Biodiversity Net Gain – An Overview

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Wild flower green roof
BNG is integral for sustainable developments

Biodiversity Net Gain (BNG) is the UK Government’s strategy towards land developments that aims to provide the maintenance and recovery of nature. As a driver for encouraging sustainable development, BNG should be regarded as an essential movement that mitigates harm and enables a positive ecological impact that delivers improvements through habitat creation.

However, as with any large-scale initiative, there are a number of aspects to consider regarding BNG. Understanding the principles of the strategy, how it will operate, and who it specifically applies to, is crucial for breaking it down into digestible aspects that assess how BNG can be successfully achieved and what solutions exist to provide assistance.

What is BNG?

The word ‘biodiversity’ originates from the term ‘biological diversity’ – referring to the variety of all living animals, insects, plants, bacteria and fungi in a specified environment. A habitat, meanwhile, is understood to be the natural home and encompassing resources used by the living organisms, animals and plants in a specified area. 

Ariel shot of a construction site in the UK countryside
Construction heavily exploits our countryside

BNG was conceived primarily as a means to protect natural habitats from land developments and sustain biodiversity outputs in the UK for the long-term. Under the new strategy, all planning applications will be required to demonstrate how their operation will leave the biodiversity of a site in a measurably improved state than in pre-development.

As part of requirements introduced with the Environment Act 2021, the stipulations for BNG are scheduled to come into effect from November 2023. Unless specifically exempt, a minimum output of 10% BNG will apply to all developments regulated by the Town and Country Planning Act 1990. In addition, all new or existing habitats will need to be secured for a minimum 30 year period. 

BNG will primarily apply to land managers, developers and local authorities. Respective parties must seek to minimise disruptions to existing natural habitats in their work. If there are restrictions present that hinder this, new on-site or off-site habitats must be generated as a solution.

Why was BNG Introduced?

The most recent State of Nature report by the National Biodiversity Network (NBN) found that, despite active legislation intended to protect biodiversity and wildlife, the UK has seen a 13% decline in the average abundance of wildlife since the 1970s. 41% of species have decreased in this time, with 27% found in fewer places. 

Wetlands in the UK
BNG serves to protect UK habitats like wetlands

The biggest drivers of change in the UK are primarily climate change, agricultural management, pollution and urbanisation, with the impact of invasive non-native species, such as grey squirrels, muntjac and rhododendron, also considered significant. In terms of land management, 72% of UK land is now managed for agriculture, while 1600 miles of road were constructed in Great Britain from years of 2006 – 2018. The UK is also consistently ranked among the worst countries in Europe for our quality of water management. In 2022, there were over 389,000 discharges of untreated sewage into our rivers. 

The troubling figures raised in the report state that 133 of 8431 species have already become extinct from Great Britain alone. Ultimately, there are limited mechanisms in place to value, sustain and enhance wildlife habitats beyond protected sites in the UK, such as National Parks and Areas of Outstanding Natural Beauty (AONB). As a consequence, most habitats continue to be lost to land development and are devoid of their intrinsic natural ability to connect and thrive. The principles of BNG, therefore, aim to create essential new habitats, as well as enhancing those that already exist. 

The Principles of BNG 

In order to minimise their impact on biodiversity, it is suggested that developers and respective parties should strive to provide conservation that exceeds the minimum 10% BNG requirement.

Employing a mitigation hierarchy will help to mitigate the elements of environmental harm from a development, as well as any negative impacts that cannot be counterbalanced elsewhere. This process will also help to recognise the risks and variables involved in achieving BNG, and developers should seek to involve all stakeholders in their plans in order to determine a practical means of securing measurable net gains.

Sedum greenroof storage units
On-site unit integration is fundamental for BNG

Incorporating economical and societal factors will help to cover aspects of sustainability, enabling a development to focus on establishing reliable solutions that generate long-term environmental benefits. Primarily, developers should seek to deliver BNG through on-site units that enable habitat creation and enhancement of landscaping elements and green infrastructure. If this isn’t possible as a result of specific conditions or site-restraints, BNG can be offset and delivered off-site through habitat creation.

The Government is also scheduled to introduce the concept of statutory credits, which will be made available for purchase some time in the future. However, these are intended as a last resort in the event that on-site or off-site units cannot be used to deliver BNG. Ultimately, all outcomes, regardless of the mode of delivery, should be communicated with transparency.

