When a daylight and sunlight assessment is required

No national rule requires a daylight and sunlight assessment. Whether you need one is decided by the local planning authority's validation list, published under article 11 of the Development Management Procedure Order 2015, and those lists differ considerably from one council to the next. Read yours before you commission anything, because the difference between needing a full BRE 209 study and needing a one-page note with a section drawing is thousands of pounds and several weeks.

The second question is different and more interesting: whether the scheme will survive without one.

Local validation lists and what triggers a daylight study

A local list sets out the documents an authority requires in addition to the national requirements, and it has to be published, kept under review and justified against local policy. The daylight entry usually turns on one of four triggers.

Some lists work from the number of dwellings, requiring an assessment for anything above a stated figure. Others use proximity, requiring one where any part of the proposal sits within a given distance of a residential boundary or a habitable room window. Height appears in many lists, catching anything above two or three storeys or anything exceeding the ridge height of its neighbours. And a few use context, requiring an assessment in conservation areas or town centre locations where existing daylight is already constrained.

In London, borough lists tend to be the most demanding, reflecting the London Plan's requirements on housing quality and daylight for new homes. Outside London the pattern is looser and the officer's judgement carries more weight.

Find the list on the council's website under validation requirements, and check the date. Lists are meant to be reviewed regularly, and an out-of-date one is arguable.

Scale and proximity thresholds councils use

Where the list is silent or vague, officers fall back on the geometry. Three informal tests come up again and again, and knowing them tells you whether the council is likely to ask.

The 25 degree test from BRE 209 asks whether the proposed building, seen from the centre of a neighbour's window, subtends an angle of more than 25 degrees above the horizontal. If it does not, daylight to that window is unlikely to be materially affected and no further analysis is needed. This is the test doing most of the work on ordinary schemes.

The 45 degree test appears in most councils' householder design guides, applied in both plan and section from the nearest edge of the neighbour's habitable room window. It is a design guide convention rather than a BRE criterion, but it is the one an officer will quote at you on a two-storey side extension.

And separation distance conventions, usually expressed as a minimum between facing habitable room windows, deal with overlooking rather than daylight. Our note on the 21 metre separation convention explains where that number comes from and how far it can be pushed.

Householder extensions and the informal tests

For a rear extension on a semi-detached house, a full assessment is rarely required and rarely helpful. What is required is a drawing that lets the officer see the relationship.

Produce a section through the boundary showing the existing and proposed heights, the neighbour's nearest habitable room window with its cill and head levels, and the 45 degree line drawn from the centre of that window. Add a plan-view 45 degree line from the same window. If the extension sits below both lines, say so in one sentence in the covering letter and move on.

If it breaks the line, do the numbers rather than hoping. A vertical sky component calculation for the affected window, with the before and after values, converts an argument about fairness into a number the officer can weigh. That is a short piece of work compared with a full multi-window study.

New homes and internal daylight

The direction of the assessment reverses for new residential accommodation. Instead of asking what the scheme does to the neighbours, the council asks whether the rooms you are creating will be adequately lit.

Current practice follows BRE 209 and the UK national annex to BS EN 17037, which express the requirement as target illuminances achieved over at least half the room area for at least half the daylight hours: 100 lux for bedrooms, 150 lux for living rooms and 200 lux for kitchens. That is a climate-based daylight modelling exercise, not a hand calculation, and it needs the room layouts, the glazing specification and the surrounding context modelled.

Single-aspect north-facing flats, deep plan units and rooms behind a balcony are where schemes fail. Finding that out at planning stage is uncomfortable. Finding it out at technical design, after the massing is consented, is worse.

Where an assessment is worth doing anyway

Three situations justify commissioning the work even when the validation list does not demand it.

Where a neighbour has objected on light and the officer is wavering, a short assessment showing compliance ends the argument. Where the site already sits in a constrained context, the baseline figures may be so low that the proportional test in BRE 209 is easily met, and the report becomes your best evidence rather than a risk. Our note on daylight assessment in low baseline conditions covers how that works.

And where amenity space is being overshadowed rather than windows being blocked, the relevant work is different again. Sun on ground analysis tests whether at least half the amenity area receives two hours of sunlight on 21 March, and an Overshadowing Study is the document that reports it. The two hour test is a common reason gardens-based objections succeed.

What to submit when no assessment is required

Do not leave the topic blank. Validation officers are checking for a response to the list, and silence reads as an omission.

A short daylight and sunlight note, two pages, does the job. State the relationship between the proposal and each neighbouring property with a habitable room window facing the site. Include the section and plan drawings with the 45 degree lines. Say which BRE 209 test you have applied and what the result is. And say plainly that no full assessment has been provided because the proposal passes the relevant screening test, rather than because nobody looked.

That note belongs in the Design and Access Statement where one is required, or as a standalone appendix where it is not.

Cost and programme

The work splits into two stages and pricing it means knowing which one you need.

Screening is quick. An experienced assessor can run the 25 degree test and report on a handful of windows from drawings and a site visit. A full assessment needs a measured survey of the site and the surrounding buildings, a 3D model, window-by-window analysis and a written report, and it is a matter of weeks rather than days, particularly if the neighbouring room uses have to be established from the street.

A Daylight and Sunlight Assessment is a report type councils expect to be authored by someone suitably qualified, and professional review is strongly advised before submission. Drafting the structure and the policy narrative early, then having the modelling and the conclusions checked by the assessor, is the sensible division of labour.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.