PROJECT NAME BNG viability for local government CLIENTs Kent County Council, Horsham District Council Project period 2021


With the passage of the Environment Act into legislation in November 2021, new developments will be required to deliver a minimum 10% net gain in biodiversity, which will become mandatory once secondary legislation is implemented, expected in November 2023. This legislation follows the Government’s 25 Year Environment Plan and growing recognition of the critical state of biodiversity in the UK, with many local authorities recently declaring Ecological and Climate emergencies.

Local authorities are having to assess the potential impact of the 10% mandatory biodiversity net gain on the feasibility of delivering extremely challenging targets for new housebuilding and economic development. In addition, many authorities are recognising that this still may not be enough to halt and reverse the loss of biodiversity in their areas and are seeking to explore the feasibility of requiring minimum net gain of higher than 10% through the planning process. Critical to the success of any Biodiversity Net Gain policy is to understand the practicalities of its implementation, and to avoid unintended consequences of development becoming unviable and not being able to deliver affordable housing or other social planning contributions.


Temple have been working with local government bodies to develop an understanding of the minimum biodiversity net gain targets that would be reasonable, achievable and feasible, to inform planning policy decisions. Two key projects in recent months include a county-wide feasibility study for Kent County Council and a district-wide assessment for Horsham District Council.

Temple worked with a development viability partner to provide Kent County Council with the evidence base to inform a potential policy to raise the minimum Biodiversity Net Gain requirement above the mandatory 10% across the county and constituent Local Authorities. The basis of this work was in recognition that Kent has suffered serious and sustained declines in wildlife and biodiversity largely due to pressure from developments, and it sought to put a greater emphasis on improving the value of existing biodiversity, as well as robustly implementing a large-scale nature recovery strategy.

To support this study, Temple undertook desk-based research, collecting detailed cost data for different types and scales of habitat enhancements, supplemented by real project examples. This exercise enabled ‘cost per biodiversity unit’ values to be applied to different development typology scenarios, and allowed variation between on-site and offsite delivery approaches to be understood.

Stakeholder engagement was an important part of this study, to critically test the approach and to secure buy-in from a cross section of parties. Temple contributed to a number of round table workshops involving KCC, several Kent based Local Authorities, Natural England, the Environment Agency and private property developers.


The outcome of the work was broadly that increasing Biodiversity Net Gain targets can be achieved without materially affecting viability on many sites, and this may achieve improved ecosystem services values, leading to multiple beneficial outcomes locally. There were also many specific sites where an increased target could not easily be achieved, and in those situations a contribution to offsite offsetting could have notable benefits, as long as these donor sites were equitable to the local communities affected and targeted to achieve the biggest gains reflecting the local wildlife priorities.

Where the work for Kent County Council used generic typologies to review effects on viability at a broad county-wide level, Temple’s work with Horsham District Council used similar principals, but focused on specific development typologies and sites in the emerging local plan at the District level.

The study reviewed potential allocations in the Local Plan, and representative potential windfall sites, and proposed and reasonable development scenarios to model reasonably achievable levels of Biodiversity Net Gain. These values were used to inform what change in net-to-gross ratios would be needed to deliver different target levels of net gain, along with costs for alternatively delivering any shortfall offsite.

The project also included a ‘green call for sites’ – an investigation into the capacity within the district to deliver any necessary off-site habitat enhancement to offset biodiversity losses through development. This involved developing a questionnaire, hosted on a dedicated website, which was disseminated to local landowners to collect information on the availability of land for biodiversity enhancement to offset the impacts of developments and deliver biodiversity net gain in the district.

The study showed that the majority of sites would be able to deliver near to, or in excess of 12% net gain, with the relative impact of additional costs or reduction in development capacity being unlikely to be significant in most cases.

The green call for sites showed that there is ample capacity and willingness among landowners within the District to deliver any offsetting that would be required arising from developments in the local plan.

Key Contact

David Coote Associate Director - Ecology