Daylight & sunlight in the built environment
BRE BR 209 provides guidance for assessing the effect of development on daylight and sunlight, both within new buildings and to neighbouring properties.
At Lighting Analysts, we use the BRE methodology to help design teams understand the daylight implications of site layout, building height, massing and separation distances at an early stage, as well as to undertake detailed planning assessments where required.
Daylight and sunlight analysis is most valuable when it is undertaken early enough to influence the design.
We use modelling to investigate building height, massing, spacing, orientation and overshadowing, helping the design team identify potential problems while the site layout is still flexible.
We help inform the site layout — not just assess its impact.
BR 209 considers daylight and sunlight at several different scales, including new buildings, neighbouring properties and external spaces. It is intended to be used alongside BS EN 17037 for daylight provision within new buildings.
Existing Buildings
The effect of a proposed development on daylight and sunlight reaching neighbouring buildings can be assessed using metrics including:
Vertical Sky Component — VSC
Measures the amount of diffuse skylight available at the centre of a window.
No Sky Line — NSL
Assesses how far daylight penetrates within the room behind the window.
Annual Probable Sunlight Hours — APSH
Assesses the potential availability of direct sunlight to windows throughout the year.
Winter Probable Sunlight Hours — WPSH
Considers sunlight availability during the winter period.
Not every development requires detailed numerical analysis.
BR 209 includes simple rule-of-thumb tests that can be used to identify whether a proposed development is likely to have a material effect on neighbouring daylight.
BRE 3x Rule
Where the proposed development is at least three times its height above the centre of the existing window away from that window, the resulting loss of light is generally expected to be small.
Where the development fails this test, the 25° rule-of-thumb test can be applied.
25° Test
The vertical angle subtended by the proposed development is measured from the centre of the affected window.
Where the complete development remains below 25°, a substantial effect on diffuse skylight is considered unlikely.
Where these initial tests are not satisfied, more detailed VSC and daylight-distribution analysis may be appropriate.
Where detailed analysis is required, Vertical Sky Component and No Sky Line calculations can be used to compare the existing and proposed conditions.
A commonly referenced BR 209 benchmark is a VSC of 27%, together with consideration of whether the proposed value remains at least 0.8 times the existing value. NSL assessment similarly considers the change in the area of the room that retains a view of the sky.
The numerical results should always be considered in the context of the existing building, site constraints and wider planning considerations rather than being interpreted as isolated pass/fail criteria.
For windows where direct sunlight is relevant, BR 209 uses Annual and Winter Probable Sunlight Hours to assess the change in sunlight availability.
The analysis compares existing and proposed conditions and considers both annual and winter sunlight exposure.
Orientation is fundamental to this assessment, as sunlight availability varies considerably between façades.
Development can also affect sunlight reaching gardens, courtyards, playgrounds and other external amenity spaces.
BR 209 recommends that at least half of a garden or amenity area should receive at least two hours of sunlight on 21 March. Shadow studies can also be used to illustrate how overshadowing changes throughout the day and year.
Site planning affects not only neighbouring properties but also the daylight available within the proposed development itself.
Building spacing, orientation, massing, courtyards, external obstruction and landscape can all influence how much daylight reaches façades and penetrates into occupied rooms.
BR 209 is intended to be used alongside the daylight recommendations of BS EN 17037 for assessing daylight provision within new buildings.
Services
Early Site Layout Review
Initial assessment of massing, spacing, orientation and neighbouring context to identify potential daylight and sunlight constraints before the design becomes fixed.
Neighbouring Daylight Assessment
Detailed assessment of the potential impact of a proposed development on daylight reaching existing neighbouring buildings, including VSC and NSL analysis where appropriate.
Sunlight Assessment
Assessment of annual and winter sunlight availability to relevant neighbouring windows.
Overshadowing & Amenity Assessment
Sun-on-ground and shadow analysis for gardens, courtyards, playgrounds and other external amenity spaces.
Planning Reports
Clear project-specific reports setting out the assessment methodology, assumptions, results and conclusions for submission as part of the planning process.
Talk to us
Early daylight and sunlight analysis can help establish appropriate building heights, spacing and massing before these become embedded within the design.
We can provide early-stage site planning advice, detailed BRE 209 assessment and planning reporting for new developments.