharge the applicant. Agree the scope and the reviewer's independence in writing before the work starts.

How officers handle unresolved disagreement

Officers are not modellers. Faced with two reports and no agreement, they fall back on three things: which report is better evidenced, what the site context justifies, and where the planning balance lands.

Context does real work here. BRE guidance itself accepts that in dense urban areas the numerical targets may be unachievable and that alternative targets can be set, provided the reasoning is explained. A scheme in a town centre that fails vertical sky component but achieves reasonable internal daylight can still be recommended for approval, and the Overshadowing Study and amenity assessment carry weight alongside it.

Give the officer a paragraph they can lift into the report: what is agreed, what is not, why the difference exists, and why the residual harm is acceptable in this context. Officers under time pressure use the words in front of them.

One honest uncertainty: there is no settled convention on cumulative assessment with unimplemented permissions, and different inspectors have taken different approaches. State your position and the reasoning, and do not present it as a rule.

PlanScope drafts the narrative and policy sections that sit around a specialist daylight assessment, structured to UK planning policy, with modelled results left for a qualified assessor to supply. Councils expect a suitably qualified author for daylight work, and professional review is strongly advised. See how professional review works or browse the full report list.

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.