Annual Probable Sunlight Hours (APSH) explained

Annual Probable Sunlight Hours (APSH) is the BRE 209 sunlight metric. It is the percentage of the year's available sunlight hours that can reach a window or amenity space at a given location, taking nearby obstructions into account. BRE 209 treats a south-facing living-room window as receiving adequate sunlight if it sees at least 25% of APSH across the year and at least 5% during the winter half-year. Where the existing baseline is already at or below those figures, BRE applies the 0.8 ratio test — the same threshold used for VSC.

What is APSH?

APSH is defined in BRE 209 as the long-run-average number of hours of direct sunlight a point can receive in a year, expressed as a percentage of the total possible sunlight hours for that latitude. The "probable" qualifier accounts for the long-run cloud cover at the site — in practice, calculation tools use a fixed national figure for England, so the metric is geometric rather than meteorological once the latitude is set.

Two values are reported: the annual figure (sun hours over the whole year) and the winter figure (sun hours between 21 September and 21 March). The split matters because the winter sun is the one residents notice most — a south-facing window can hit the 25% annual target through summer alone and still be in shadow for the whole winter.

Which windows does APSH apply to?

BRE 209 only requires APSH testing where the window:

• Faces within 90 degrees of due south, and
• Serves a main living room (living room, lounge, living-dining room).

Kitchens and bedrooms are not tested for sunlight under BRE 209, on the rationale that kitchens are used in the morning and evening when direct sun is less critical, and bedrooms are mainly used at night. North-facing windows are excluded because the geometry makes direct sun impossible in the UK.

Outside those filters, APSH is also used to test amenity space (gardens, parks, courtyards) but with a completely different test — the open-space test — which is described separately below.

How is APSH calculated?

APSH is calculated by sun-path analysis from a reference point at the centre of the window, on the outside face, for the latitude of the site. The principle:

1. Plot the annual sun path at the site latitude on a stereographic projection or equivalent.
2. Project all obstructing surfaces (existing buildings, the proposed development, trees of significant size) onto the same projection from the reference point.
3. For each hour of the year, determine whether the sun is above the horizon and unobstructed.
4. Sum the unobstructed sun hours and express as a percentage of the total possible sun hours for the latitude (1486 hours per year for central England, of which 486 fall in the winter half-year).

Modern reports use specialist software (IES, ReLux, Cadline, or Radiance-based tools) to automate the sun-path analysis. The output is a pair of numbers per window: APSH-annual and APSH-winter, before and after the development.

The thresholds in detail

The open-space (amenity) test

The amenity-space test sits inside BRE 209 alongside the window-based APSH test, but it has different mechanics. The test asks: on 21 March (the spring equinox), does at least half of the amenity area receive at least two hours of direct sun? It is calculated by overlaying the equinox shadow on a plan of the amenity area; the output is a percentage of the area that meets the 2-hour rule, before and after the development.

For full mechanics of the equinox shadow analysis (and why 21 March is chosen rather than midwinter or midsummer) see our note on shadow analysis at the equinox.

Common pitfalls and challenges

• Wrong room classification. If the applicant has assessed your window as serving a "kitchen" or "bedroom", APSH is not run at all. Check the receptor schedule against the actual room use.
• Annual figure reported, winter figure missing. BRE 209 requires both, and a scheme can pass the annual test while obliterating winter sun. Always check that both numbers appear in the table.
• Orientation filter applied too aggressively. The 90-degree-from-south filter is sometimes read narrowly to exclude windows facing south-east or south-west, but those orientations are explicitly within scope.
• Open-space test omitted. Where the scheme overshadows a garden or park, the open-space equinox test should appear separately from the window APSH test. If only window APSH is reported, the amenity impact has not been assessed.
• Cumulative baseline. Including consented but unbuilt nearby schemes in the baseline reduces the apparent APSH loss. Check what is in the baseline model.

How to challenge an APSH calculation in an objection

1. Name the room and window, and confirm it is a main living room facing south.
2. State the four numbers the applicant reports: APSH-annual existing, APSH-annual proposed, APSH-winter existing, APSH-winter proposed. Cite the page and table number.
3. Compare to the absolute thresholds (25% and 5%) and to the 0.8 ratio. Show the arithmetic.
4. Cite BRE 209 paragraph 2.2.11 (the sunlight section) explicitly, alongside any Local Plan or London Plan policy that requires residential amenity to be protected.
5. If the scheme overshadows a garden, park or play space, ask separately for the open-space equinox figures and compare to the 50% / 2-hour test.
6. Conclude. "The proposal fails the BRE 209 sunlight test on both the annual and winter limbs, and conflicts with [policy reference]. The application should be refused on sunlight grounds."

Frequently asked questions

What APSH value is acceptable in planning?
BRE 209 treats a south-facing living-room window as receiving adequate sunlight at 25% of APSH-annual or more, with at least 5% in the winter half-year. Below those values, the 0.8 ratio test is applied.

Which windows are tested?
Living-room windows facing within 90 degrees of due south. Bedrooms, kitchens and north-facing rooms are not tested for sunlight under BRE 209.

What is the open-space test?
For amenity areas (gardens, parks, play space): at least 50% of the area should receive at least two hours of direct sun on 21 March, the spring equinox.

How do I challenge an APSH calculation?
Check the room classification, confirm both the annual and winter figures are reported, and (where amenity space is affected) ask separately for the open-space test results. Errors in any of these three are common grounds for challenge.

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