Daylight saving

It is generally accepted that people like to live and work in buildings with good levels of natural light. Extensive research has been undertaken into the effect of daylight and sunlight on health, and the design of natural light admittance into buildings is now a complex combination of architecture and engineering. The increase in environmentally conscious design, such as the use of atria to facilitate natural daylight and ventilation, has seen the access of daylight and sunlight become a key design issue.

The effect on the daylight and sunlight amenity enjoyed by neighbouring properties is now routinely considered by London planning authorities when an application is submitted for development of an adjoining site. Frequently, the local authority will also ask for analysis that proves that the light levels to residential accommodation in new developments are satisfactory.

As the level of importance placed on environmental issues continues to increase, the number of local authorities outside the capital requesting daylight and sunlight studies in support of planning applications has also risen.

The extent of local authority requirements and guidance on compliance can vary, but their main concern is for residential neighbours. The most widely accepted method of analysis is in accordance with the Building Research Establishment Report 209: Site Layout Planning for Daylight and Sunlight – A Guide to Good Practice. Whilst the target-values provided in this report can often be attained by low-rise suburban developments, they are difficult to meet when developing compact urban sites.

The report states that the guidelines it provides are ‘…not mandatory and should not be seen as an instrument of planning policy.’ It also suggests that ‘Different criteria may be used, based on the requirements for daylight in an area viewed against other site layout constraints.’

The introduction advises that ‘in an historic city centre, a high degree of obstruction may be unavoidable if new developments are to match the height and proportions of existing buildings.’

The BRE report is predicated on a suburban development model i.e. the construction of houses on typical suburban residential streets. Whilst it does make some reference to its application in an urban environment, the BRE Report provides no alternative target values, nor any real guidance on acceptable degrees of tolerance that could be applied when considering developments in inner cities. This is compounded by the fact that many homes in inner cities currently have light levels significantly below the target values.

The level of daylight received at a window face is expressed as a percentage called a ‘vertical sky component’ (VSC). As an example, a window positioned 2m above ground, facing a continuous obstruction of altitude 25 degrees would achieve a VSC of 27 percent and have the ‘potential for good daylighting’.

In the tight-knit urban environments that are often formed on old street patterns, such as those in many areas of London, there is a high density of buildings on very narrow streets or lanes. In cases, the existing light levels, particularly to the lower floors and any basement accommodation, can be significantly below the BRE target values. These low light levels are generally accepted by owners and occupiers because they are choosing to live in such an area for reasons other than daylight and sunlight.

The light level to such accommodation only becomes an issue when an adjacent piece of land or building is considered for development. Land prices, and sometimes planning policy, will dictate a building of a certain mass; frequently larger than the existing structure. The developer will then be faced with formulating a design that complies with the BRE report by not reducing existing light levels to adjacent properties by more than 20 percent.  

When any existing levels are as low as 15 percent VSC, any reduction, however small, can appear significant in terms of the percentage reduction and will often result in non-compliance with the BRE report. The question is, would such a reduction be noticeable or important to residents?

Each case needs to be judged on its merits. The developer and their design team need expert consultancy advice to create a design that works commercially and architecturally, and also leaves the neighbours with sufficient light.

The local authority’s judgement will require flexibility in interpreting the results and the confidence to consider applying different target values to reflect the actual site and it environs. This second point is actually a direct recommendation in the BRE report and should imply that planning officers are not obliged to apply the report’s target values literally.

The questions of light levels within a development itself is dealt with in The British Standard Lighting for Buildings – Part 2: Code of Practice for Daylighting, which came into effect on 30 September 2008. The superseded 1992 code, which recommended that kitchens, lounge/dining rooms and bedrooms should achieve minimum average daylight factor (ADF) levels of 2 percent, 1.5 percent and 1 percent respectively.

The 2008 code maintains these values, but also recommends that rooms which serve more than one purpose should be assessed using a higher daylight requirement. This will mean that living/kitchen/dining rooms and kitchen/dining rooms, which are commonplace in today’s urban schemes, will require an ADF of two percent.  

With high-density urban living, designers face difficulty achieving the target values, and this clarification on the minimum value for multi-purpose rooms compounds the problem. The main obstacles include the size of a room, the position of the window, the existence of balconies and glazing transmittance.

However, these issues are not new and reference to multi-purpose rooms in the updated code is perhaps a clarification instead of a change. A frequent argument is that an ADF value of 1.5 percent is sufficient for multi-purpose rooms which contain a kitchen, as the majority of the floor area is used as living space. This is particularly so where kitchens are located at the rear of the room, furthest from the natural light, and are designed to be predominantly artificially lit.

It is often argued that the target values required by these documents are draconian, and that a separate set of guidelines is needed for inner city development. The BRE Report is now nineteen years’ old and does not provide clear guidance specific to urban areas. Revised guidance, such as a reduced scale of BRE compliance linked to the housing density detailed in the local authorities UDP and London Plan, would assist designers, developers and local authorities.

Windows to residential buildings which are over twenty-years’-old will still enjoy a right to light. This easement entitles the owner to receive a certain level of skylight through a defined aperture. For residential property, at least 50 to 55 percent of a room should be left adequately lit. If a development results in an unreasonable interference with a neighbour’s right to light then a legal nuisance will have been caused and an injunction or damages are the likely remedies.

The current emphasis placed on environmental and amenity issues during the planning process, coupled with the need to consider neighbours’ common law rights, means that light issues will continue to have a significant impact on the development of urban sites.

Paul Smith is building consultancy director at GL Hearn, a specialist property, planning and development consultancy.