Daylight assessment for a basement flat conversion

A basement flat daylight assessment is not the same exercise as a neighbour impact study. Nothing is obstructing the window from outside except the lightwell you built, and that is the whole problem. The sky angle available to a window at the base of a 1 m wide, 2.5 m deep lightwell is narrow, so the room can fail an internal daylight target even on an open site with no buildings anywhere near it.

Councils know this. Basement conversions are among the most scrutinised householder and small conversion applications, and daylight is the usual reason they are refused.

Why basement conversions fail on daylight

The current edition of BRE 209, published in 2022, moved internal daylight assessment onto the illuminance approach in BS EN 17037. The UK National Annex sets minimum target illuminances of 100 lux for bedrooms, 150 lux for living rooms and 200 lux for kitchens, to be achieved over at least half the assessment area for at least half the daylight hours in the year.

Plenty of authorities still ask for average daylight factor instead, using the older BRE figures of one per cent for a bedroom, one and a half for a living room and two for a kitchen. Check which the local plan or the validation list refers to before you commission anything, because the two methods are not interchangeable and a report using the wrong one gets sent back.

Either way, the number that matters in a basement is how much sky the window can see. The BRE guidance uses a 25 degree test: where the obstruction subtends an angle of less than 25 degrees to the horizontal, measured at the centre of the lowest window, diffuse daylight is likely to be adequate. Measure that angle from the head of your lightwell wall to the middle of the basement window and you will usually know the answer before anyone opens a modelling package.

Lightwell geometry and what it delivers

Three dimensions govern: width of the lightwell measured out from the wall, depth from ground level down to the lightwell floor, and the height of the window within that depth.

Widening the lightwell helps most. Lowering the internal floor level so the window head sits closer to ground level helps almost as much, and often costs less than it sounds if the slab is coming out anyway. Raising the window head above the surrounding ground, by dropping the external levels or introducing a retaining step, helps most of all, and is the move that turns a marginal scheme into a compliant one.

What does not help much:

• A rooflight or pavement light set flush in the lightwell floor. It admits some light but almost all of it lands on the floor of the well, not in the room.
• Light-coloured render on the lightwell walls. Real, measurable, and worth perhaps a fraction of what the client hopes.
• Mirrors and light tubes. Usable as a supplement, not as the basis of a case.

A sloping or splayed reveal on the inside of the window does more than any of those, because it widens the cone of sky visible from within the room at no external cost.

An illustrative example. A Victorian terraced house with an existing 0.9 m deep front lightwell and a window head 2.1 m below pavement level. The obstruction angle is steep, well over 25 degrees, and the room fails. Extending the well to 1.8 m and lowering the cill of the well by 300 mm brings the angle down towards the threshold. These figures are illustrative only and the geometry on any given site has to be measured.

Measuring at the window in a lightwell

Two errors turn up repeatedly in submitted reports.

The first is measuring vertical sky component at the outer face of the lightwell rather than at the window. VSC is a neighbour test in any event, and taking it at the top of the well tells you about a window that does not exist.

The second is modelling the lightwell as an open rectangle when it has a grille, a guard rail, a ladder or planting in it. Grilles over a lightwell can remove a substantial share of the light reaching the window, and a designer who omits the grille from the model will be asked to rerun it.

Model the well as built, with the actual reflectances of the surfaces, the actual glazing transmittance including any secondary glazing, and any railing. Then state those inputs in the report so an officer can see they are there. A daylight and sunlight assessment that lists its assumptions is much harder to dismiss than one that presents results alone, and councils normally expect this work from a suitably qualified author.

Room layout and borrowed light

Where the geometry cannot be improved, layout is the remaining lever. Put the habitable rooms at the window end and the bathroom, plant, store and circulation at the back.

Borrowed light through an internal glazed screen from a lit room into an unlit one is usually not accepted as making the rear space a habitable room. It can help a kitchen that opens directly onto a lit living space, because the combined area is then assessed as one space with one window wall. Open-plan is the honest version of borrowed light.

Deep plans are the enemy. A room depth more than about twice the height of the window head above the floor gets very dark at the back whatever you do at the window, and the assessment area measure in BS EN 17037 penalises exactly that geometry.

The same geometric logic applies when turning shops and offices into homes, which is covered separately in our note on daylight when converting commercial premises to homes.

Ventilation and outlook alongside daylight

Daylight is one of three tests and it rarely arrives alone.

Outlook is a planning judgement rather than a numerical standard. A habitable room whose only view is a brick wall 900 mm away will attract an objection on outlook even where the illuminance figures pass, and officers do refuse on that basis. The argument and the evidence are set out in our piece on outlook and the overbearing test.

Ventilation runs on a separate track under Approved Document F, and means of escape from a basement habitable room under Approved Document B usually requires either a suitable escape window or a protected stair. Neither is a planning matter, but both constrain the window size and position you can propose, so resolve them before the daylight model is built rather than after.

Damp and water resistance under Approved Document C, with tanking to BS 8102, will also affect the internal finished floor level and so the window head height. Everything in a basement is connected to everything else.

Local policy on below ground habitable rooms

National policy does not prohibit habitable rooms below ground. Local policy sometimes does, or restricts them heavily.

A number of authorities, particularly London boroughs with a history of basement development, carry local plan policies or supplementary planning documents dealing with basement conversions, single-aspect units and minimum daylight standards. Some set their own numerical daylight targets. Others restrict bedrooms wholly below ground while allowing living space. You have to read the adopted plan and the relevant supplementary document for the authority in question, because there is no national default to fall back on.

Where the property is in a conservation area, the lightwell, its railings and any new rooflight all become external alterations with their own tests, and the approach to rooflights and glazing in conservation areas applies.

Evidence that changes an officer's mind

Officers respond to three things on basement schemes.

A drawn section through the lightwell and the room, at a proper scale, showing the 25 degree line. More useful than any table of results, because it lets the officer check the claim themselves in ten seconds.

Results presented room by room against the standard the authority actually uses, with the assessment area shown on plan, and the failures admitted rather than buried. A report that concedes the rear bedroom does not meet the target and proposes it as a study instead will get further than one claiming everything passes.

And a comparison with the existing condition. Where a basement is already in residential use and the proposal improves the daylight, say so with numbers for both cases.

Enter your postcode on PlanScope to see which reports a basement conversion is likely to need before you commission the survey work.

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.