Annual Probable Sunlight Hours (APSH) Explained

Annual Probable Sunlight Hours (APSH) metric is one of the most important measures used to quantify sunlight exposure. This metric plays a key role in daylight and sunlight assessments, particularly when assessing the potential impact of new developments on the access to sunlight enjoyed by existing buildings.

In urban planning and architectural design, understanding sunlight access is crucial for creating liveable, energy-efficient, and psychologically beneficial environments.

In this comprehensive guide, we explain what APSH is, how to calculate it, why it matters, and how professionals apply it in daylight and sunlight assessments within the UK planning system—particularly under the framework of the BRE Guidelines (BR209).

What Is Annual Probable Sunlight Hours?

Annual Probable Sunlight Hours (APSH) measures the number of hours in a year when direct sunlight is expected to reach a specific point on a building—typically a window—based on standardised sunpath geometry.

Unlike generic measures of daylight or insolation, APSH does not represent the maximum theoretical sunlight. It measures the availability of direct sunlight at a point by accounting for physical obstructions such as buildings or trees, under clear sky conditions.

Although the term 'probable' might imply a weather-related basis, APSH does not reflect meteorological variability.

Daylight surveyors express APSH as a percentage of the total annual unobstructed sunlight hours that would be available if no obstructions existed.

Why APSH Matters in Planning and Design

The Annual Probable Sunlight Hours (APSH) test is important because:

1. Amenity Protection: It helps protect the amenity of existing buildings by quantifying whether a new development will significantly reduce the amount of direct sunlight received.

2. Compliance with Planning Policies: Local planning authorities in the UK often require sunlight assessments for planning applications involving large buildings or developments near residential properties.

3. Human Health and Comfort: Adequate access to sunlight affects mood, well-being, and can even impact physical health through vitamin D production.

4. Thermal Efficiency: Sunlight contributes to passive solar heating, which can help reduce energy demand in winter months.

5. Design Optimisation: APSH helps architects orient buildings and windows for optimal sunlight access.

How Is APSH Calculated?

The BRE Guide “Site Layout Planning for Daylight and Sunlight: A Guide to Good Practice” (BR209, 2022) outlines the methodology for calculating APSH.

Step-by-Step Breakdown

1. Reference Point: The APSH is typically assessed at a point in the centre of the outside face of a window.

2. Sunlight Availability Indicators: The BRE provides indicators (Figures A2, A3, A4 in Appendix A) for different UK latitudes. These tools simulate the probable sunpath.

3. Unobstructed APSH: The total number of sunlight hours an unshaded point would receive in a year is established—typically 1486 hours for southern England.

4. Obstruction Plotting: Buildings that might block sunlight are plotted using specialist software tools that utilise sunpath diagrams.

5. Assessment of Sunlight Loss: The number of unobstructed ‘sunlight spots’ is counted and expressed as a percentage of the total possible. This provides the APSH percentage.

APSH Standards in the BRE Guidelines

The BRE guidelines set specific target values for APSH to determine if a building enjoys adequate sunlight:

For Existing Buildings:

Minimum APSH: At least 25% of annual probable sunlight hours, including 5% between 21 September and 21 March (winter months) should be received at the centre of the main window.

Noticeable Impact: A reduction is considered noticeable if:

For New Buildings:

BE EN 17037 recommends that one habitable room, preferably a main living room, should receive at least 1.5 hours of sunlight on 21 March.

These recommendations are advisory but widely adopted by planning authorities across the UK.

When Is APSH Assessment Required?

An APSH assessment is usually necessary in the following cases:

New Developments Near Residential Buildings: Especially if any part of the new structure lies within 90° of due south of existing windows.

Extensions or Upward Developments: When additions may cast new shadows onto neighbouring dwellings.

Conservation Areas or Protected Views: Where the visual and environmental quality is highly valued.

Planning Objections and Appeals: APSH is often a determining factor in resolving disputes.

Common Factors Affecting APSH

Several elements influence APSH outcomes:

1. Orientation of the Window: South-facing windows typically receive more sunlight.

2. Obstruction Height and Proximity: Taller or closer obstructions cast longer shadows.

3. Balconies and Overhangs: These can severely limit sunlight access even if no other buildings are obstructing the view.

4. Window Reveal Depth: Deep reveals can block sunlight, reducing effective APSH.

5. Vegetation: Trees can obstruct sunlight and should be considered if dense and evergreen.

Practical Design Strategies to Improve APSH

To maximise APSH in new buildings:

Orient Living Rooms Southward: Ensures primary living spaces benefit from the highest sunlight availability.

Step Back Upper Floors: Reduces shadowing onto windows below.

Use Dual-Aspect Flats: Improves sunlight penetration and offers more varied sunlight exposure.

Stagger Balconies: Prevents upper balconies from blocking sun to lower windows.

Reduce Building Depth: Allows sunlight to reach more of the internal floor area.

Summary and Key Takeaways - Annual Probable Sunlight Hours APSH

Annual Probable Sunlight Hours (APSH) measures the likely hours of direct sunlight a window receives, accounting for cloud cover.

Assessors express it as a percentage of the total annual potential, typically using an unobstructed south-facing surface as the benchmark.

The BRE Guidelines place APSH at the core of site layout considerations, using it in planning applications to help protect residential amenity.

For existing buildings, the target is 25% APSH annually, including 5% in winter. BRE define significant impact as a reduction to less than 0.8 times the former APSH level and a total loss exceeding 4%.

New dwellings should have at least one main room that gets 1.5 hours of sunlight on 21 March.

Tools like the sunlight availability indicator or specialist 3D modelling software are used to conduct APSH assessments.

APSH is an advisory planning metric, distinct from enforceable legal rights like those associated with rights to light claims.

Need Expert Advice on Annual Probable Sunlight Hours APSH?

If you're planning a development or need support assessing its impact on sunlight and amenity, our team at Anstey Horne can help. We are experts in daylight and sunlight assessments, including APSH analysis, and have extensive experience guiding developers, architects, and planners through the intricacies of the BRE Guidelines and local planning requirements.

Contact us today to discuss your project and ensure your proposals meet the necessary daylight and sunlight standards.

For more information on all aspects of Daylight & Sunlight Assessments for planning see the collection of articles on our blog page.

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