Daylight and Sunlight Assessment or Overshadowing Study

A Daylight and Sunlight Assessment tests windows. An Overshadowing Study tests the ground. That is the whole distinction, and it decides which document a council actually needs: if the concern is a neighbour's living room going dark, the window tests apply, and if the concern is a garden losing its afternoon sun, the sun on ground analysis applies.

Plenty of small schemes need one and are sold the other.

What each study measures

The window based assessment uses three tests from the BRE guidance. Vertical sky component measures how much of the sky is visible from the centre of a window, expressed as a percentage. Daylight distribution, or the no-sky line, measures how much of the room area can see any sky at all. Annual probable sunlight hours measures the proportion of the year's available sunlight that reaches a window, with a separate winter figure.

The ground based study asks a different question: how much of an amenity area receives direct sun, and for how long, on a reference date. The usual test is the proportion of the area receiving at least two hours of sunlight on 21 March, compared before and after the development.

They are not alternatives in the sense of being interchangeable. They answer different complaints.

Window tests, and what the numbers mean

Vertical sky component is the first test and the one most often quoted. The guidance treats a value of 27 per cent or more as unlikely to cause noticeable loss, and where a window falls below that, the comparison with its former value becomes the point: a retained value of at least 0.8 times the original is the usual threshold for a loss that would not be noticed. Our note on the vertical sky component test sets out the method and its limits.

Daylight distribution matters where the room is deep or the window is high. A window can retain an acceptable vertical sky component while the back of the room loses its view of sky entirely, and the no-sky line assessment is the test that picks that up. It needs internal room dimensions, which means access or a reasonable assumption, and the assumption has to be stated.

Sunlight is assessed only for windows facing within 90 degrees of due south, using the annual and winter probable sunlight hour figures. Annual probable sunlight hours explains how the figures are derived and why a north-facing window is not assessed at all.

Amenity space tests, and the March date

The overshadowing analysis takes gardens, courtyards, play areas, roof terraces and any other space where people sit outside.

The reference date is 21 March, the spring equinox, chosen because it gives an average day rather than a best or worst case. The guidance looks for at least half the area to receive two hours or more of direct sunlight on that date, and where an existing space falls below the guideline after development, the comparison with the former value decides whether the loss is noticeable.

Two practical points. The assessment covers the neighbour's amenity space and the proposed development's own, and officers increasingly ask about the second on schemes creating new gardens or communal space. And where the space is a roof terrace or a podium, the surrounding parapets and plant enclosures have to be modelled, because they overshadow more than people expect.

Sun path diagrams for 21 June and 21 December are often produced alongside, as context rather than as tests.

The model both studies share

Both assessments run from the same three dimensional massing model, which is why commissioning them together costs less than commissioning them separately.

The model needs the existing and proposed building forms with accurate heights, the surrounding buildings within the zone of influence, ground levels, and true orientation. Its inputs come from a measured or topographical survey rather than from mapping alone, because a 300 millimetre error in a parapet height changes results at close range.

Room dimensions and window positions are the extra input that only the window tests need. Where the neighbouring interior cannot be surveyed, a stated assumption based on typical room depths for the building type is acceptable, and the report has to say so. Daylight modelling assumptions and why they matter covers how those choices affect the result.

How officers read each output

Planning officers use these documents differently from the way consultants write them.

For window tests, the officer wants a table of affected windows with the existing and proposed values, the percentage change, the room each window serves, and a plain statement of which windows fall below the guidelines. A report that buries three failures in an appendix will be read as evasive when the objector's consultant finds them.

For overshadowing, the officer wants a pair of plans showing the two hour contour before and after, the percentage of each area meeting the guideline, and the same plain statement. Colour plots help here more than they do for window tests.

Neither document decides the application. Both feed a planning judgement in which the guidance is explicitly advisory and local context carries weight. Say that in the report rather than presenting a pass as an entitlement.

There is a related habit worth dropping. Reports that quote the guidance as a standard, using words such as compliant or breach, overstate what it is. The guidance itself says the numerical values are not mandatory and should be applied flexibly, particularly in dense urban settings where meeting them everywhere would prevent any development at all. An officer who has read that passage will notice a report that pretends otherwise, and an inspector certainly will.

Programme and cost differences

An overshadowing study is the simpler exercise. It needs no internal dimensions, no assumptions about neighbouring room layouts and no access, so it can be produced from a survey and a model alone.

The window based assessment needs more. Access to neighbouring properties is sometimes refused, room layouts have to be assumed, and the analysis is run window by window, which on a scheme facing a terrace of eight houses means a great many windows. Expect a longer programme and a higher fee.

Where budget is tight, the sequence that works is to run the BRE initial check first. If no obstruction subtends an angle above 25 degrees to the horizontal, measured from the centre of the lowest affected window, diffuse daylight is unlikely to be materially affected and the fuller window analysis may be unnecessary. That check costs an hour of a consultant's time and settles a surprising number of householder cases.

Choosing for a householder application

For a rear extension on a suburban plot, the usual answer is neither document in full.

Most councils assess householder schemes against a 45 degree rule taken in plan and section from the neighbour's nearest habitable room window, supported by the BRE initial check. Where the extension passes both, a short paragraph in the design and access statement and a section drawing showing the geometry is enough.

Commission the fuller work in three situations. Where the neighbour has objected on light and the officer has asked for evidence. Where the extension is two storeys and close to a boundary with a habitable room window. Or where the scheme creates a new dwelling whose own amenity space or windows will be assessed. In those cases a Daylight and Sunlight Assessment is the right document, with an Overshadowing Study where gardens are the issue rather than rooms.

Councils usually expect a suitably qualified author for both, and professional review is strongly advised before submission.

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.