Choosing the baseline for a daylight assessment

The baseline you model decides the numbers, and the numbers decide the argument. Two daylight assessments of the same scheme can report a 12 per cent reduction and a 34 per cent reduction to the same window, with neither consultant doing anything wrong, because one measured against the buildings standing today and the other measured against a consented scheme that has not been built. Choosing the baseline is a planning judgement dressed as a technical input, and it should be settled with the case officer long before the report is written.

Why the scenario matters more than the metric

Vertical sky component, no sky line and annual probable sunlight hours are all comparative measures. Each expresses a change from a starting condition, and the 0.8 rule in BRE 209 asks whether the proposed value falls below 0.8 times the former value.

Change the former value and you change every result in the table. A neighbouring window facing an empty yard has a high starting value, so almost any development in front of it produces a large proportional reduction. The same window facing a consented four-storey block already has a low starting value, and the marginal effect of your scheme looks modest.

Neither picture is dishonest. They answer different questions. The officer's real question is what the occupier of that room will experience if your scheme is built, and that depends on what else is going to happen around them.

The three scenarios that appear in practice

Most reports work with some combination of the following.

The existing scenario is the one that should always be reported, because it describes the actual change to real people. The consented scenario is legitimate and useful, though it invites a question about likelihood: an extant permission with a lapsed reserved matters approval and no site activity is a different proposition from one under construction.

Cumulative assessment belongs in areas where the plan anticipates coordinated change, such as an allocated regeneration area or an opportunity area with a masterplan. Presenting a cumulative baseline in a suburban context, where nothing else is coming forward, reads as an attempt to dilute the effect.

The fourth row is the contentious one. Modelling a notional policy-compliant building on the application site, and comparing your scheme against that rather than against the existing low-rise structure, can be a reasonable way to show that the site was always going to be developed. It can also look like an argument for whatever massing you happen to want. If you use it, present it as a secondary sensitivity test alongside the existing baseline, never instead of it.

Presenting cumulative effects honestly

Where more than one scenario is reported, the temptation is to lead with the flattering one. Resist it.

Set out the existing-baseline results first and in full. Then present each additional scenario as a separate table with the same receptor references, so a reader can trace one window across all of them. Explain in a sentence what each scenario assumes and why it was included.

Receptor numbering is worth getting right early. Number every assessed window on a plan, keep the same reference in every table and in every appendix, and use it in the Design and Access Statement when discussing design responses. Reports that renumber between sections are the ones that lose arguments they should win.

Where a scenario produces materially different conclusions, say so plainly. An assessment that concludes "the scheme complies" without noting that compliance depends on a neighbouring permission being implemented is storing up a problem for committee.

Agreeing the baseline with the case officer

The productive moment is pre-application, before any modelling has been paid for. What to put in a short scoping note:

1. The proposed receptors, shown on a plan with the numbering you intend to use.
2. Which windows are assumed to serve habitable rooms, and the basis for that assumption.
3. The scenarios you propose to model and the reason for each.
4. The source of the surrounding building geometry, whether a measured survey, photogrammetry or mapping data.
5. The treatment of any consented schemes, with application references.
6. The metrics and any local plan standards that apply instead of, or alongside, the national guidance.

Ask for a written response. An officer who agrees the scope in an email has effectively agreed not to argue about it later, and if the scope is refused you have found out at the point where changing it costs almost nothing.

Where a local plan or a borough guidance document sets its own daylight standards, those take precedence over generic guidance, and some London boroughs are specific about which scenarios they will accept. Check before you model rather than after.

Documenting the assumptions

Every assessment carries a set of judgements that are invisible in the results table. Write them down.

The list that matters: room depths and whether they were measured or assumed, window dimensions and their source, the reflectance values applied to internal surfaces, the treatment of balconies and overhangs, whether obscured or secondary glazing was accounted for, how rooms with more than one window were handled, and what was done where access to a property was refused.

Refused access is the honest gap in most urban assessments. State it. Model the room on a reasonable assumption, say what the assumption was, and flag the sensitivity. A report that quietly models an assumed 4 m deep room and never mentions it is one cross-examination away from trouble.

A Daylight and Sunlight Assessment that includes an assumptions register as an appendix is easier for an officer to accept and far easier to defend later. The same discipline applies to an Overshadowing Study, where the ground model, the assessment date and the definition of the amenity area all shape the outcome.

Where a contested baseline goes wrong

The failure pattern is predictable. The applicant models a consented baseline, the objector's consultant models the existing one, and the two reports never engage with each other. The officer, faced with irreconcilable numbers and no way to test them, gives limited weight to both and decides on qualitative grounds instead. All the technical work is wasted.

Avoiding that costs one email at pre-application stage. Where agreement proves impossible, model both, report both, and argue the weight in the planning statement rather than embedding the argument in the modelling assumptions where nobody can see it.

An illustrative example: a mid-rise scheme opposite a consented but unstarted block reports existing-baseline reductions that breach the guide targets at six windows, and consented-baseline reductions that breach at one. Reporting only the second figure would be indefensible. Reporting both, with an explanation of the consented scheme's status, is the version an officer can actually use.

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, and daylight modelling should be prepared or checked by a suitably qualified consultant. We do not guarantee planning approval. Enter your postcode on PlanScope to see which amenity documents a scheme is likely to need.