Biodiversity Net Gain in the city

7th February 2024

Over the past half century our cities have seen a steady decline in biodiversity. Development has removed green spaces from city centres and plant variety has diminished as development becomes more uniform.

The State of Nature report, published in 2019, states that, since the 1970’s, the UK has experienced approximately 13% decline in the abundance of wildlife. This loss has occurred despite multiple legislations and policies to prevent such a loss.

Although specific habitats and species have enjoyed protection, there were limited procedures in place that allow the value of habitats to be assessed and maintenance and enhancement plans to be put in place.  As a result, development continued to result in habitat loss and nature’s ability to thrive was diminished.

The new Biodiversity Net Gain legislation, coming in to force on 12th February 2024, aims to reverse this loss.  In this article we explore this legislation and what it means to the both the city scape and the Landscape Architecture profession.

What is biodiversity?

In a nutshell, biodiversity refers to all the different kinds of life you’ll find in one area—the variety of animals, plants, fungi, and microorganisms, like bacteria, that make up our natural world”.  Each of these organisms interact with each other in a complex network of interdependent relationships, maintaining a delicate balance and supporting the life within the ecosystem. As such, a change to any part of the biodiverse network can have a ‘domino’s affect’ on other parts.

From a Landscape Architecture point of view biodiversity refers to the existing ecosystem of an area which landscape changes and developments are being planned for. As protecting the environment and preserving key habitats becomes more and more important, our role in protecting and increasing biodiversity in any new development is a key consideration of our profession.

What is Biodiversity Net Gain?

Biodiversity Net Gain (BNG) is an approach to development, land management and/or marine development that aims to leave the existing natural ecosystem in a quantifiably better state than it was previously.

BNG delivers measurable improvements for biodiversity by creating or enhancing habitats in association with development. BNG can be achieved on-site, off-site or through a combination of on-site and off-site measures. BNG aims to create new habitat as well as enhance existing habitats, ensuring the ecological connectivity they provide for wildlife is retained and improved”.

BNG, does not just focus on maintaining the status quo.  The policy aims to “create new habitat as well as enhance existing habitats.” From 12th February 2024 onwards most new developments will need to deliver a minimum BNG of 10% .

What is the timeline for BNG?

From 12th February 2024 the Environment Act 2021 will make BNG mandatory for all but small sites (with some exemptions), then from 2nd April 2024 small sites will also fall under the legislation.  From these dates, in England, BNG will become mandatory under Schedule 7A of the Town and Country Planning Act 1990 (as inserted by Schedule 14 of the Environment Act 2021).

How is BNG measured?

BNG is measured using the Biodiversity Metric. This tool uses a standardised statutory biodiversity metric formula to measure biodiversity at the time of development and biodiversity upon completion.  Four key factors underpin this comparison: habitat size; condition; distinctiveness; and location.

As defined by CIEEM, “BNG involves the use of a metric as a proxy for recognising the negative impacts on habitats arising from a development and calculating how much new or restored habitat, and of what types is required to deliver sufficient net gain.”

For the purposes of BNG, biodiversity is measured in standardised biodiversity ‘units’. A habitat will contain a number of biodiversity ‘unit’s, depending on things like its:

  • Size
  • Distinctiveness
  • Diversity
  • Quality
  • Location
  • Strategic importance to the local area
  • Type

Biodiversity ‘units’ can be lost through development or generated through work to create and enhance habitats.

The tool is used to calculate how many ‘units’ a habitat contains prior development and how many will be required post development to both replace the units lost and achieve a BNG of 10%.

In addition, biodiversity metric ‘units’ can help to determine the value of an area of land. This is sometimes referred to as biodiversity banking.

This tool should be used by a competent person, normally an ecologist.  As Landscape Architects, we will need to work closely with Ecologists and Developers to understand the point at which we are starting from, the point we need to reach and how we can design for nature to ensure we reach, and potentially exceed, the requirements. Consulting with planning authorities will be key early on in proceedings.

How will BNG work in practice?

BNG goes beyond planting a few trees or shrubs; developers will be required to measurably demonstrate exactly how they will increase the biodiversity value of a site. Moreover, BNG will need to be “trackable, with goals and targets set and easy ways to monitor, evaluate and report results to local planning authorities.

Crucially, a development site may not be the appropriate place to implement an increase of BNG, this may mean that developers need to rely on a combination of ‘biodiverse’ designs and/or offsite mitigation schemes in order for plans to be granted planning permission.

The process that will be followed will be similar to below:

  • A field survey will be undertaken, normally by an ecologist, to collect pre-development habitat data.
  • Post-development habitat data will be defined using landscape plans.
  • The Biodiversity Metric will be used to convert pre-development and post post-development habitat data biodiversity ‘units’.
  • Landscape Architects, Ecologists, Developers and Planners will work together to increase the number of ‘units’ possible on the site of development.
  • Additional biodiversity units may then be added, if required, to post-development data using offsite compensation or biodiversity credits.

This calculation will be required prior to planning permission being granted (or as a condition of planning permission). It will therefore ensure the development plans have the ability to achieve a net gain for biodiversity.

How can Landscape Architects support the achievement of BNG?

