Choosing a daylight and sunlight consultant
Two competent practitioners should agree on a daylight result. When they do not, the cause is almost always inputs. What to ask before you appoint.
Daylight and sunlight is the one technical discipline where the quality of the consultant changes the answer. The method in BRE 209 is published and the arithmetic is not difficult, but the results depend almost entirely on how the model was built, which rooms were assumed behind which windows, and which numbers the author chose to put in the summary table. Two competent practitioners modelling the same scheme should agree. In practice they often do not, and the difference is usually traceable to inputs rather than to method.
So when you are appointing, ask about the model before you ask about the fee.
What the BRE guidance actually requires
The third edition of BRE 209, published in 2022, sets out the methods councils rely on. For daylight to neighbouring windows, vertical sky component and the no sky line, with the familiar numerical targets of 27 per cent vertical sky component and retention of at least 0.8 times the former value. For sunlight to neighbouring windows, annual probable sunlight hours, applied to windows within 90 degrees of due south. For gardens and amenity areas, the two-hour sunlight test on 21 March across at least half the area.
For daylight inside new dwellings, the 2022 edition moved to the methods in BS EN 17037, working in illuminance and spatial daylight autonomy rather than the older average daylight factor. Plenty of assessments still present daylight factors, and plenty of council officers still ask for them, so be explicit in the brief about which method you want and why.
The guidance says of itself that its numerical targets are advisory and should be applied flexibly according to context. That flexibility is real and it is routinely overstated by applicants and understated by objectors. A consultant who tells you the targets are either absolute or irrelevant is telling you something about their advocacy rather than about the guidance.
Software, models and verification
Ask how the existing context was modelled. A model built from a measured topographic survey with verified building heights is a different thing from one traced off aerial imagery and extruded to an assumed storey height.
Ask what sits behind the neighbouring windows. Room depths and layouts drive the no sky line result more than anything else, and where the consultant could not get inside, the assumption has to be stated. Our note on assessing daylight to a property you cannot access covers the conventions councils accept and the ones they challenge.
Ask about verification. Reputable practices run a sanity check against a simple hand calculation or a known geometry before trusting the output, and they will say so without being pushed.
Finally, ask to see the model. A consultant who will not share the geometry file with a reviewer is creating a problem for you at consultation stage.
Competence for neighbour assessment and for internal daylight
These are related but not identical skills, and a practice can be strong at one and weak at the other.
Neighbour assessment is about geometry, context and the fair application of the BRE tests, including the judgement calls about which windows are habitable, which are served by balconies, and where a baseline is already unusually poor. It is argued at committee and sometimes at appeal, so the author needs to be able to stand behind the figures in writing.
Internal daylight for new homes is a design tool as much as a compliance exercise. It drives window sizes, reveal depths, balcony arrangements and room layouts, and it needs to be run early enough to change them. Running it at the end produces a report that says the scheme fails and a design team with no time to respond.
There is no statutory licence for this work. Councils expect a suitably qualified author, typically a chartered surveyor or a specialist practice with demonstrable experience of comparable schemes, and professional review is strongly advised before submission. You can find reviewers through the professional directory.
Reading an assessment critically
| What to look for | Why it matters |
|---|---|
| Both vertical sky component and no sky line reported | Reporting one without the other hides half the daylight picture |
| Absolute values alongside percentage change | A window falling from 8 to 6 per cent vertical sky component is a small percentage loss from an already poor baseline |
| Window schedule with room use identified | Bathrooms and circulation are not habitable and should be excluded, but not quietly |
| Sunlight results for all windows within 90 degrees of south | Selective inclusion is the commonest form of presentational bias |
| The existing baseline scenario described | Comparing against a notional cleared site rather than the building actually there changes everything |
| Sensitivity to balconies and recessed windows | These depress baseline values and the report should say so rather than treat them as neutral |
Our piece on reading the results tables goes through a worked table line by line.
Where results get presented selectively
The most common is reporting only the windows that pass and summarising the rest as broadly compliant. The second is quoting the 0.8 relative test while ignoring that the absolute value has fallen well below 27 per cent. The third is describing a result as acceptable in an urban context without saying what the context actually is or citing any policy that endorses a different target.
Dense urban settings do justify a different reading of the guidance, and in parts of London this is standard practice rather than special pleading. But it has to be reasoned against local policy and comparable schemes, as our note on BRE 209 in dense London contexts explains.
Objectors do the mirror image of all this, so read both sides with the same scepticism.
Instructing at the right design stage
- Run an obstruction angle test on the initial massing, before elevations exist. It costs little and it tells you which neighbours are in play.
- Commission the neighbour assessment when the massing is fixed but before planning drawings are finalised, so that a setback or a chamfer is still possible.
- Run internal daylight analysis alongside the unit layouts, not after them.
- Reassess if the massing changes by more than a storey or the building line moves.
- Keep the same consultant through to any appeal, because the person who built the model is the person who can defend it.
The early massing test is the step design teams skip and regret, and we set out how it works in the note on obstruction angles.
Cost drivers and what to give the consultant
Fees track the number of surrounding properties assessed, whether internal layouts are available or have to be assumed, whether a measured survey is needed, and whether BS EN 17037 internal analysis is required across a large residential scheme. Appeal support is priced separately and should be.
Give the consultant the topographic survey, the massing model in a usable format, the accommodation schedule, any internal layouts of neighbouring properties you hold, and the planning history of the surrounding buildings. Withholding the awkward neighbour does not make them disappear.
A daylight and sunlight assessment drafted to the right structure, with the policy framing already in place, gives the consultant a head start on the reporting and keeps the fee on the analysis where it belongs.
For specialist technical reports, qualified professional input may be required. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
What qualifications should a daylight consultant hold?
There is no statutory licence for this work. Councils expect a suitably qualified author, typically a chartered surveyor or a specialist practice with demonstrable experience of comparable schemes in similar contexts. Ask for recent examples that went to committee or appeal, and for confirmation that the author will stand behind the figures in writing.
What are the BRE 209 numerical targets?
For daylight to neighbouring windows, 27 per cent vertical sky component or retention of at least 0.8 times the former value, with an equivalent relative test for the no sky line. For sunlight, annual probable sunlight hours applied to windows within 90 degrees of due south. For gardens, two hours of sunlight on 21 March across at least half the area.
Is the average daylight factor still used?
The 2022 edition of BRE 209 points to the methods in BS EN 17037 for daylight inside new dwellings, working in illuminance and spatial daylight autonomy. Many assessments still present daylight factors and many officers still ask for them, so state in the brief which method you want and make sure the report explains its choice.
When should the assessment be commissioned?
Run an obstruction angle test on the initial massing before elevations exist, then commission the neighbour assessment once the massing is fixed but before drawings are finalised. Internal daylight analysis should run alongside unit layouts. An assessment delivered after the design is frozen reports failure without leaving time to fix it.
How do I tell whether an assessment is presented fairly?
Check that both vertical sky component and no sky line results appear, that absolute values are given alongside percentage changes, that the window schedule identifies room uses, and that all windows within 90 degrees of south are included in the sunlight analysis. Selective inclusion is the commonest form of presentational bias.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.