What is Annual Probable Sunlight Hours?
Annual Probable Sunlight Hours (APSH) is a measure of the frequency that sunlight is likely to be present at a specific point.
According to the BRE the maximum APSH available at a given location is defined as the long-term average of the total number of hours during a year in which direct sunlight reaches the unobstructed ground (when clouds are accounted for).
What is APSH used for?
Within the BRE Guide APSH is used to assess the degree to which a proposed development could impact on the levels of sunlight available to the accommodation in neighbouring properties.
What do you include in APSH testing?
According to the BRE, APSH testing is intended for all the main living rooms within dwellings and also for any attached conservatories. The BRE advise that APSH levels for kitchens and bedrooms are typically not tested unless they comprise a living area within an open plan layout. Non-domestic accommodation is typically not included in testing unless it is deemed that the accommodation has a special requirement for sunlight.
Where and how is APSH measured?
APSH is measured at a discrete point located at the centre of the window being tested. For full height windows (sliding doors, etc.) the point is located at a height of 1.6m above finished floor level.
Provided no double counting exists it is permissible to determine the APSH for a room or room zone with reference to the aggregate of all the APSH levels predicted for the individual windows which serve it. Double counting is typically assumed unless the windows under consideration are diametrically opposed.
Where a room is served by a number of windows, but none are diametrically opposed, it is permissible to determine the APSH for the subject room with reference to the highest of the individual APSH levels identified.
APSH testing is always carried out as a desktop study. While it is possible to calculate APSH using graphic based manual calculations it is more typical nowadays for this testing to be carried out using computer simulations.
Impact testing is carried out by predicting APSH levels at sensitive windows in the neighbouring environment for the existing scenario (with existing buildings on the development site) and then again with the proposed development in place.
How is the impact on neighbouring properties determined?
According to the BRE the sunlighting of an existing room will be adversely affected if:
- the annual probable sunlight hours it receives is less than 25% of the maximum possible for the location in question and less than 0.8 times its former value, or the annual probable sunlight hours registering during winter months (21st September to 21st of March) is less than 5% of the maximum possible and less than 0.8 times it’s former value during that period.
- and also, the absolute reduction in annual probable sunlight hours registering over the whole year is greater than 4%.
How should the results of APSH testing be interpreted?
According to the BRE the assessment of impact will depend on a combination of factors and there is no simple rule of thumb that can be applied. The BRE Guide advises that the magnitude of the departure from advisory minimums, the sensitivity of the receptor to loss of light and the existence of mitigating factors are all aspects that need to be considered when attempting to determine the significance of an identified impact.
Beyond the guidance provided in the BRE Guide (which states that “the advice given here is not mandatory”) it is important to recognise that in most planning jurisdictions even significant impacts can be deemed acceptable in circumstances where a countervailing planning objective can be articulated.
Should trees be included in APSH testing?
According to the BRE, for the case of impact assessments, it is usual to ignore the effect of existing trees. The BRE make this recommendation because daylight is at its scarcest and most valuable in winter when most trees will not be in leaf. Having regard to this guidance it is reasonable to assume that the overshadowing effect of trees can be omitted from an impact assessment but that the presence of evergreen trees may be an exception to this rule.
Does APSH analysis have any shortcomings?
APSH testing provides a simple indication of the period that some level of sunlight is likely to be visible within a space. As the measure relates more directly to the period of availability rather than the illuminances achieved it would be a mistake to assume that APSH can provide a meaningful indication of the sunlight amenity that would be perceived by a human observer within a room.
A critical shortcoming of APSH analysis is that it is insensitive to many of the important factors that influence the sunlighting of internal spaces. These factors include window size, the optical properties of window glazing as well as the size and layout of the room a window serves.
Having regard to the above it is instructive to note that a large room, with dark furnishings, served by a small heavily tinted window will have the same APSH as a small room, with light furnishings, served by a large clear window in an equivalent location.
From the perspective of development management, it may be helpful to regard APSH as a measure which provides an indication of the capacity that exists for windows and their associated accommodation to receive light directly from the sun. In this sense APSH can be regarded as similar in nature to the capacity of other services that a development site will need, for example grid, water and drainage capacity, etc.
