Tunbridge Wells Borough Council: Deep Dive into a BNG development

Tunbridge Wells Borough Council (BC)’s introduction of no net-loss of biodiversity requirements for all major developments since 2010 and a proactive approach to BNG since 2019 has provided insights for the creation of a Biodiversity Net Gain (BNG) policy in their new Local Plan. Please note - Councils developing BNG approaches in advance of its mandatory introduction may have to adapt their approaches to comply with the details of legal requirements in November 2023.


Tunbridge Wells BC began considering biodiversity earlier than many Local Authorities within its planning process back in 2010. The adopted Local Development Framework (LDF) committed the Council to:

conserve biodiversity, in particular as identified in the BAPs, by adopting a robust policy of no net loss of important habitats or species, either directly or indirectly’

To do this, planning applications were subject to a hierarchical approach to nature conservation and the protection of biodiversity and geodiversity, applied across the sites and habitats of national, regional and local importance within Tunbridge Wells. The objective was to avoid no net loss of biodiversity and Tunbridge Wells used Landscape Ecological Management Plans (LEMPS), secured through Section 106 agreements, to implement this.

Following adoption of the LDF, Tunbridge Wells’ continuing commitment to the environment includes;


Image showing key dates in Tunbridge Well's policy development

The project

A planning application for the Turnden site was submitted to Tunbridge Wells in March 2020. This proposed the construction of 165 new dwellings with associated access, car parking, recycling storage, landscaping, earthworks and other associated works.

The land proposed concerned mainly grassland horse paddocks of a previous equestrian centre as well as hedgerows, ancient woodland and two ponds. The developers emphasised the sustainability of the development, arguing that it would increase biodiversity through significant restoration and enhancement of the local landscape including publicly accessible recreational land, on-site open space, and child play areas.

The developer submitted a BNG report alongside the planning application. Defra Biodiversity Metric 2.0 was used to calculate both the biodiversity value of the site’s existing condition and the proposed condition of the site post-development. The original BNG report calculated a net gain of 17.74%.


The challenge for the Council in considering this application concerned the application of the Biodiversity Metric and the area which net gain was expected to be delivered.

There were several concerns raised concerning the BNG document submitted with the planning application. The most pertinent was made by the High Weald Area of Outstanding Natural Beauty (AONB) team – the Turnden site lies within the AONB and so the AONB team were an important consultee for the application.  

While the AONB team recognised the BNG calculation report as ‘robust, thorough (…) and accepted good practice guidance’, their concerns surrounded the grassland condition being considered as ‘improved grassland or semi improved’ and how this was translated into the metric when they considered that there was insufficient botanical evidence. As such the AONB team felt that the baseline calculation for biodiversity on the site was under scoring its true value.

Concerns were also raised regarding the anticipated location of BNG delivery. This was because a substantial portion of the net gain was to be delivered immediately adjacent to the development but outside of the red line boundary submitted with the original BNG report.

The solutions

The developer responded through a re-submission of the BNG report which confirmed the grassland on the proposed development site as ‘semi-improved grassland’ and gave detailed reasoning as to its classification as modified grassland in the Metric. The amended submission also included all land proposed for BNG within the legal agreement for the development and the revised calculations showed the proposal would create a 24.95% net gain.

However, the AONB team maintained their previous concerns about the application, stating that the re-classification of the grassland as semi-improved still failed to recognise the important role that this habitat provides in the wider environment, including acting as a safe passage for a number of protected species.

A further review of the Metric by the applicant in preparation for the Public Inquiry took a  more precautionary approach to the calculations and resulted in a final amendment to the BNG calculations, reducing net gain to an uplift of 21.6%. The application approved by Tunbridge Wells Planning Committee was called in by the Secretary of State for reasons other than BNG but there were separate sessions at the subsequent Public Inquiry on BNG. The decision of the Secretary of State is still pending.

Lessons learned

BNG implementation often involves many stakeholders with competing interpretations of the metric: The Turnden development planning application process exemplifies how the introduction of BNG is likely to bring in multiple stakeholders with different perspectives on the application of the Biodiversity Metric. In this case, differences of opinion led to several amendments to the BNG report over several months and the attention to these details was critical in progressing the scheme by the Council.

Implementing BNG benefits from preparation: The Turnden project exemplifies the benefits of being prepared for potential appeals on the introduction of BNG requirements for planning applications. It also highlights the benefits of having planning policy in place to support the Council’s position prior to BNG becoming mandatory. Tunbridge Wells’ development plan provided a basis to open dialogue with developers on their application of the Biodiversity Metric and seek to find a solution to concerns and the draft local plan was referred to as a substantial policy document throughout the planning application.

Aid future policy development:

To address some of the issues arising as they assess BNG reports, the Council is now drafting a supplementary planning document (SPD) with more specific requirements for applicants, alongside a new emerging plan. This sets out information requirements on BNG and detailed advice on the way that the Biodiversity Metric should be applied. Tunbridge Wells BC is also using its implementation of an interim BNG policy as a learning point ahead of introduction of mandatory net gain issued under the Environment Act.

Advice for other authorities

Just get started! While the introduction of BNG requirements has raised issues with planning applications, Tunbridge Wells have found the learning invaluable in identifying possible issues the Council may have when mandatory BNG is implemented next year.

Be prepared to learn and change: It is only after several years of experience applying biodiversity requirements that the planning team have taken the decision to develop an SPD on BNG. The learning and experience they have gained are helping to inform the SPD and planners feel it would have been a mistake to try and develop detailed guidance earlier.