Briefing a daylight consultant: the inputs to supply

A daylight and sunlight consultant can only produce a defensible assessment from a correctly georeferenced massing model, a measured survey of the neighbouring buildings and an agreed list of affected properties. Supply those three things properly and the first run of results arrives in a fortnight. Supply a SketchUp file with no coordinates, no levels and guessed window positions and you will pay twice, once for the modelling and again for the rework when the officer asks where the cill heights came from.

The brief is worth writing down. A one page instruction setting out the scope, the deliverables and the assumptions saves the argument that otherwise happens at stage three.

Drawings and models to provide

The consultant needs the proposed scheme as a three dimensional model, and the format matters less than the setting out. Any of the common modelling formats will import. What has to be right is the origin, the rotation and the vertical datum.

Provide the model with the site origin tied to a known ordnance survey coordinate and with true north set correctly, not project north. A model rotated to suit a drawing sheet produces sunlight results that are wrong by whatever the rotation is, and the error is invisible in the output.

Levels come next. Supply a topographical survey to ordnance datum covering the site and the ground immediately around it, including the ground at the base of each neighbouring building. Daylight results depend on the relative height of a window above the obstruction, and a half metre error in site levels moves every vertical sky component figure.

Also supply proposed general arrangement plans and sections at a readable scale, an elevation study, and the existing site model showing what will be demolished. The consultant has to run a baseline using the existing condition, so the existing building has to be modelled as accurately as the proposal.

Surveying neighbouring windows and room uses

This is the part that takes time and the part clients try to skip.

Every window that may be affected needs a position, a width, a height, a cill level and a head level, all tied to the same datum as the site survey. Where access to the neighbouring property is available, the room behind each window should be measured, with its depth, its use and the number of windows serving it. Room depth changes the daylight distribution result entirely.

Access is usually refused. Where it is, the consultant works from an external measured survey and states the assumptions: an assumed room depth, an assumed single aspect, an assumed residential use. Those assumptions must be in the report, because an objector's consultant will test them.

Room use is the judgement that matters most. BRE 209 applies its sunlight tests to main living rooms and treats bedrooms and kitchens as less sensitive, so the assumed use of each room changes which tests apply. Where the use is uncertain, external observation helps, and estate agent floor plans in the public domain are a legitimate source.

Write to the neighbours early asking for internal access, and keep the correspondence. A refusal on file justifies the assumptions. Silence does not look as good at appeal.

Reflectance and transmittance assumptions

The guidance carries default values, and a report should state the ones it has used rather than leaving them implicit.

The assumptions that usually need stating are the reflectance of external obstructions, the average internal surface reflectance of the room being assessed, the diffuse transmittance of the glazing, and the maintenance factor applied for dirt on glass. BRE 209 and the supporting British Standards give defaults for each.

Where the proposal uses a lighter facade material than the existing building, an inter-reflected component argument is sometimes run. Treat it with caution. Officers are sceptical of a daylight case that depends on the reflectance of a facade that could be repainted, and a result that only passes with an elevated reflectance assumption will be challenged.

For new accommodation inside the scheme, the glazing specification matters directly. A solar control unit with a low light transmittance can push internal daylight below target, and that is a coordination point between the facade consultant and the daylight consultant rather than something to discover at the end, in the same way that plan rearrangement rather than bigger windows usually fixes daylight to single aspect flats.

Agreeing the scope of affected properties

Deciding which neighbours to assess is a scoping decision that should be made with the local planning authority rather than by the consultant alone.

The usual approach is to include every residential property where the proposal would subtend more than 25 degrees at the lowest window, plus any property specifically raised at pre-application. Some authorities ask for a fixed radius. Some ask for a named list. Local amenity policies vary in how they apply the guidance, which is the subject of local plan amenity policies and how they use BRE 209.

Get the list agreed in writing at pre-application. A scheme that assesses 40 windows and is then asked to assess another building at committee loses weeks.

Amenity areas need scoping too. Gardens, courtyards, play areas and allotments are assessed for sunlight on the ground rather than at a window, and the test is a different one. Say explicitly which open spaces are in scope.

Deliverables and file formats

Specify the deliverables in the instruction, because a report that arrives without the underlying tables is hard to defend.

• A written report explaining the method, the assumptions and the results against each test.
• Tabulated results per window and per room, with the existing and proposed values and the ratio between them.
• Plans showing the assessed windows numbered to match the tables.
• The no-sky line plots for daylight distribution.
• Sunlight results for the relevant rooms, and sun on ground results for amenity areas.
• The three dimensional model file in an editable format.
• A short non-technical summary the planning statement can quote.

Ask for the model file explicitly. If the scheme changes after submission, or the assessment has to be rerun for an appeal, owning the model avoids paying for it twice.

Programme and when to run the first test

Run a first test on massing, at the point where the building envelope is roughly fixed and before the facade design starts. That test is quick, it costs little relative to the design cost at that stage, and it is the only point at which a bad result can be fixed by moving mass rather than by argument.

A second run goes in once the design is fixed, with the full survey data and the agreed scope, feeding the submission. Allow three to four weeks for that from receipt of complete information, and longer where a measured survey of neighbours is included.

Construction phase effects are a separate question, and a temporary obstruction such as scaffolding or a tower crane is treated differently from a permanent one, as temporary loss of daylight during construction explains. Scope it in only where the programme is long enough to matter.

A formal Daylight and Sunlight Assessment carries a named author, and councils expect the work to come from a practitioner with the relevant competence, so an AI-assisted draft is a starting point for that specialist rather than a replacement for the modelling.

Comparing quotations

Fee comparisons on this work are usually comparing different scopes rather than different prices.

Check five things in each quotation. The number of neighbouring properties and windows included. Whether a measured survey of neighbouring buildings is in the fee or excluded. Whether internal daylight to the proposed units is included alongside the neighbour assessment. How many design iterations are covered before extra fees apply. And whether support at committee, in response to an objector's report, or at appeal is included or charged separately.

The last one is where budgets break. A scheme with an organised objection will generate a rebuttal, a response to the rebuttal and possibly an appeal statement, and that work can exceed the original assessment fee. Agree a rate for it at the outset rather than negotiating under pressure.

Where overshadowing of gardens and open space is a live issue, confirm whether an Overshadowing Study is inside the fee or a separate instruction.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

Enter your postcode on PlanScope to see which amenity and daylight documents a scheme 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.