Assessing the Benefits

In addition to encouraging sustainable developments, BNG links more broadly to efforts to address the climate emergency that exists domestically and worldwide. Developing a greener approach to infrastructure development and providing improved access to green spaces and nature not only raises biodiversity levels and improves air quality, but contributes towards better mental and physical wellbeing for the general population.   

Trees enhancing urban area
Green elements soften an urban landscape

BNG enables the production of beneficial resources, enhancing a site’s visual appearance and delivering habitat creation on a significant scale. Developing land that aids natural resources and combats the effects of climate change, also helps to facilitate a more scientific and analytical understanding of the natural world. In turn, providing increased awareness of the importance of retaining essential habitats on an intended development site. 

The Department of Food and Rural Affairs (DEFRA) claims that an estimated 75% of net gains will be achieved on-site through enhancement schemes and initiatives. However, permitting the means to compensate for an inability to improve on-site biodiversity through off-site establishment, enables developers to meet the required biodiversity level through alternative means and still deliver net gains.

How is BNG Calculated?

BNG utilises a metric system that assesses the values of an area’s ecosystem by considering the relative features of a specific habitat in order to calculate its biodiversity value. The metric is then used to calculate how a development will impact the biodiversity value of the proposed site before any approval is made.

The aspects considered by the metric are – the type of habitat, whether on or off-site; the size of habitat, in hectares or kilometres; the condition of each area of habitat, and any specific considerations raised by the proposed site itself. 

Biodiversity assessment
Initial appraisals determine biodiversity value

The metric can be asserted by – ecologists carrying out a biodiversity assessment; developers who have commissioned an assessment; planning authorities interpreting outputs as part of a planning application, in addition to communities assessing the impacts of a local development. 

Assessments for BNG are required in the planning permission stage. This accounts for any recent or deliberate harm to the biodiversity value of a site to be taken into account. The assessment is preceded by a preliminary ecological appraisal, involving a baseline ecological impact assessment that will help to identify any priority habitats encompassed in the metric. 

If the site receives planning permission and works go ahead without complication, further surveys will need to be conducted once the development is complete, in order to determine whether BNG has been achieved. BNG will be successful if the score is higher in post-development than it was in the planning phase. 

Do Green Roofs Apply to BNG?

Green roofs are a highly beneficial addition for land developments and the scope to which they have been utilised has grown in recent years. Green roofs establish a habitat for flora and fauna and can play an integral role in enhancing biodiversity, especially in urban environments. In order to assess the quality of green roofs and their output in regard to developing new habitats, the biodiversity metric utilises a criteria that assesses the distinctiveness, condition and strategic significance of a unit. 

Green Roof Canopy
Enhance outputs through additional habitat

Distinctiveness considers how distinct and unique the habitat is. Rare habitats with a dense collection of plant life will score higher than a more common green roof with mostly moss and grass. Alongside assessing the condition of the green roof, distinctiveness is the most crucial aspect when considering using green roofs to aid BNG. Assessing strategic significance in the metric ascertains whether the right habitats are provided in the right places.

The condition score will rate the biodiversity value of the habitat against others of a similar type. Using this criteria helps to ensure the habitat meets the correct standard. For example, an intensive green roof should have a minimum 50% native and 30% non-native wildflowers with 70% of the roof area consisting of soil and vegetation, including any water features. Achieving a ‘Good’ condition for a green roof will depend on whether it is combined with elements of additional habitat. 

urbanspec Green Roof Structures

Sedum and wild flower greenroof structure
Sedum plants are highly suited to green roofs

Aided by a lower structural height that helps to reduce continual maintenance costs, the configurable design platform of urbanspec external stores can be easily adapted to support the integration of a green roof system. From a visual perspective, specifying a green roof design can enable an external store to blend in with its environment, coordinate with green spaces on a development, and soften an urban landscape. 

Providing environmental benefits, green roof structures are proven to help to reduce the ‘urban heat island effect’ by absorbing noise, trapping dust, and breaking down gaseous pollutants. In addition, they reduce water run-off and recycle CO₂ in the atmosphere, with 1m₂ of green roof can absorb as much as 5 kg of CO₂ annually.

Green roof structures can be specified as either extensive or intensive. Extensive systems support primarily sedum plants, while the intensive version provides an increased substrate depth, supporting a mix of sedum and wildflower plants.