Southwest Environmental Limited

ADF Daylight Assessment

Whilst BR209 for the most part concentrates on impact on neighboring properties, there is some consideration given to the lighting levels within the proposed development.

There are standards set for average daylight factors which are: 1% for bedrooms and 2% for kitchens and living rooms. In many cases Local Authorities, particularly London Borough Planning Authorities will pick higher values.

Some councils such as Plymouth rely on Window Size to Floor Area comparisons, which is also quite effective.

The ADF value is created by "blending" together Internal Walls Areas, Glazing transparency, Open Sky Angle and Surface Reflectivity.

We are typically asked to do these for basements, or lower ground floor proposals.  Image: David Lally, via Wikimedia Commons

The truth is that if you have a moderately sized window, that is not too badly "over shadowed" it is very likely that your room will pass, the ADF Assessment. Problems arise during retro fit projects, particularly where there are light wells, or glazed grates where there is a very narrow open sky angle, and this has a negative effect on calculations.

ADF Daylight Advice - Site Layout

If you have been asked to check daylight levels in a proposed development then please get in touch, and we can have a quick look at plans and tell you what the score is.

We can provide allsorts of day lighting design advice to anybody who needs it. Most designers /  architects are aware of how to make a room lighter (bigger, taller windows, lessen over shadowing) but sometimes it is good to have specialist on you side to prove a point.

We will of course first check you design to see if it meets ADF criteria:

- 1% for Bedrooms

-1.5% for Living Rooms

-2% for Kitchens

If these values are met. Then there is no need to change anything. If is worth nothing that some planning officers ask for ADF values above those mentioned in BRE Guide 209.

ADF Daylight Advice - Interior Layout

Increasing window size is a very good first move, but in retrofits this is not always possible. When windows are overshadowed or are within a light well (have a narrow sky angle) then ADF values can be reduced.

However interior layout can be changed to helps. This might include built in wardrobes in a bedroom, or a walk in larder in kitchen so as to reduce floor area, and increase proportionate lighting.

It can also involve swapping rooms around, or combining rooms. These are all allowable measures to increase daylighting and can result in better living accommodation, with lower energy consumption owing to increased natural lighting.

The below section was written by a machine.

Technical Archive: The Legacy of Average Daylight Factor (ADF)

This page serves as a historical resource regarding the Average Daylight Factor (ADF) metric. For many years, ADF was the primary quantitative measure used in the UK to assess the adequacy of natural light within proposed developments. However, as building science and planning requirements have evolved, simple ADF calculations have been largely phased out in favor of more sophisticated, time-dependent analyses.

Transition to BS EN 17037 and CBDM

Current industry standards have moved away from static measurements like ADF because they do not account for geographic location, orientation, or varying sky conditions throughout the year. For all current planning applications and compliance checks, please refer to our updated guidance on Sunlight and Daylight Assessments. Modern assessments now typically utilize:

• Climate-Based Daylight Modelling (CBDM): A detailed simulation that uses local weather data to predict lighting levels for every hour of the year.
• BS EN 17037 Compliance: The European and British standard that provides a more holistic view of daylighting, including views to the outside and glare protection.
• Spatial Daylight Autonomy (sDA): Measuring the percentage of a space that receives sufficient daylight for a certain amount of annual operating hours.

Historical Context of ADF

Under legacy guidance such as the older versions of BR 209, ADF was often calculated to ensure minimum thresholds were met—typically 2% for kitchens, 1.5% for living rooms, and 1% for bedrooms. While these figures provided a basic benchmark for internal amenity, they are no longer considered sufficient by most local planning authorities. Modern sustainability strategies now require a deeper level of technical evidence to ensure occupant well-being and energy efficiency.

Technical Support for Modern Standards

Although this specific methodology is archived, the requirement for robust natural light analysis remains a critical part of the Environmental Impact Assessment (EIA) process and residential planning. Southwest Environmental Limited continues to provide industry-leading technical authority on the latest standards to ensure your project meets current regulatory expectations. With over 900 projects completed nationwide, we offer the professional insight (IES/FGS) needed to navigate the complexities of modern environmental design.