What Is Biodiversity Net Gain? A Developer's Complete Guide

Since February 2024, every planning application in England must demonstrate a minimum 10% biodiversity net gain (BNG). This isn't optional — it's the law, introduced through the Environment Act 2021 and enforced by local planning authorities.

If you're a developer, landowner, or anyone involved in the planning process, this guide explains exactly what BNG means for your project, what it costs, and how to comply without losing weeks to ecological surveys.

What Is Biodiversity Net Gain?

Biodiversity Net Gain is a legal requirement that developments must leave biodiversity — the variety of plant and animal life — in a measurably better state than before the development took place.

In practice, this means:

• Your site's baseline biodiversity value is measured in "biodiversity units" using the Statutory Biodiversity Metric
• Your post-development plan must show at least 10% more units than the baseline
• This must be maintained for at least 30 years

Think of it like carbon offsetting, but for nature. If you build on grassland, you need to create or enhance enough habitat elsewhere to more than replace what was lost.

Who Does It Apply To?

Since April 2024, BNG applies to virtually all planning permissions in England, including:

• Major developments (10+ dwellings or 1+ hectare) — since February 2024
• Small developments (1-9 dwellings, under 1 hectare) — since April 2024

Some exemptions exist for householder applications (extensions, loft conversions), permitted development, and urgent Crown developments. But if you're building new homes or commercial space, you almost certainly need BNG.

The Statutory Biodiversity Metric

The metric is the official tool for calculating biodiversity units. Currently at the current statutory version, it considers:

Factor	What It Measures
Habitat type	What's growing there (woodland, grassland, wetland, etc.)
Area or length	Size in hectares (area habitats) or kilometres (hedgerows, watercourses)
Distinctiveness	How ecologically valuable the habitat type is (V.Low to V.High)
Condition	How healthy the habitat is (Poor, Moderate, Good)
Strategic significance	Whether it's identified in local nature recovery strategies

The formula multiplies these factors together. Higher quality habitats generate more biodiversity units — which means losing them costs more to replace.

Three Separate Modules

The metric calculates three types of biodiversity units separately:

1. Area habitat units — for woodlands, grasslands, urban habitats, etc.
2. Hedgerow units — measured in linear metres
3. Watercourse units — rivers, streams, ditches, canals

The 10% gain target must be met in each module independently. You can't offset lost hedgerows with extra grassland.

Small Sites Metric (SSM)

The Small Sites Metric is a simplified statutory route that may apply to certain eligible small developments. Eligibility is project-specific and must be checked against current government guidance. Key differences can include:

• Fewer habitat types to choose from
• Distinctiveness is auto-assigned (you don't need to score it)
• Competence still matters — confirm eligibility and whether professional input is required
• Simpler condition assessment

Who can complete formal SSM work depends on current competence requirements and the evidence needed for the site. Professional ecological input may still be required.

How Much Does BNG Cost?

This is where developers get nervous. The costs vary enormously depending on your site:

Cost area	What affects it
Initial screening	BioGain's core early-stage screen is currently free
Ecological surveys and metric work	Site, season, habitats, species risk, scope and evidence requirements
On-site delivery	Design, land take, habitat type, establishment risk and long-term management
Off-site units or statutory credits	Actual residual need, location, habitat type, trading rules and current market or statutory prices

A practical approach: screen early, confirm which metric may apply, then obtain the surveys and competent professional input required for the project. Screening helps frame questions; it does not determine formal eligibility or replace evidence.

What You Need to Submit

For a planning application with BNG, your Local Planning Authority will typically require:

1. Completed biodiversity metric — the statutory biodiversity metric calculation tool (or SSM for small sites)
2. Biodiversity Gain Plan — showing how you'll achieve the 10% gain
3. Habitat Management and Monitoring Plan — how habitats will be managed for 30 years
4. Pre-development habitat map — what's on the site now
5. Post-development habitat map — what you plan to create/retain

The Mitigation Hierarchy

BNG doesn't mean you can bulldoze ancient woodland and plant some wildflowers. There's a strict hierarchy:

1. Avoid — Design around existing habitats where possible
2. Minimise — Reduce the impact on habitats you can't avoid
3. Restore — Rehabilitate damaged habitats on-site
4. Offset — Create new habitats to compensate for unavoidable losses

LPAs will want to see evidence that you've followed this hierarchy before approving off-site solutions.

Trading Rules

You can't replace high-value habitats with low-value ones. The statutory biodiversity metric enforces trading rules:

• Medium distinctiveness losses must be replaced by medium+ habitats in the same broad habitat type
• High/Very High distinctiveness losses require like-for-like or better replacement
• Low/Very Low habitats have more flexible trading options
• Trading rules apply until you reach "no net loss" — after that, you can use any habitat type to hit the 10% target

Common Mistakes

1. Not checking early enough — Finding out your site has ancient woodland at planning stage is an expensive surprise. Screen the site early.
2. Ignoring hedgerows — They're a separate module in the metric. Removing hedgerows requires hedgerow replacement, not just grassland.
3. Assuming off-site delivery has a standard price — Costs vary by location, habitat, trading rules, availability and the actual residual need.
4. Using the wrong metric version — Make sure you're using Statutory Biodiversity Metric (the current version), not an older version.
5. Forgetting the 30-year commitment — Habitat management plans need funding and monitoring for three decades.

Key Dates

Date	What Happened
November 2021	Environment Act receives Royal Assent
November 2023	Statutory Biodiversity Metric published
February 2024	BNG mandatory for major developments
April 2024	BNG mandatory for small developments
2025	Statutory Biodiversity Metric updated, Small Sites Metric revised

Getting Started

If you're a developer with a site in England, here's the practical path:

1. Screen your site — Use BioGain for a free initial review of available data
2. Check metric eligibility — Confirm the applicable rules and competence requirements using current guidance
3. Investigate the baseline — Verify habitats, condition and units before relying on them in design
4. Design for gain — Green roofs, SuDS, native planting, hedgerow retention all contribute
5. Get competent professional input — Match surveys and expertise to the site's habitats, species risk and formal evidence requirements

Early screening can surface questions sooner, but it does not predict the eventual cost or complexity of formal compliance.