Rooms lit only by a rooflight and the daylight tests

A rooflight-only room cannot be assessed with vertical sky component, because there is no vertical plane to measure. The test is defined at a point on the outside face of a window, looking outward at the visible sky, and a horizontal opening in a roof has no outside face in that sense. So for a room lit only from above, the assessment has to move to internal illuminance, and the reference to BRE 209 in a local plan policy does not change that.

Architects run into this at two moments. When a basement or lower ground floor flat is lit by a light well and a rooflight, and when a deep rear extension leaves the original back room dependent on glazing in the new roof.

Why vertical sky component cannot be applied to a rooflight

BRE 209 builds its obstruction tests around vertical glazing. Vertical sky component measures the proportion of the unobstructed overcast sky illuminance reaching a reference point on a vertical plane at the centre of the window. Annual probable sunlight hours is measured at the same point. Both need an orientation and a vertical surface, and both are designed to compare an existing and a proposed case at a neighbour's window. The full mechanics of that test are set out under what 27 per cent measures.

One BRE 209 test does still work. The no-sky line, or daylight distribution, is the boundary on the working plane between the part of the room that can see any sky and the part that cannot. A rooflight usually gives a large sky-visible area directly beneath it, and the no-sky line plot is a useful diagram for a top-lit room, particularly where the rooflight sits at one end of a deep plan.

What people sometimes attempt is a vertical sky component calculated on a notional vertical plane at the head of a light well. It has no standing, it produces a number that looks like a familiar metric, and a consultee reviewing the report will say so.

Assessing a top-lit room

The right approach is the illuminance route that BRE 209 (2022) adopted from BS EN 17037: target illuminance achieved over a stated fraction of the reference plane for a stated fraction of daylight hours, with the UK National Annex targets for dwellings. The changes the 2022 edition made to the treatment of new interiors are covered under the BRE 209 changes for new interiors.

Horizontal glazing performs well in that calculation. An unobstructed rooflight sees the whole sky vault rather than half of it, so per square metre of glass it admits considerably more daylight under an overcast sky than a vertical window of the same area. That is the physics, and it is why a modest rooflight can carry a room that would fail on a window.

Three things to get right in the model.

• Model the shaft as well as the aperture. A rooflight over a 600 mm ceiling void behaves differently from one at the top of a 3 m light well, and the reflectance of the shaft walls does real work.
• Use the glazing transmittance of the actual unit. Rooflights are often triple glazed, sometimes with a diffusing interlayer, and the transmittance can be well below that of a clear double-glazed window.
• Apply a maintenance factor. Horizontal glass soils faster than vertical glass and is cleaned less often.

Report the no-sky line plot alongside the illuminance results. Together they answer both the quantity question and the distribution question, and a daylight and sunlight assessment that gives only an area-weighted average hides the corner of the room nobody will use.

Rooflights in a rear extension and the room behind

This is the commonest version of the problem, and it appears in householder applications every week. A 4 m single-storey rear extension is added across the full width of the house. The extension gets a glazed roof or a run of rooflights and works well. The original rear room, now internal, is left with a doorway or an opening into the extension and no daylight of its own.

The planning issue is limited, because the amenity of the applicant's own retained room is not usually a matter the council refuses on, and BRE 209 is not directed at it. The habitability issue is real. A retained room with borrowed light through a 2 m opening will be dark for most of the year, and it is the part of the finished project clients complain about.

Two design responses work. Keep the original opening as wide as the structure allows, ideally the full width of the room, and put the rooflights directly over the junction rather than at the far end of the extension. Or accept that the retained room becomes a utility, store or plant space and plan it that way from the start.

Where the extension overshadows a neighbouring window, the ordinary BRE 209 tests apply to that window, and the effect of an overhang or projection above it is covered under balconies and overhangs and the daylight loss below.

Below-ground rooms and what local policy expects

There is no national daylight standard for basement habitable rooms. The Nationally Described Space Standard deals with floor areas and head heights rather than daylight, and the requirement comes through adopted local plan policy on residential amenity and, in London, through the Mayor's housing quality and standards policies and guidance.

Those policies commonly ask for habitable rooms to have adequate daylight and an outlook. Daylight and outlook are separate things, and this is the point that decides basement schemes. A rooflight can deliver the illuminance target. It cannot deliver a view out, and a bedroom whose only opening is in the ceiling will be resisted on outlook grounds even where the daylight modelling passes.

So for a lower ground floor flat, the arrangement that gets permission usually combines a light well with a vertical window or glazed door giving both daylight and outlook, plus rooflights to reach the back of the plan. Show the light well's width-to-depth ratio on the section. A narrow slot against a boundary wall will be read as a ventilation shaft rather than a source of amenity.

Modelling assumptions that flatter a rooflight

Four assumptions inflate results, and a reviewer will look for all of them.

1. Treating the rooflight as a frameless aperture. Kerbs, upstands and frames can take a meaningful share of the structural opening, so model the clear glazed area.
2. Ignoring obstruction above. Parapets, plant enclosures, adjoining higher buildings and a proposed upper storey all cut the sky the rooflight sees, and in a courtyard the effect is severe.
3. Over-optimistic surface reflectance. A light well with dark brick walls is not a white shaft, and a 0.8 reflectance assumption on external surfaces is not defensible.
4. Omitting any external shading the overheating strategy relies on. If the Part O case depends on solar control glass or external blinds, the daylight model has to include them.

State every one of these in the methodology, with the values used. An assessment that passes on optimistic inputs and fails on realistic ones is worse than one that reports a modest shortfall honestly, because the first gets challenged and the second gets weighed.

Ventilation, overheating and escape

A rooflight brings three other regimes with it, and they pull in different directions.

Overheating comes first. Horizontal and near-horizontal glazing receives high solar gains in summer, and Part O of the Building Regulations limits glazing areas and requires either the simplified method or dynamic thermal modelling. A large unshaded rooflight over a south-facing top-floor room is one of the reliable ways to fail an overheating risk assessment, so resolve the glazing area with the thermal modeller before the planning drawings are finalised.

Ventilation is easier, because an openable rooflight gives good stack ventilation and helps the purge rates in Approved Document F. Specify whether it opens and by how much.

Means of escape is the one that catches basements. Approved Document B expects an escape window or door from a habitable basement room, or a protected stair, and a rooflight is a poor escape opening because of the height of the sill above the floor and the difficulty of reaching it. Solve that on the section at planning stage rather than discovering it at building control.

What officers look for on the section

Case officers assess top-lit rooms from the section drawing more than from the plan, so draw it properly and annotate it.

Include the floor-to-ceiling height, the depth and width of any light well with the ratio stated, the position of the rooflight relative to the room, the shaft depth and its finish, the head height at the back of the plan, and the room's intended use. Add a note on the daylight result for that room and the target it was tested against.

Where the scheme has several top-lit rooms, a short schedule listing each one with its use, its glazed area, its daylight result and its outlook arrangement saves the officer reading the whole assessment. That schedule is often what gets copied into the committee report.

Councils normally expect daylight work to be done by someone experienced in the current methods, and professional review before submission is strongly advised. PlanScope drafts the report structure, the methodology narrative and the policy framing, and marks every figure it cannot source as to be confirmed. Enter your postcode to see which reports your site is likely to need.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.