Biodiversity by design

23rd June 2023


Is biodiversity net gain (BNG) the key to unlocking sustainable development? Faye Durkin reports

As an ecological consultant I spent the early part of my career working with developers, conducting ecological surveys of proposed development sites and helping to design mitigation and compensation schemes. Often, the design process was progressed with ecology being an afterthought or an issue to be dealt with, rather than being viewed as an integral part of a scheme. Surveys were conducted and proposals modified to squeeze ecology in around the edges with protected species often being viewed as a ‘constraint’ and the mitigation and compensation requirements merely boxes to be ticked.

The drivers behind site design were most often function, aesthetics and cost, with the biodiversity value of green space either not considered at all or not considered to be important. This often led to two types of green infrastructure within schemes; those areas designed for people and those designed for wildlife. The two were seen as mutually exclusive with areas designed and managed for amenity needed to be ‘neat and tidy’ and those managed for wildlife fenced off and kept separate to deter people away.

In the 2010s things began to change with the introduction of BREEAM and Code for Sustainable Homes assessments. The land use and ecology section of these assessments required developers to consider the impact of a scheme on biodiversity and not just on protected species and habitats. However, the early versions of these calculators were very simplistic with biodiversity measured in the literal sense of species per metre squared without due regard for habitat type, condition or composition.

Despite the simplicity of the calculation used to determine biodiversity value, BREEAM and Code for Sustainable Homes assessments were an important first step towards more sustainable development design. Developers were rewarded for going beyond legal compliance. Funding mechanisms encouraged developers to achieve BREEAM excellent and schemes were seeking ways in which green infrastructure could deliver multiple benefits by providing areas that were attractive for people, but which also had benefits for wildlife.

Now in the 2020s we have the Defra Metric which has been trialled and tested for over 10 years with Version 4.0 launched this spring. The metric uses habitat type and condition as a proxy for biodiversity value. Based on the UK Habitat Classification system it essentially ranks the habitats according to their wildlife value.

Although 10% biodiversity net gain (BNG) will not be mandatory for the vast majority of developments until November 2023 (April 2024 for small scale projects and November 2025 for NSIPs) many local authorities have been ahead of the legislation and have been requiring at least no net loss for a number of years due to the National Planning Policy Framework 2016 requirement for this, and the 2019 amendment which required developments to deliver a net gain of biodiversity.

This has meant that designing schemes with biodiversity in mind has quickly become common place amongst developers with ecologists now involved far sooner in the process than ever before.

Alongside the Metric are the 10 Good Practice Principles developed by Ciria, CIEEM and IEMA which add important qualitative detail to the implementation of BNG over and above the quantitative numerical assessment of pre and post development biodiversity value delivered by the Metric (biodiversity-net-gain-principles-and-guidance-for-uk-construction-and-developments (iema.net)).

Principle 1 is: Apply the Mitigation Hierarchy. Developers must first seek to avoid the impact (i.e. by retaining and protecting the most valuable areas of habitat). This can mean that schemes become economically unviable; the land is more ‘valuable’ as high-quality habitat and the compensation needed to off-set the impact and deliver the additional 10% becomes more costly than the development. Or it might mean that those parts of the site that are most valuable are retained and protected or enhanced as part of a scheme and integrated into the landscape design.

Where impacts cannot be avoided, they must then be mitigated for on-site through the landscape design process with ecologists working as part of the project design team to inform the decision-making process in terms of suitable habitats to integrate into part of the project.

There can be a tendency at this stage for the wider design team to focus on the Metric and attempts made to select more valuable habitats without due regard to the likelihood of their ongoing success. The driver behind this is often to try and achieve 10% BNG onsite and therefore avoid costly offsite compensation. However, Principle 6 is: Achieve the best outcomes for biodiversity. It is therefore important for the whole design team to work together to deliver a scheme that functions well for both people and wildlife by providing habitats that can be maintained in the long term.

The offsite compensation market has developed rapidly over the past few years. The options available to developers are locally specific with the market and supporting systems more established in those local authorities that have had local BNG policy for some time. Some local authorities are administering the compensation market directly whilst a range of options are available in other areas such as schemes provided by Wildlife Trusts, Land Agents and specific habitat banks.


The Biodiversity and Natural Capital Steering Group have hosted a series of BNG focussed webinars which are accessible to IEMA members via the website. The three-part series looking at the design, implementation and management of BNG has been developed into an Insights document which provides readers will lessons learned from a range of case studies and answers frequently asked BNG questions (IEMA - Biodiversity and natural capital).


For some developments it will not be possible to identify suitable local compensation and so Natural England are developing a biodiversity credit trading platform. This will be a final port of call for developers and those seeking to purchase credits will be expected to demonstrate that they have followed the mitigation hierarchy. For projects using the credit trading platform there will not be a direct link between the impact site and compensation site with developers purchasing credits through the platform and not needing to identify a specific project to offset their project.

The regulatory framework which underpins BNG has been rapidly developing and will continue to do so. In recent months there has been long awaited guidance on stacking, bundling and additionality which will provide landowners with much needed clarity regarding which co-benefits can be grouped together and which can’t. Whilst this is a complex issue the information released so far has helped and Principle 7 (additionality) is clear, BNG needs to be over and above that which exceed existing obligations.


The BaNC will be delivering a webinar focused on stacking and bundling in summer 2023 – register for the newsletters to find out more.



Ecologists are now much more involved in the design process, and this is unlocking a more sustainable approach to development where the benefits to both people and wildlife are recognised. Principles 3 (Be inclusive and equitable) and 9 (Optimise sustainability) further demonstrate how delivering BNG is ultimately helping to deliver more sustainable developments with opportunities to involve stakeholders and provide developments which results in better social, economic and environmental outcomes.

General awareness regarding the climate emergency and the need to build back better and greener after COVID has been evident with the public realising just how important access to greenspace was when a daily walk was the only opportunity for recreation. There is therefore an appetite within the market for sustainable development with high-quality green infrastructure.

BNG is swiftly evolving into environmental net gain (ENG) which allows for ecosystem service co-benefits (i.e. the benefits to nature and people) to be captured when measuring biodiversity change evidencing overall improvement in the natural environment performance delivered through nature-based solutions with several tools (e.g. EBNT) now available to help measure the potential impacts of development and to help drive better decision making. BNG and the Metric have been the key that has finally unlocked sustainable development design as it has provided a tool to facilitate conversations around ecologically led design. I’m excited to see what the future holds as BNG evolves into ENG and we can further explore the benefits of nature-based solutions and biodiverse design.


Since its launch in September 2022 the focus of the BaNC network has been biodiversity given the timing of the Environment Act and mandatory BNG. Over the coming months this focus will gradually shift towards natural capital, starting with an Insights from Practitioners (the Good, the Bad and the Ugly).



Faye Durkin is Head of Ecology and Natural Capital (North) at Greengage

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