Assessing daylight to a lower ground floor flat
A window with a very low starting value fails proportional daylight tests easily. Here is how to model the lightwell, weigh the loss and present the analysis so it can be checked.
A window at or below pavement level starts with a vertical sky component so low that the usual proportional test stops behaving sensibly. Assessing daylight to a lower ground floor flat means reporting absolute values alongside percentage change, because a window with a very low existing value can lose a large proportion of it from an alteration that makes no perceptible difference in the room, while another window can pass the proportional test and still be dark. Reviewers should expect both figures, and a clear statement of what the room is used for.
Why below grade windows behave differently
Vertical sky component measures the amount of visible sky at the centre of a window. A window in a lightwell, or one set below the pavement with railings and a retaining wall in front of it, sees a narrow strip of sky before any new development is proposed.
The guidance in BRE 209 uses two linked ideas: a target value, and a proportional test where the value after development should not fall below 0.8 times the value before. On a window starting near the target, that combination works well. On a window with a very low starting value, a small absolute reduction crosses the proportional threshold easily.
The arithmetic is unforgiving at the bottom of the scale. A fall of one percentage point is a large proportion of a small number, and the same fall at a first floor window would not register as a breach.
Modelling the lightwell properly
Most disputes about lower ground floor windows are really disputes about the model.
- The lightwell walls, their depth and their width at the window.
- Railings, balustrades and grating at pavement level, and whether they were modelled at all.
- Projecting steps, bridges over the lightwell and bin storage, which obstruct more sky than their size suggests.
- Vaults extending under the pavement, which change where the window actually sits.
- Surface reflectance assumptions for the lightwell floor and walls.
A model that omits the railings overstates the existing value, which then makes the proposed change look proportionally smaller. One that omits the lightwell altogether is worthless. Ask for the model views as well as the results table, and check them against photographs taken from inside the room.
Photographs are underused in these cases. A photograph from the window sill outward, with the railings and the opposite wall in frame, tells an officer more about the existing condition than three pages of tabulated values, and it makes an overstated baseline obvious.
Room use matters as much. A lower ground floor room used as a utility space, a store or a circulation area is not assessed in the same way as a habitable room, and the assessment should say which rooms were treated as habitable and on what evidence.
The baseline problem when existing conditions are already poor
The comparison is against the existing condition, which in a dense street may already be well below any target. Two competing readings follow, and both appear in reports.
The first says a resident who already has very little daylight is more sensitive to losing any of it, so the proportional test should be applied strictly. The second says the guidelines were written for suburban conditions and applying them rigidly in a city centre produces results that do not describe how people experience the room.
BRE 209 in its 2022 edition supports flexibility, and expects the assessor to consider context rather than to report a pass or fail. That is an invitation to reason, not a licence to dismiss a breach. The honest position is that neither reading settles a case on its own and the planning judgement has to be made on the whole picture.
Weighting a small absolute loss
The questions that carry weight in the judgement are practical ones.
| Question | Why it matters |
|---|---|
| Is this the only window serving the room? | A room with a second window is affected differently |
| What is the room used for? | A bedroom and a living room are weighted differently |
| What is the absolute value before and after? | A change between two very low values is different from a change from a good value |
| How much of the loss is caused by the lightwell itself? | Existing self-obstruction is not caused by the proposal |
| Are there other affected rooms in the same property? | Cumulative effect on a dwelling is a separate point |
Reporting only the percentage change in these cases is the most common criticism a reviewer should make. Reporting only absolute values is the mirror error, because it hides a real loss at a window that was previously acceptable.
Where the proposal affects gardens or shared amenity space as well, an Overshadowing Study covers the ground level effects that residents often care about more than the numbers at a window.
Design measures that improve the position
The lightwell is usually the variable with the most in it. Widening it, increasing its depth at the window, splaying the reveals, and replacing solid guarding with open railings all increase the sky visible from the window.
Other measures worth testing include lowering the cill where structure allows, using light coloured finishes on the lightwell walls, removing or relocating a bridge, and adding a rooflight where the room extends under the pavement or under a rear terrace. Rooflights change the analysis substantially, and rooms lit only by a rooflight sets out the tests that then apply.
On a proposed conversion, layout is the real tool. Put the habitable rooms where the daylight is and the storage where it is not, rather than driving a standard flat plan through a floor that cannot support it. A Daylight and Sunlight Assessment run early, at concept stage, usually pays for itself in avoided redesign.
Presenting the analysis so it can be checked
- State the metrics used and the edition of the guidance.
- Give absolute values before and after for every window, including those that pass.
- Give the proportional change alongside, and say which windows breach which test.
- Identify room uses, the number of windows serving each room, and the evidence for both.
- Describe the lightwell geometry and what was included in the model.
- Set out design changes already made in response to the analysis.
And say what you could not establish. Where internal layouts were unavailable and assumptions were made from external survey, write that down. An assessment that admits an assumption is more useful to an officer than one that presents every figure with equal confidence.
One further point for reviewers. Check whether the windows assessed are the ones that actually serve the rooms in question, because basement flats are frequently reconfigured without record, and a window shown on a historic drawing as serving a bedroom may now light a hallway.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.
PlanScope drafts amenity documents structured to the recognised methodology, and councils expect a suitably qualified author, so professional review is strongly advised. Enter your postcode on PlanScope to see which reports your site needs.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Why do lower ground floor windows fail proportional daylight tests?
Because the proportional test compares the value after development with the value before, and a window in a lightwell starts with very little visible sky. A fall of a single percentage point can cross the threshold, even though the same fall at an upper floor window would not register as a breach. Report absolute values alongside.
What must a lightwell model include?
The lightwell walls, their depth and width at the window, railings and grating at pavement level, projecting steps and bridges, vaults under the pavement and reflectance assumptions for the surfaces. A model that omits railings overstates the existing value and makes the proposed loss look proportionally smaller than it is.
Does poor existing daylight make a loss more or less acceptable?
Both arguments are made. One says residents with little daylight are more sensitive to losing any of it. The other says the numerical guidelines were written for suburban conditions and mislead in dense settings. The guidance expects context to be considered, and the planning judgement rests on the whole picture rather than a single test.
Which design changes improve daylight to a basement flat?
Widening the lightwell, increasing its depth at the window, splaying reveals, replacing solid guarding with open railings, using light coloured finishes, lowering the cill where structure allows and adding a rooflight where the room extends under a pavement or terrace. Layout changes in a conversion often achieve more than any of these.
How should room use be treated in the assessment?
State which rooms were treated as habitable and on what evidence, since utility spaces, stores and circulation areas are not assessed in the same way as living rooms and bedrooms. Where internal layouts were unavailable and assumptions were made from external survey, say so rather than presenting the assumption as fact.
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.