Room use assumptions and how they change results

Daylight room use assumptions decide more results than the tests do. Before a single vertical sky component figure is calculated, the assessor has fixed which rooms count, what each one is used for, and where the internal partitions sit. Change a kitchen to a living room, or split an open plan space into two notional rooms, and the same building produces a different conclusion. BRE 209 sets the tests. The room schedule sets what the tests are applied to, and it is the part of the report most often left undocumented.

Which rooms the guidance treats as sensitive

BRE 209 directs the analysis at rooms where daylight is expected, and the hierarchy it describes places main living rooms first, then dining rooms and kitchens, with bedrooms treated as less demanding.

That hierarchy carries through to both families of test. The vertical sky component and no-sky line calculations are applied to windows serving rooms that need daylight. Annual probable sunlight hours are assessed for windows within 90 degrees of due south serving main living rooms, and the guidance treats sunlight to bedrooms and kitchens as less important.

So the same window loss produces a different verdict depending on the label attached to the room behind it. An assessor who records a ground-floor room as a bedroom has removed it from the sunlight analysis altogether. That may be perfectly correct. It needs to be visible in the report either way.

Kitchens, bathrooms and circulation

Bathrooms, cloakrooms, toilets, stores, garages, halls, stairs and landings are not normally analysed. Nobody argues about those.

Kitchens are where the disagreement lives. A separate galley kitchen in a Victorian terrace is a room where daylight matters but sunlight is treated as less important. The difficulty is that the assessor rarely knows whether the room is a kitchen at all, because they are standing outside looking at a window. Assumptions get made from window size, position, the presence of an extract vent and the age of the building, and those assumptions are reasonable. They are still assumptions.

Where existing conditions are already poor, the choice of room use interacts with the baseline, and the combined effect can flip a conclusion from acceptable to unacceptable. That interaction sits alongside assessing daylight where existing conditions are already poor, because the two methodological choices compound.

Open plan living, kitchen and dining rooms

An open plan living, kitchen and dining space is one room with one no-sky line, and the deep part of the plan behind the kitchen drags the daylight distribution result down.

Two habits produce inflated results here. The first is assessing only the front portion of an open plan space as though a partition existed. The second is treating a room served by windows on two elevations as two separate rooms, each assessed against its own window, which double counts the sky visible from the overlap.

Neither is defensible where the layout is known from drawings. In new-build assessments the layouts are known, so there is no excuse. In assessments of neighbouring properties the layout usually is not known, and the honest approach is to model the room as a single space, state the assumption, and run a sensitivity test showing what happens if the space is in fact partitioned.

Where the assumed layout comes from

For the proposed development, the architectural drawings are the source and the room schedule should cite drawing numbers and revisions.

For neighbouring properties, the realistic sources are a measured survey where access is granted, previous planning application drawings on the council's register, historic building control records, marketing floorplans, and external observation of window positions, cill heights and elevational rhythm. Each carries a different reliability and the report should say which was used for which property.

Access requests are refused more often than granted, and an assessor who has asked and been refused should record that. A neighbour objecting on daylight grounds who declined access has weakened their own position, which is one of the points that surfaces in daylight evidence in a neighbour objection.

Aerial and street-level imagery helps with window positions and extensions built since the last application. It tells you nothing about internal partitions, and reports that imply otherwise should be challenged.

How one changed assumption moves the result

Take an illustrative first-floor room in a semi-detached house, with a single south-facing window, currently receiving annual probable sunlight hours above the guidance figures and losing a proportion of them to a proposed extension next door.

Recorded as a living room, the loss is assessed against the sunlight criteria and, if it falls below both the annual and winter thresholds and drops by more than a quarter of its former value, it registers as a noticeable reduction. Recorded as a bedroom, the guidance treats the sunlight loss as of less importance and the same physical change may be reported as acceptable.

Nothing about the building changed. The figures are illustrative and the point is structural: a single-word entry in a schedule can decide the finding. That is precisely why the schedule belongs in the report rather than in the assessor's working file.

Disclosure in the report

A defensible daylight and sunlight report states, for every assessed property, the source of the layout, the assumed use of each room, the windows serving each room, and any room deliberately excluded with the reason.

It also states what was not known. Where an assumption could reasonably have gone the other way, the report should say which way it went and why, and where the outcome is sensitive to that choice, it should quantify the alternative. A short sensitivity table costs an afternoon and removes the most common line of attack at committee or appeal.

Excluding rooms without saying so is the failing that damages credibility most, because it looks like concealment even when it was carelessness. A Daylight and Sunlight Assessment that discloses its assumptions openly survives scrutiny better than one with slightly better numbers and nothing to show for them.

Reviewer checks on the room schedule

Working through someone else's assessment, these are the checks that find real problems.

1. Does every assessed window map to a named room with a stated use, and does the count of rooms match the count of windows sensibly for the building type?
2. Are open plan spaces modelled as single rooms, with the no-sky line run across the whole floor area?
3. Are rooms with windows on two elevations handled as one room, rather than counted twice?
4. Do the assumed uses match the drawings for the proposed scheme, revision for revision?
5. Where a room has been excluded, is the reason recorded, and is it one the guidance supports?
6. Has any room use changed between the submitted assessment and an earlier version, and if so, was the change explained?
7. Are room depths and window head heights consistent with the elevations, since both drive daylight distribution more than window area does?

Where several of these fail, the numbers themselves are rarely worth recalculating. Ask for a corrected schedule first, then re-run. And where the assessment supports an argument about shadow on gardens or amenity space rather than inside rooms, check whether an Overshadowing Study is the more appropriate document, because the room schedule question does not arise in the same form.

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 is work councils expect from a suitably qualified assessor. We do not guarantee planning approval. To see which amenity documents a site is likely to need, enter your postcode on PlanScope.