Biodiversity Net Gain (BNG) Policy Overview

Natalie Duffus, Department of Biology, University of Oxford

Date: October 2025

Summary

  • Biodiversity Net Gain (BNG) creates a mandatory requirement for nearly all construction projects to measure and improve biodiversity, making it one of the most ambitious ecological compensation policies in the world.
  • Effective governance of BNG is essential to ensure that uncertain future biodiversity gains materialise.
  • BNG uses a habitat-based metric, so additional species-based monitoring and mitigation are needed for the best environmental outcomes
  • Developers must compensate for biodiversity loss first on-site, and then off-site once on-site actions are exhausted. Demand for off-site units appears lower than expected (Duffus et al., 2025b).
  • Community acceptance of BNG measures depends on factors such as public access and species richness (Butler et al., 2025).
    Globally, BNG is influencing policies in countries including Sweden, Oman, Saudi Arabia, the U.S. and Singapore (Duffus et al., 2025a).
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The Statutory Biodiversity Metric

BNG is delivered through the Statutory Biodiversity Metric (Defra, 2024) — a habitat-based proxy for biodiversity.
It measures biodiversity before and after development, requiring a 10% net improvement maintained for at least 30 years.

The metric combines:

  • Habitat area
  • Habitat distinctiveness (conservation value)
  • Habitat condition (ecological quality)

Higher scores are assigned to habitats supporting Local Nature Recovery Strategy (LNRS) objectives.

The Biodiversity Gain Hierarchy

Developers must follow a strict order of actions (DEFRA, 2024):

  1. Avoid and minimise impacts on valuable habitats.

  2. Enhance or create habitats on-site.

  3. Offset remaining losses off-site, through the BNG market.

If off-site delivery is not possible, developers can purchase statutory biodiversity credits from government schemes.

Governance and Compliance

BNG involves offsetting—habitats are lost now, while compensating habitats may take decades to mature.
Strong governance is essential to ensure future biodiversity gains materialise.

Off-site BNG projects

  • Secured via Section 106 agreements or conservation covenants with responsible bodies.
  • Gains must last 30 years and are listed on a public register (Defra, 2025a).
  • This transparency ensures a high degree of accountability.

On-site BNG projects

  • Only significant gains require legal agreements or registration.
  • Smaller on-site gains may not be recorded, creating lower transparency and weaker accountability.

BNG: The Story So Far

In the first 18 months of BNG’s rollout:

  • 112 off-site projects were registered by September 2025, covering over 4,000 hectares of enhanced or created habitat (Defra, 2025a).

  • Yet, by June 2025, less than 2% of registered offset land had been sold (Duffus et al., 2025b).

Many larger developments achieve 10% BNG entirely on-site, helped by metric flexibility allowing smaller high-quality habitats to offset larger low-quality ones (zu Ermgassen et al., 2021; Rampling et al., 2023).

However, on-site habitats often fall into governance gaps, meaning weaker compliance monitoring (Chapman et al., 2024).

Ecological Outcomes

Studies show the statutory metric is an imperfect proxy for actual biodiversity outcomes (Duffus et al., 2025a, 2025c; Hawkins et al., 2022; Marshall et al., 2024).
It must therefore be supplemented with species-based monitoring.

Current delivery is skewed toward fast-maturing habitats (grasslands) rather than slower, complex ones (woodlands, wetlands) (Duffus et al., 2025b; Miles et al., 2025).

BNG can enhance community greenspace, though offsetting remains controversial (Apostolopoulou, 2020; Sullivan & Hannis, 2015). Acceptance often depends on public access and species richness (Butler et al., 2025).

While there are tensions between nature and social goals, these can be resolved project-by-project (Mancini et al., 2024; Atkins et al., 2025).

The Future of BNG

BNG remains dynamic. A Defra consultation (2025b) proposed simplifying rules for small and medium developments but drew criticism for potentially weakening ecological outcomes (Leverhulme Centre for Nature Recovery, 2025).

The policy sits alongside the Planning and Infrastructure Bill (PIB, 2025), which introduces a Nature Restoration Fund (NRF) to streamline mitigation.
The Office for Environmental Protection (2025) has raised concerns that PIB could reduce environmental safeguards.

BNG therefore continues to play a key compensatory role in England’s nature recovery framework — and its metric is influencing global policy (Duffus et al., 2025a).