Daylight Calculations: Comprehensive assessments of daylight, sunlight, and solar gain; internal daylight evaluation; and overshadowing reports. 

Daylight calculations for planning are part of a comprehensive assessment used to evaluate the impact of a proposed development on the natural light received by both existing neighbouring properties and the future occupants of the new building. 

These assessments are essential for urban planning applications, particularly in dense areas, to ensure a balance between new development and the preservation of quality living conditions. They help determine potential conflicts regarding a “right to light” and inform sustainable design choices by maximizing natural light to reduce reliance on artificial lighting and minimise solar gain where it would cause overheating. 

The analysis is conducted using 3D modeling software and follows the guidance of the UK’s Building Research Establishment (BRE) Report 209: Site Layout Planning for Daylight and Sunlight and the British Standard BS EN 17037. 

Key Components of the Assessments

The reports use several key metrics to assess different aspects of natural light: 

Impact on Existing Neighbouring Properties (Overshadowing Reports) 

Vertical Sky Component (VSC): Measures the amount of sky visible from the centre of a window on an existing building. A VSC of less than 27% (and a reduction of more than 20% of its original value) suggests a potentially noticeable impact on the neighbour’s daylight.

No-Sky Line (NSL) / Daylight Distribution: Maps the parts of an existing room that receive direct daylight from the sky. If the area receiving light is reduced significantly (e.g., by more than 20%), the room may appear gloomier.

Annual Probable Sunlight Hours (APSH): Assesses the amount of direct sunlight received by existing windows that face within 90 degrees of south. The BRE recommends that a main room should receive at least 25% of annual sunlight hours (and 5% of winter sunlight hours), and a significant reduction from the existing value may be unacceptable.

Sunlight to Amenity Areas: Assesses the overshadowing of gardens and public open spaces. The guideline suggests that at least 50% of the area should receive a minimum of two hours of sunlight on the spring equinox (21 March). 

Internal Daylight Evaluation (Proposed Development)

Average Daylight Factor (ADF): A calculation used to determine if proposed habitable rooms will receive adequate overall levels of natural light. Typical recommended minimum values are 1.5% for living rooms, 2% for kitchens, and 1% for bedrooms.

Climate-Based Daylight Modelling (CBDM) / Illuminance Levels: A more sophisticated approach (often used for BREEAM certification) that uses climate data to predict actual light levels (lux) throughout the year, ensuring sufficient light over a specified working plane.

Solar Gain Analysis: Although daylight is the primary focus, the reports also indirectly inform the design to manage excessive solar gain (heat from the sun), which can lead to overheating, by optimising window sizes, orientation, and potential shading solutions. 

Use in Planning

The resulting report, complete with 3D renderings and data tables, allows the local planning authority to make an informed decision, balancing new development with the amenity and quality of life of surrounding residents. Early engagement and assessment help designers mitigate issues and ensure a higher chance of planning approval.