Room use assumptions in a daylight assessment

Two assessors can model the same window on the same neighbouring flat and reach opposite conclusions about acceptability, without either making an arithmetic error. The difference is what they assumed the room behind the window is for. Building Research Establishment guidance weights living rooms most heavily, treats kitchens and dining rooms as important, and is explicit that bedrooms matter less, particularly for sunlight. Change the assumed use and the same numerical result moves from material harm to a minor effect.

Which makes the assumptions schedule the first thing a reviewer should open, and the last thing most assessments include.

What the guidance actually says about rooms

The site layout guidance is framed around main habitable rooms. For daylight, living rooms, dining rooms and kitchens are the primary concern, with a caution against blocking daylight to bedrooms rather than an equal test. For sunlight, the emphasis narrows further: living rooms and conservatories are the focus, and kitchens and bedrooms are described as less important.

Non-habitable rooms sit outside the assessment entirely. Bathrooms, hallways, landings, utility rooms, garages and circulation space are not assessed, and windows serving them can be excluded provided the exclusion is stated and justified.

That hierarchy is guidance rather than a rule, and it exists because it reflects when people occupy rooms. A north-facing bedroom losing winter sunlight hours affects a household less than the same loss to the room they sit in during the evening. An assessment that treats every window identically has discarded the reasoning the guidance is built on.

Where the assumptions come from

Internal access to neighbouring properties is the exception rather than the norm. Owners decline, leaseholders cannot give access to common parts, and on a scheme affecting 200 windows across six blocks, access is impractical even where goodwill exists.

So the assessor works from outside, and the inference chain runs roughly like this: window size and proportion, cill height, position within the elevation, floor level, the presence of net curtains or blinds, whether the window is obscure glazed, the building type and its likely original layout, and any external evidence of subdivision.

Each step is reasonable. Stacked together they produce a room use schedule that is a set of educated guesses, and the honest way to present it is as exactly that.

Sources of evidence worth using

Other sources are better than external observation and are routinely not consulted.

• Previous planning applications on the affected buildings, which often contain floor plans. The authority's public register holds them, and for a converted building there is usually a change of use or extension application with drawings.
• Building control records and, for blocks converted under prior approval, the prior approval submission.
• Estate agent floor plans from recent listings, which are widely available and generally accurate as to room use even where dimensions are approximate.
• The original architect's drawings for post-war blocks, sometimes held in the local archive.
• A written enquiry to the managing agent of a block, which can produce a standard floor plan for every flat in one email.

Where these produce a plan, say so and cite it. An assessment that identifies room uses from a dated floor plan is in a different evidential position from one that identifies them from a photograph of the elevation.

The problem that access solves and nothing else does

Vertical sky component is measured at the outside face of the window and needs no internal information at all. Daylight distribution does. Calculating the no sky line within a room requires the room depth, width, height and the position of the window within it, and none of those can be measured from the street.

Assessments that lack internal access therefore tend to report vertical sky component and stop. That is defensible if it is stated. It is not defensible to present a distribution result derived from an assumed room geometry without saying that the geometry was assumed, because the result looks identical to a measured one in the output table.

The same applies to the illuminance-based methods introduced in the more recent revision of the guidance and in the European daylight standard. They depend on room surfaces, reflectances and geometry, all of which are invented when access is refused.

Recording the assumptions so they can be tested

Put a schedule in the appendix with one row per window and these columns: building, elevation, floor, window reference, assumed room use, basis for the assumption, source document and date, and whether internal access was obtained.

Then state, in the main text rather than a footnote, how many windows were internally surveyed and how many were assumed. A Daylight and Sunlight Assessment reporting that 12 of 340 windows were internally surveyed is telling the reader something important about the weight its results carry, and reporting it plainly is better practice than burying it.

Where a window has been excluded as non-habitable, give the reason in the same row. Obscure glazing is a good reason. "Appears to be a bathroom" is a reason that should be marked as an inference.

Challenging an assumption

For an objector or a reviewing officer, the route is straightforward and it works.

Ask for the assumptions schedule if it is not in the report. Ask how many windows were internally surveyed. Then, for the specific windows you care about, provide better evidence: your own floor plan, a photograph from inside, or an offer of access to the assessor. An offer of access is the strongest move, because refusing it is untenable and accepting it means the assessment has to be updated.

If the corrected use changes a window from a bedroom to a living room, the sunlight tests apply where they previously did not, and the weight attached to a given percentage loss changes. Set that out numerically rather than as a complaint about methodology.

Amenity space is a separate question with its own test, and the two hour sun on ground test does not depend on any room use assumption at all, which sometimes makes it the more productive ground.

Sensitivity testing is the honest answer

The best assessments run the analysis twice: once on the assumed uses, once on a worst case in which every ambiguous window is treated as a living room. Where the conclusion holds under both, the assumption has stopped mattering and the report can say so in a sentence.

Where the conclusion flips, that is worth knowing before the committee report is written rather than at appeal. It also gives the applicant a reason to pursue access to the handful of properties that actually drive the result, which is usually a small number.

This costs an extra model run. It removes the most common line of attack on the report, and it is the difference between an assessment that reads as analysis and one that reads as advocacy. Transient effects deserve the same treatment, since how transient overshadowing is assessed also turns on which spaces are treated as sensitive.

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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. For specialist technical reports, qualified professional input may be required.