What is Biodiversity Net Gain?

What is Biodiversity Net Gain?

Biodiversity net gain (BNG) is a development approach that requires projects to deliver a measurable improvement in biodiversity compared to the pre-development baseline. Rather than merely minimizing ecological damage, BNG mandates that the residual biodiversity value after development—including on-site retention, on-site enhancement, and off-site compensation—exceeds the original value by a specified percentage. England's Environment Act 2021 made a minimum 10% BNG mandatory for most planning permissions from February 2024 (small sites from April 2024).

Why It Matters

BNG represents a paradigm shift in how development interacts with nature. The traditional approach—avoid, minimize, and compensate for biodiversity loss—aimed to prevent net loss but rarely achieved it. Studies of biodiversity offsets under older frameworks found that the majority failed to deliver equivalent biodiversity outcomes. BNG raises the bar by requiring demonstrated improvement, not just mitigation.

The mandatory 10% BNG requirement in England affects most development requiring planning permission—housing, commercial, infrastructure, and energy projects—though householder applications, developments with minimal habitat impacts and, from August 6, 2026, most sites of 0.2 hectares or less are exempt. Developers must submit a biodiversity metric calculation demonstrating how they will achieve the required gain, and the commitment is secured through a 30-year management and monitoring plan. This has created a new market for biodiversity units and habitat banking.

The financial implications are significant. Developers who cannot achieve BNG on-site can buy off-site biodiversity units from habitat banks; statutory biodiversity credits, sold by the government only as a last resort, cost £42,000 to £650,000 per credit depending on habitat, and each unit needed requires two credits. This creates a strong incentive to deliver BNG through good design and on-site habitat creation rather than purchasing expensive credits. The market for off-site habitat banking, where landowners create or enhance habitats and sell the resulting biodiversity units, is growing rapidly.

Other jurisdictions are watching England's implementation closely. Scotland's National Planning Framework 4 (2023) requires major developments to show significant biodiversity enhancement, and Wales' planning policy requires a net benefit for biodiversity. The EU's Nature Restoration Law requires member states to restore degraded ecosystems, which may evolve toward BNG-style requirements for development. The concept is likely to become a global standard for reconciling development with nature conservation.

How It Works / Key Components

BNG is measured using the statutory biodiversity metric (which replaced Biodiversity Metric 4.0 when BNG became mandatory in February 2024; a simplified small sites metric is available for minor development), a habitat-based calculator that assigns biodiversity units to land parcels based on habitat type, condition, and strategic significance. The metric is designed to be applied by ecologists at the pre-development stage, producing a baseline biodiversity value against which post-development outcomes are measured.

The mitigation hierarchy remains central: avoid impacts where possible, minimize unavoidable impacts, and compensate for residual losses. BNG adds a fourth step—enhancement—requiring the final outcome to exceed the baseline by at least 10%. Developers must demonstrate that they have followed the hierarchy rather than jumping directly to off-site compensation, although from August 2026 minor developments above the 0.2-hectare exemption may go straight to off-site gains.

On-site measures include retaining existing high-value habitats, enhancing degraded habitats, creating new habitats as part of landscaping and green infrastructure, and incorporating biodiversity features into building design (green roofs, swift bricks, bat boxes). Off-site measures involve creating or enhancing habitats on land outside the development boundary, secured through conservation covenants or planning obligations.

Habitat management and monitoring plans ensure that created or enhanced habitats achieve their target condition. Plans must cover a minimum 30-year management period, with regular monitoring and reporting. If habitats fail to reach target condition, remedial action is required. This long-term commitment distinguishes BNG from historical mitigation approaches that often lacked follow-through.

Council Fire's Approach

Council Fire integrates biodiversity net gain into development planning from the earliest stages, ensuring BNG requirements enhance rather than constrain project outcomes. We conduct baseline assessments, optimize site designs to maximize on-site BNG delivery, evaluate off-site options where needed, and structure long-term management plans that satisfy regulatory requirements.

Frequently Asked Questions

Does BNG apply to all developments?

In England, mandatory BNG applies to most developments requiring planning permission. Householder applications and developments with minimal habitat impacts are exempt, and from August 6, 2026 most sites of 0.2 hectares or less are exempt too, unless onsite priority habitat is harmed. Nationally significant infrastructure projects come under a separate BNG regime from November 2, 2026.

How much does BNG add to development costs?

Defra's 2019 impact assessment estimated net gain delivery costs at 0.1–0.8% of build costs for brownfield housing and 0.1–3.9% for greenfield housing, depending on region and delivery approach. Costs are higher when developers rely on off-site credits due to poor on-site design. Early ecological input during masterplanning significantly reduces costs.

What is a biodiversity unit?

A biodiversity unit is the currency of BNG measurement—a composite score reflecting habitat area, type, condition, and strategic significance. One unit does not equal one hectare; high-quality habitats in strategic locations generate more units per hectare than degraded habitats in non-strategic locations.