What the 2022 edition of BRE 209 changed for new interiors

The third edition of BRE 209, published in 2022, changed how daylight inside proposed dwellings is assessed. The headline for anyone reading a report is that average daylight factor has been superseded for new interiors, and illuminance-based assessment aligned with the current daylight standard has taken its place. The tests applied to existing neighbouring buildings are largely as they were.

If you are commissioning or reviewing an assessment, that split is the thing to hold onto. The BRE 209 2022 changes affect the inside of your own scheme far more than they affect what you owe the neighbours.

What the third edition kept and what it replaced

The neighbour tests survived. Vertical sky component, daylight distribution using the no-sky line, and annual and winter probable sunlight hours remain the framework for assessing the effect of a new building on existing windows, along with the familiar proportional test against the former value.

What went was the average daylight factor method for assessing the interiors of proposed dwellings. Daylight factor is a ratio of internal to unobstructed external diffuse illuminance under an overcast sky. It takes no account of orientation, of where the sun actually is, or of the climate at the site. A north-facing room and a south-facing room with identical geometry return the same daylight factor, which is not how they feel to live in.

The third edition points instead to illuminance targets achieved for a proportion of the daylight hours across the year, which is a climate-based calculation.

Illuminance-based assessment for proposed interiors

An illuminance-based assessment answers a different question: how much daylight reaches a given fraction of the working plane, for what proportion of the daylight hours in the year, at this location.

That means real climate data for the site, a modelled reference plane at a stated height, stated surface reflectances, and a calculation grid. It also means the answer changes with orientation and with the obstruction opposite, which is exactly what a designer wants to know.

The practical effect is that rooms with deep plans, single aspect north-facing living rooms, and rooms under balconies show up worse than they did under the old method, and well-oriented rooms show up better. A Daylight and Sunlight Assessment run on the new basis will not reproduce the old numbers, and nobody should expect it to.

The relationship with the current daylight standard

BRE 209's approach for new interiors follows the current European daylight standard as adopted in the UK, with a national annex that sets target illuminances for dwelling room types. Kitchens carry the highest target, living rooms sit in the middle and bedrooms the lowest, each to be met across a stated share of the reference plane for at least half the daylight hours.

Two points cause confusion. The standard's own recommended target and the UK national annex values are not the same, so a report has to say which it applied. And the standard offers both a climate-based route and a simplified daylight-factor-equivalent route, so two assessments can both claim to follow it and produce different-looking evidence.

The background to those targets is set out in the explainer on BS EN 17037 daylight targets for new homes, which is worth reading alongside any report you are checking.

There is also a sunlight limb to the standard, expressed as a number of sunlight hours on a stated date to at least one habitable room in a dwelling. Single aspect north-facing flats cannot meet it, and a scheme with a lot of them should address that directly rather than hoping the officer only reads the daylight table.

Reference plane and room use

The reference plane sits at a stated height above floor level, inset from the walls, and the assessment area is the room it serves. Where a room is open plan, the split between kitchen, dining and living zones changes which target applies to which part of the grid. Two assessors can model the same flat and disagree on the answer purely because they drew the zone boundaries differently, so the report should show the zoning on a plan.

Why daylight factor reports still circulate

Three reasons, all of them practical.

Local plan policies written before 2022 often name average daylight factor explicitly, and a policy does not update itself when a guidance document does. Some borough design guidance still sets daylight factor thresholds. And daylight factor is quicker and less costly to calculate, which matters on a small scheme.

None of that makes a daylight factor report wrong, and it does make it incomplete on a scheme where orientation is doing real work. The pragmatic answer on a contested site is to report the climate-based results as the primary assessment and include the daylight factor figures where a local policy asks for them.

Software and climate data assumptions

Ask for these to be stated in every report, because they change the answer:

• The climate file used, its location and its vintage
• The reference plane height and the offset from the walls
• Surface reflectances for walls, floors and ceilings, and whether furniture was modelled
• Whether glazing transmittance is a manufacturer figure or a default, and whether a maintenance factor was applied
• Whether balconies, reveals, brise soleil and external obstructions were modelled in three dimensions
• The calculation grid spacing

Reflectance assumptions deserve particular attention. A model run with light finishes throughout produces results that a dark scheme will not deliver, and the specification should match the model or the model should be rerun.

Balconies are the other recurring issue. Deep balconies above windows remove a large share of the sky visible from the room below, and the effect is far more visible in a climate-based result than it was in a daylight factor. The note on balconies and overhangs goes through the geometry.

What to ask a consultant to confirm

Before you accept a report, get written answers to five questions. Which edition of BRE 209 was used. Which daylight standard and which set of target values. Whether the results are climate-based or daylight-factor-equivalent. Whether the modelled reflectances and glazing match the specification. And which rooms fail, by how much, and what was tested to fix them.

A report that answers all five is defensible at appeal. One that presents a single summary table with no method section is not, however good the numbers look.

Where a scheme also affects neighbouring windows, keep the two assessments in one document with a clear internal division, so the officer does not have to work out which method applies where. Sun on ground and amenity space testing sits alongside it, and on a dense site an Overshadowing Study usually travels with the daylight work.

How councils are responding to the change

Unevenly, which is the honest answer. Authorities with recently adopted plans or updated design guidance tend to ask for the climate-based method. Authorities working to older policies often still cite daylight factor, and their validation lists say so.

There is a second wrinkle in London, where borough housing design guidance and the London Plan's residential quality policies drive a lot of daylight evidence. Where that guidance predates the third edition, the assessment has to satisfy the policy as written while also being defensible on current method, which usually means two result tables rather than one.

Nobody enjoys paying for two sets of modelling. The alternative is a refusal reason citing the policy you did not answer, and then an appeal where you produce the numbers anyway.

The safest course is to check the adopted local plan and any supplementary design guidance for the specific wording, then report on the basis the policy names while also reporting the current method. That costs a little more modelling time and removes an argument.

At appeal, inspectors look at what the guidance says now rather than at what a policy written years earlier assumed. A report that only gives the older method on a site where daylight is contested leaves you defending a method that the guidance itself has moved on from.

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.