Who can do a daylight assessment and how to check one

Nobody is licensed to carry out a daylight and sunlight assessment. The BRE guide on site layout planning for daylight and sunlight is advisory, there is no protected title for the people who apply it, and a local planning authority will accept an assessment from whoever the applicant appoints. So the question of who can do a daylight assessment is answered by competence and by evidence of it, not by a register.

That matters because the guidance itself says its numerical values are not to be applied as mandatory limits, and interpreting them properly is the whole skill. An assessment that reports numbers without judgement is worth very little in a contested case.

What the guidance actually says about application

The guide sets out methods and numerical guidelines for daylight to existing and proposed buildings, sunlight to buildings and to open space, and the effect of obstructions. It also says, in terms, that the figures are advisory and that they should be applied flexibly, with the context of the site and the character of the area taken into account.

That last point is where competent practitioners and spreadsheet operators diverge. A dense urban site will not meet suburban values, and an assessment that reports widespread non-compliance without explaining the context invites either a refusal or an argument the applicant did not need to have.

The third edition of the guide also changed the internal daylight method, moving away from average daylight factor towards the illuminance-based approach in the relevant European standard. An assessment using the older method is not automatically wrong, but it has to say which edition and which method it used, and an assessment that does not say is the first thing to query.

Report the results in the Daylight and Sunlight Assessment, which councils expect from a suitably qualified author and where professional review is strongly advised.

Modelling competence and software

The calculations are geometric, and the model is the assessment. Competence is mostly about building a model that reflects reality.

Three things distinguish a reliable model. It is built from a measured survey rather than from online imagery, with the surveyed heights and positions of surrounding buildings recorded. It includes the features that block light, meaning balconies, overhangs, window reveals, boundary walls, parapets and projecting bays, rather than treating facades as flat planes. And it is checked against photographs from the assessed windows, which is the simplest sanity test available.

Software matters less than inputs. Any of the established calculation tools will produce the same vertical sky component for the same geometry. What varies between consultants is what went into the geometry, and that is what a reviewer should interrogate.

Ask for the model, not just the tables. A consultant who will not share the model or a clear set of plans and sections showing what was assumed is telling you something.

Where a rights of light surveyor differs

These are two different disciplines and they are regularly confused by clients.

A daylight and sunlight assessment is planning evidence, applying advisory guidance to amenity, assessed against the planning balance and weighed with everything else. A rights of light analysis is a legal exercise about an easement acquired over time, measured by a different method, with remedies in the civil courts including damages and, in principle, an injunction.

The practical consequences are stark. A scheme can be acceptable in planning terms and still face a rights of light claim from a neighbour, and planning permission is no defence to that claim. Conversely a neighbour with no enforceable easement can still object effectively on planning amenity grounds.

On a scheme adjoining a building with long-established windows, instruct both. The daylight consultant produces the planning evidence and a rights of light surveyor advises on the legal exposure and on insurance, and the two analyses do not substitute for each other.

Assumptions that change the result

Six assumptions move the numbers, and each should be stated explicitly in the report:

• the baseline. Whether the existing condition or an extant consented scheme is used as the comparison, which can change a large relative loss into a modest one
• the room uses behind the assessed windows, which on a neighbouring building are often unknown and have to be inferred, with the inference stated
• which rooms are counted. Kitchens, bathrooms, circulation space and non-habitable rooms are treated differently, and excluding a room needs a reason
• the surface reflectance and the room depths assumed for internal daylight calculations
• whether the surrounding context is modelled as existing, as consented or as allocated in the development plan
• the window geometry, including reveal depth and any obscured or fixed glazing

Changing the baseline is the single most common source of dispute, because it is legitimate to assess against a consented scheme and it is also legitimate for an objector to point out that the consented scheme was never built.

Reviewing an assessment critically

Work through it in this order when the report lands on your desk:

1. Find the edition of the guidance and the method used for internal daylight, and check both are current or justified.
2. Check the model source. Measured survey or desktop, and what was surveyed.
3. Read the baseline statement, and look for any reliance on a consented scheme.
4. Look at the schedule of rooms assessed and ask what was excluded and why.
5. Read the retained values, not only the percentage changes. A room losing 40 per cent of a very high value may still be well lit, and a room losing 20 per cent of a poor value may not be.
6. Check the sunlight results separately, including sunlight to amenity space, which is often skipped on urban schemes.
7. Look for the context paragraph. If there is no discussion of the character of the area and the density of the surroundings, the report has done the arithmetic and not the assessment.

Where the authority's own consultant reaches a different conclusion, the difference is almost always in the assumptions rather than the calculation, and what happens when the council's expert disagrees with your consultant sets out how that plays out.

Conditions and verification

Daylight results are rarely secured by condition, because the geometry is fixed by the approved drawings. Three exceptions come up.

A condition requiring internal layouts to accord with those assessed, where the applicant's own daylight compliance depended on particular room arrangements. A condition on obscured or fixed glazing where overlooking mitigation also affected the daylight analysis, which can create a conflict worth checking before it is imposed. And a condition requiring a revised assessment where a reserved matter or a subsequent amendment changes the massing.

On an outline application, the realistic position is a strategy and a parameter-based assessment, with a condition requiring a full assessment at reserved matters against the approved parameters. Say that plainly rather than producing a detailed assessment of a scheme that does not exist yet.

Appointment before the massing is fixed

Appoint the consultant at concept stage, to test massing options rather than to assess a settled design. Two or three massing studies run early will identify which elevation is the problem and how much height or depth the scheme can carry, and that information is worth more than a compliant report on a scheme you cannot build.

Then expect a second appointment for the submission assessment, and budget for a third if the authority instructs its own reviewer and a response is needed. Where you want an independent view on an inherited assessment, PlanScope's professional directory is one route for requesting a review and recording its status. Naming the author and the date on the report matters here as much as anywhere, for the reasons set out in why a planning report should name its author and date.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This article is general guidance rather than advice on a specific site, and rights of light is a legal question, so take advice from a solicitor and a qualified surveyor where an easement may exist.