At Outerspace our vision is ‘Enhancing Lives, Celebrating Nature’.  At the heart of everything we do is the connectivity of humans with nature, together with a deep understanding of the spirit of a place. We do not believe that one can be enhanced independently of the other.

Typically, when designing a high-density residential scheme, for example, we will include areas of both private and public space, with planting that provide some gains for nature (such as nectar rich, berry producing planting etc.). As is often the case plans change during the early stages, and changes to development layouts will require landscape designs to alter and move in correlation.  BNG will require further stages in this ongoing iterative process to ensure the final calculations achieve a design that effectively meet BNG requirements.  To some extent Landscape Architecture studios based in London have already been doing this as we work withing the requirements of Urban Greening Factor.  In fact, it could well be the case that BNG and Urban Greening factor end up playing ‘top trumps’ against each other so the one with the highest requirement needs to be met, which is a win win situation for nature in London.

As Landscape Architects, we play a key role in supporting our clients in Landscape Planning, Landscape Design and Landscape Management across a wide variety of different projects. We are therefore uniquely placed to offer holistic advice on both how to achieve BNG within a site but also how to achieve this in a way which enhances the daily interactions of the people who use it.

At present Outerspace is looking to hire our first inhouse Ecologist. Ecology and Landscape Architecture sit hand in hand as complimentary disciplines.  Ecology’s main focus is nature, Landscape Architects are passionate about connecting humans with nature.  Embedding an inhouse shared design process where our Ecologist and Landscape Architects are involved from the very beginning will support our clients throughout the BNG process.

How can we achieve BNG in our projects?

The Building with Nature standard provides detailed ‘how to’ guidance for putting nature at the heart of development and a peer-reviewed accreditation process.  Their highlighted cases studies show how even inner-city streets can be greener and biodiverse whilst also bringing people in contact with nature and each other.

As Landscape Architects it is our duty to constantly learn and grown, increasing our knowledge of technical skills but also other projects, both by us and other Landscape Architect studios, so as to implement designs that work.

To achieve BNG we will achieve the 10% increase in projects by using our extensive knowledge of planting, SUDS’s an urban design, alongside Continued Professional Development, to include as wide a variety of species as possible in projects, creating environments where nature can thrive.

Arguably the biggest barrier to achieving BNG on a landscaping project, is the client’s desire for an easy-care garden.  “Low maintenance” is often at the top of the list of requirements as the costs of managing a landscape long term are factored in. However, a low maintenance landscape can still include an abundance of plants and habitats.

We have a range of design features which can increase BNG without necessarily adding significantly to the maintenance requirements. Such features include (but are not limited to):

  • Green Roofs, these can be accessible to the public (with required safety barriers) or closed off, in some schemes we may include habitats such as bee hives or ‘bug hotels’ to encourage a network ‘insect islands’ across a development.
  • Living Walls/Green Walls
  • Wildflower areas or meadow planting used to replace traditional lawns.
  • Designing Ponds and Wetland areas. It is often difficult for these to be accepted due to Health and Safety concerns, however the biodiversity advantages such features bring to a design, if properly secured, are massive.
  • Including rain gardens and SUD’s into our inner city plans.
  • Using Perennial Planting Schemes to reduce the need to plant replacements
  • Placing planters in areas with no access to soil beds.  Such techniques can dramatically change a landscape dominated by hard landscaping and were used to striking affect in our Moretown scheme.
  • Creating space for hedges, thus creating safe areas for small birds and insects.
  • Specifying a wide variety of trees.
  • Working with developers to preserve soil.

As you can see from the links to our other journal articles, Outerspace have a depth of knowledge inhouse and are passionate about truly connecting humans and nature.


Conflicting Policies with achieving BNG in London

Within London there are other policies (especially for residential development) that Landscape Architects have to address to achieve a successful planning application:

Play and amenity: The GLA have prescriptive requirements on the physical amount of play to be accommodated within every site. This is based upon the number of units being delivered, with play for 0-5 year olds being compulsory and increasingly 5-11 year olds. Unless planned carefully, these areas can easily be excluded from the BNG calculation.

Health and Safety and Maintenance: One of the biggest biodiversity wins is water, but there is often a concern by developers and local authorities alike regarding risk and maintenance. Wildflower meadows are also highly valuable but in landscapes that are both intensively used and often in shade these are also difficult to maintain.

Secure by Design: Naturalistic landscape elements, such as hedgerows and wilder scrub planting, is great for Biodiversity but are usually frowned upon by the Security By Design Officer who are concerned about site lines and surveillance.


In summary BNG is an amazing opportunity to genuinely safeguard and enhance the biodiversity of city landscapes for the benefit of future generations. Landscape Architects have a range of features that can be used successfully, but together with other pressures on the landscape such as play and amenity, we require the conviction and investment of developers. In the longer term perhaps there’ll be an understanding by landowners and developers buying land that sites can no longer be ‘maxxed out’ and that the value of land needs to take this into consideration.

We welcome the new legislation and hope that through its introduction we can continue, increase and improve on the work we do, with the aim of leaving the planet in a better place for our children than we found it when we first stepped into our profession.

Written by Kate Kershaw (Practice Manager)