Annual probable sunlight hours explained

Annual probable sunlight hours, usually shortened to APSH, is the measure used to judge whether a new building will take too much sunlight from a neighbouring window. It counts the hours of sunlight a point could receive over a year, weighted by how likely the sun is to be shining at each position in the sky, and expresses the result as a percentage of the total available at that location.

If a neighbour's scheme is being assessed for sunlight, this is the number in the table. Two figures usually appear: one for the whole year and one for the winter months alone.

What the annual and winter measures represent

The annual figure covers the full year. The winter figure covers the period from 21 September to 21 March, when the sun is low and a modest obstruction takes a disproportionate share of what is available.

Both are needed because they answer different questions. A courtyard that gets pleasant afternoon sun in July can be in shadow from October to March, and an annual figure alone hides that. Councils look at the winter number when the affected room is a living room, because that is the period when sunlight in a living room matters most to the people using it.

The BRE guidance suggests that sunlight may be adversely affected where a window receives less than 25 per cent of annual probable sunlight hours, or less than 5 per cent during the winter period, and receives less than 0.8 times its former value, and loses more than 4 per cent of annual probable sunlight hours overall. All the limbs work together. A window falling below 25 per cent but retaining more than 0.8 of what it had is not treated as adversely affected.

Which windows the test applies to

Only main windows to habitable rooms, and only where the window faces within 90 degrees of due south.

That last point surprises people and it is not arbitrary. A window facing due north never receives direct sunlight in this country, so a new building opposite it cannot take away sunlight that was never there. The 90 degree rule draws the line at windows oriented anywhere from due east, through south, to due west.

Check the orientation before assuming a test applies. A rear elevation facing north-east is outside the sunlight assessment entirely, though it will still be assessed for daylight, which is a separate calculation on a separate measure.

Living rooms carry the most weight. Kitchens and bedrooms are assessed where they are main habitable rooms, but the guidance recognises that sunlight matters less in a bedroom than in the room where people sit in the afternoon.

How the figures are worked out

The measurement point is the centre of the window on the outside face of the wall, the same position used for daylight.

The assessor models the path of the sun across the sky over the year, and for each position works out whether the point can see the sun or whether something blocks it. Each position is weighted by the probability that the sun is actually shining at that time, drawn from long-term weather records. That weighting is what makes the hours probable rather than theoretical.

Software does the work now, but the underlying method is the same as the printed sunlight availability indicator that preceded it, and a competent assessor can still explain a result by pointing at a sky diagram. If a report gives you numbers with no plan showing which window is which, ask for one.

Reading a results table

A usable table has a row for each assessed window and these columns: the window reference, the room it serves, the room use, the existing annual APSH, the proposed annual APSH, the ratio, then the same three for the winter period.

Work through it in this order.

1. Check which windows were assessed and confirm the ones that matter to you are on the list.
2. Check the orientation, since a window omitted for facing north should be identified as such rather than simply missing.
3. Look at the ratio rather than the raw percentage. A fall from 60 per cent to 52 per cent is a large-sounding change and a ratio of 0.87.
4. Look at the winter row separately. Annual results can pass while winter results fail, and for a living room that is the material finding.
5. Check the room use against your own knowledge of the property. Assessors work from drawings and sometimes label rooms wrongly.

Where the affected space is a garden rather than a window, a different test applies: whether at least half the area receives at least two hours of sunlight on 21 March. That is the subject of an Overshadowing Study rather than the window calculation.

Where a failure is acceptable in context

The guidance is advisory and its numerical values are meant to be applied with judgement. Councils balance a sunlight loss against the benefits of the development and against the existing character of the area.

A shortfall carries more weight where the affected room is the main living room of a family home, where the loss is large in proportion as well as in absolute terms, and where the applicant could have avoided it by stepping the massing back.

It carries less weight where the room is a bedroom or a secondary space, where the existing sunlight is already limited by other buildings, where the property is in a dense urban area in which the guidance's assumptions do not fit, and where the loss falls on a window that also serves a room with a second aspect.

None of that means an objection is pointless. It means an objection that identifies the room, its use and the winter figure is far more useful to a case officer than one that says the development will block the light.

Design changes that improve sunlight

If you are the applicant rather than the neighbour, the levers are these.

Height on the southern side of the affected window does the most damage, so reducing or stepping that part of the massing produces the biggest improvement. Moving mass east or west helps less than people expect, because the sun is highest and strongest in the south.

Increasing separation helps in proportion to the obstruction angle rather than in proportion to the distance, so the first few metres are worth more than the last few.

Chamfering or setting back the upper storeys on the southern elevation is usually the most efficient change, since it removes the part of the building that blocks the winter sun without losing much floor area.

And where the layout allows, put the non-habitable parts of your own building opposite the affected windows.

Illustrative example: a two-storey rear extension on the south side of a semi-detached pair. The neighbour's ground floor living room window faces south-west and currently receives a comfortable annual figure with an adequate winter one. The proposed extension takes the winter figure below 5 per cent and the ratio to 0.7. Pulling the first floor back by 1.5 metres from the boundary, and dropping the eaves by 400 mm, restores the winter figure to an acceptable level while keeping most of the additional floor area. That is the conversation to have with the applicant before the objection deadline.

Councils usually expect a suitably qualified author for a Daylight and Sunlight Assessment, and professional review before submission is strongly advised. Enter your postcode on PlanScope to see which reports your own project is likely to need.

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.