3D Design Bureau

Breaking Down The Jargon: Daylight And Sunlight Assessments terminology

Understanding Daylight and Sunlight assessments can be difficult. They involve a huge amount of work from skilled analysts to understand the large quantity of data and to collate it into a clear and concise understanding of how a proposed property development will perform when built.

With such complex studies, there comes a significant amount of terminology. This can be challenging to follow and sometimes lead to confusion within a design team or at planning authority level.

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This blog aims to help anyone involved with a daylight and sunlight assessment understand the lingo of industry professionals such as 3DDB.  

  • Skylight:

Non-directional ambient light cast from the sky and environment.

  • Sunlight:

Direct parallel rays of light emitted from the sun.

  • Daylight:

Combined skylight and sunlight.

  • Baseline Model State:

The development site is in its existing state. Neither the proposed development nor any extant developments in close proximity to the site. This model state is used when generating the baseline results for all the existing neighbouring properties.

  • Proposed Development Model State:

The proposed development is modelled into the existing environment. This model state is used when assessing the effect of the proposed development on the existing neighbouring properties, as well as assessments carried out within the proposed development itself.

  • Cumulative Proposed Development Model State:

The proposed development has been modelled into the existing environment along with any extant schemes in close proximity to the subject site. This model state is used when assessing the effect of both proposed developments on the existing neighbouring properties.

  • Vertical Sky Component (VSC):

The ratio of that part of illuminance, at a point on a given vertical plane, that is received directly from an overcast sky model, to illuminance on a horizontal plane due to an unobstructed hemisphere of this sky. Usually, the ‘given vertical plane’ is the outside of a window wall. The VSC does not include reflected light, either from the ground or from other buildings.

  • Annual Probable Sunlight Hours (APSH) / Winter Probable Sunlight Hours (WPSH):

Annual Probable Sunlight Hours (APSH) and Winter Probable Sunlight Hours are a measure of sunlight that a given window may expect over a year period (1 Jan – 31 Dec), or the winter period (21 Sep – 21 Mar) respectively. It can be defined as the ratio between the annual or winter sunlight hours in a specific location, and the hours of sunlight an assessment point on a window actually receives. North-facing windows may receive sunlight on only a handful of occasions in a year, and windows facing eastwards or westwards will receive sunlight only at certain times of the day. Taking this into account, the BRE Guidelines suggest that windows with orientation within 90 degrees of due south should be assessed.

  • Sun On Ground (SOG):

Assessment of what portion of a garden or amenity space is capable of receiving 2 hours or more of direct sunlight on a given date between February 1st and March 21st.

  • Sunlight Exposure (SE):

The number of hours a room can expect to receive direct sunlight on a given date between February 1st and March 21st at a given point on the windows.

  • Spatial Daylight Autonomy (SDA):

Spatial Daylight Autonomy assesses whether a space receives sufficient daylight on a working plane during standard operating hours on an annual basis. For compliance, the target value is achieved across 50% of the working plane for half of the occupied period.

  • Working plane:

Horizontal, vertical or inclined plane in which a visual task lies. Normally the working plane may be taken to be horizontal, 850 mm above the floor in houses and factories, and 700 mm above the floor in offices. The plane is offset 300mm from the room boundaries under BRE 209 criteria, and 500mm from the room boundaries under I.S. EN 17037 criteria.

  • LKD:

Living / Kitchen / Dining room.

  • BRE Target Value:

When assessing the effect a proposed development would have on a neighbouring property, a target value will be applied. This applied target value is generated as per the criteria set out for each study in the BRE Guidelines.

If you need advice on your project with regard to daylight and sunlight, get in touch with our team who will be more than happy to shed more light on your project! For more information on daylight and sunlight assessments, you can also visit the articles below or book a free presentation with the team by contacting us at info@3ddesignbureau.com or +353 1 2880186.

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Nicholas Polley - Managing Director at 3D Design Bureau


Nicholas Polley
Managing Director
at 3D Design Bureau


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